Advanced beneficiation of bastnaesite ore through centrifugal concentration and froth flotation

(1)

ADVANCED BENEFICIATION OF BASTNAESITE ORE THROUGH CENTRIFUGAL CONCENTRATION AND FROTH FLOTATION

by Doug Schriner

(2)

ii

A thesis submitted to the Faculty and the Board of Trustees of the Colorado School of Mines in partial fulfillment of the requirements for the degree of Master of Science (Metallurgical and Materials Engineering).

Golden, Colorado Date ______________ Signed: ________________________ Doug Schriner Signed: ________________________ Dr. Corby Anderson Thesis Advisor Golden, Colorado Date ______________ Signed: ________________________ Dr. Ivar Reimanis Professor and Head Department of Metallurgical and Materials Engineering

(3)

iii

ABSTRACT

Gravity separation and flotation studies have been conducted on Molycorp bastnaesite ore in order to determine if new beneficiation schemes present a more selective and more economical alternative than that which is currently employed at Mountain Pass. Literature on bastnaesite, monazite, barite, and calcite flotation and gravity concentration principles was surveyed. Flotation reagent additions were determined using components that have shown preferential floatability of bastnaesite and monazite over the gangue minerals. Hallimond Tube microflotation tests were performed on crushed and ground ore samples. Heavy liquid separation with sodium polytungstate was used to investigate the effectiveness of gravity separation on the ore. Shaking table and Falcon concentrator tests were performed to gravity concentrate the ore. A gravity-concentrated feed was floated and compared with a non-concentrated ore feed to illustrate the benefit of preconcentration. An economic analysis was generated for flotation plants operating with and without gravity preconcentration that would sell products with two distinct grades and recoveries.

Qualitative microflotation tests produced little selective separation of the rare earth minerals (bastnaesite, parisite, and monazite) from the gangue (calcite, barite, dolomite, and quartz). Heavy liquid tests illustrated the sink/float behavior of the

minerals at different specific gravities of separation. Their results suggest that at higher specific gravities the calcite floats while the bastnaesite and barite sink. Shaking table tests showed potential to effect such a separation, but optimum conditions were not determined. A Falcon centrifugal concentrator was used to carry out tests according to a Design of Experiments matrix generated with Stat Ease Design Expert 9. The best conditions from those trials were determined, and the tests were repeated to verify the desirability of those parameters. Bench flotation was then used to compare the standard feed at plant conditions to a feed consisting of the blended gravity concentrates. The flotation results showed that the preconcentrated feed outperformed the typical plant feed. Economic analysis of a plant with and without gravity preconcentration shows that gravity preconcentration, although more capital-intensive, will yield a higher annual profit and a better 10-year net present value.

(4)

iv

LIST OF FIGURES

Figure 1.1. Beneficiation Flowsheet at Mountain Pass (as of 1991) [2] ... 2 Figure 1.2. Molycorp REO Flotation Circuit ... 3 Figure 1.3. REO Export Prices from China. Data retrieved from

www.metal-pages.com ... 4 Figure 1.4. Sichuan Mianning REE Deposit Flowsheet. [10] ... 7 Figure 1.5. Egyptian Beach Sand Beneficiation Flowsheet. The first percentage

given relates to the original weight and the second represents the assay of monazite. [12] ... 8 Figure 2.1. Visual Representation of Young's Equation, showing the contact

angle of a hydrophobic (left) and hydrophilic (right) surface. [19] ... 10 Figure 2.2. Illustration of the geometry used to determine the contact angle of

water on carbon. [19] ... 11 Figure 2.3. Aqueous Equilibria of cerium species. [22] ... 12 Figure 2.4. Schematic of electrical double layer (from Somasundaran 1975).

[23] ... 13 Figure 2.5. Zeta Potential of bastnaesite and monazite. (Luo Jiake 1984, from

[16]) ... 15 Figure 2.6. Zeta potential of barite, bastnaesite, and calcite in pure water.

(Smith 1986, from [16]). ... 16 Figure 2.7. Oleate adsorption onto bastnaesite as a function of pH with and

without pre-boiling. (Smith 1986 from [16] ... 17 Figure 2.8. Adsorption isotherm of oleate on calcite at pH 9.3. [21] ... 18 Figure 2.9. Adsorption Density of hydroxamate on barite, calcite, and

bastnaesite at 21°C and pH 9.3. [22] ... 18 Figure 2.10. Schematic Drawing of a Modified Hallimond Tube. [31] ... 20 Figure 2.11. Effect of pulp temperature on oleate flotation. ... 20 Figure 2.12. The flotation recoveries of Mountain Pass ore as a function of

conditioning temperature for fatty acids (left) and hydroxamate

(7)

vii

Figure 2.13. Calcite recovery as a function of fatty acid addition for various fatty

acids at pH 9.7. [34] ... 24

Figure 2.14. Calcite recovery as a function of pH for various fatty acids at a constant collector addition of 10-3 mol/liter. [34] ... 24

Figure 2.15. Structure of oleic acid ... 24

Figure 2.16. Bastnaesite recovery by sodium oleate as a function of pH. [26] ... 25

Figure 2.17. Oleate concentration required to float salt-type minerals.[21] ... 26

Figure 2.18. A mechanism of hydroxamate adsorption to an ion on a mineral surface. [20] ... 26

Figure 2.19. The structure of potassium octyl hydroxamate. [22] ... 27

Figure 2.20. Modified hydroxamic acid chelating surface cerium(III). [37] ... 27

Figure 2.21. Hallimond tube flotation results on bastnaesite with K-octyl hydroxamate as collector. [22] ... 28

Figure 2.22. Bastnaesite ore recovery (as total weight, including gangue, recovered) by K-octyl hydroxamate as a function of pH. [22] ... 28

Figure 2.23: Bastnaesite recovery as a function of ammonium lignin sulfonate concentration. [26] ... 30

Figure 2.24. Recovery of calcite as a function of increased concentration of metal salts. [33] ... 30

Figure 2.25: Effect of sodium metasilicate for oleate (top) and hydroxamate (bottom) flotation. [38] ... 31

Figure 2.26. Adsorption and Floatability of sodium oleate and sodium silicate on calcite and fluorite as a function of sodium silicate concentration. [21] ... 32

Figure 2.27. Floatability of different minerals versus Na2SiO3. (Luo Jiake 1984, from [16] ... 32

Figure 2.28. Applicability of Beneficiation Equipment to a given feed size. [41] ... 35

Figure 2.29. Effect of contaminating slimes and suspension viscosity on the cleaning of Roslyn coal. [42] ... 36 Figure 2.30. XRD Patterns of Molycorp Bastnaesite size fractions upgraded with

(8)

viii

diffraction peaks have been normalized to the maximum peak

intensity for each pattern. [40] ... 36

Figure 2.31. Density of aqueous sodium polytungstate solution as a function of mass percent. [43] ... 37

Figure 2.32. Deister shaking table deck. Dark circles represent low-s.g. particles. White circles represent high-s.g. particles. The diameter of the circles corresponds of the particle size. [45] ... 38

Figure 2.33. Fluid Motion over riffles on a Deister deck. [45] ... 38

Figure 2.34. Wilfley shaking table ... 39

Figure 2.35. Knelson Concentrator schematic. [46] ... 40

Figure 2.36. Cumulative gold retained as a function of particle size. [47] ... 41

Figure 2.37. Cross-Section of a Falcon Concentrator ... 42

Figure 3.1. Modified Hallimond Tube used for microflotation tests. ... 45

Figure 4.1. Particle size distributions for ground ore samples ... 49

Figure 4.2. False-Color image of bastnaesite ore +50 mesh size fraction. Values represent surface area percentages. ... 52

Figure 4.3. False-Color image of 90-minute ground bastnaesite ore 200 x 400 mesh size fraction. Values represent surface area percentages. ... 52

Figure 4.4. Cumulative REE mineral recovery by liberation class from MLA. ... 54

Figure 4.5. Molycorp ore zeta potential ... 54

Figure 4.6. Elemental recovery as a function of collector concentration ... 56

Figure 4.7. Concentrate grade as a function of collector concentration. ... 56

Figure 4.8. Elemental recovery as a function of ammonium lignin sulfonate concentration. ... 57

Figure 4.9. Concentrate grade as a function of ammonium lignin sulfonate concentration. ... 57

Figure 4.10. Elemental recovery as a function of sodium silicate concentration. .... 58

(9)

ix

Figure 4.12. Elemental recovery as a function of copper (II) nitrate

concentration. ... 59

Figure 4.13. Concentrate grade as a function of copper (II) nitrate concentration. . 60

Figure 4.14. WHIMS concentrate TREE grade (left)and recovery (right). ... 62

Figure 4.15. Barium distribution between WHIMS magnetic and non-magnetic fractions. ... 62

Figure 4.16. Calcium recoveries to HLA products. ... 63

Figure 4.17. Mass recovery as a function of fluid density. ... 64

Figure 4.18. Shaking Table Streams ... 65

Figure 4.19. Gravity concentration circuit. ... 67

Figure 4.20. Desirability Surface for Falcon tests with 50-micron feed size. ... 69

Figure 4.21. TREE Grade vs Recovery of Falcon Concentrates. ... 70

Figure 5.1. Sensitivity Analysis for the gravity-flotation concentrator. ... 73

Figure A.1. Cumulative particle size analysis of ground Molycorp bastnaesite ore. ... 83

Figure A.2. Liberation of Bastnaesite according to size fraction. Left: +100 mesh, Center: 100 x 325 mesh, Right: 325 x 400 mesh. ... 85

Figure A.3. False-color image showing particle association of the +100 mesh size fraction. ... 85

Figure A.4. False-color image showing particle association of the 100 x 325 mesh size fraction... 86

Figure A.5. False-color image showing particle association of the 325 x 400 mesh size fraction... 86

Figure A.6. Measured and WPPF-calculated diffractograms and residual plots for the bastnaesite ore. ... 87

Figure A.7. Bastnaesite ore diffractogram with candidate phases. ... 87

Figure A.8. QEMSCAN mineral mass abundance in each size fraction. ... 90

Figure B.1. Mineral locking for bastnaesite. ... 93

(10)

x

Figure B.3. Mineral locking for monazite. ... 94 Figure B.4. Mineral locking for calcite. ... 94 Figure B.5. False-color image showing particle association of the +100 mesh

size fraction. ... 95 Figure B.6. False-color image showing particle association of the +100 mesh

size fraction. ... 95 Figure B.7. False-color image showing particle association of the +100 mesh

size fraction. ... 96 Figure B.8. Measured and WPPF-calculated diffractograms and residual plot

for the original bastnaesite ore. ... 96 Figure B.9. Bastnaesite ore diffractogram with candidate phases. ... 97 Figure D.1. Sensitivity Analysis for the flotation concentrator. ... 101

(11)

xi

LIST OF TABLES

Table 1.1. Flotation Conditioning Steps at Mountain Pass. [3] ... 2

Table 1.2. Characteristics of RER scrubbing tests. [14] ... 7

Table 2.1. Bastnaesite Points of Zero Charge (PZC). [24] ... 14

Table 2.2. Monazite Points of Zero Charge (PZC). [24] ... 14

Table 2.3. Points of Zero Charge (PZC) of alkaline-earth semisoluble salt minerals. [24] ... 15

Table 2.4. Specific Gravity (SG) and Concentration Criterion (CC) for major components of Molycorp ore. ... 34

Table 3.1. Microflotation Reagents ... 44

Table 3.2. WHIMS DOE Parameters ... 46

Table 3.3. Falcon DOE Parameters ... 47

Table 3.4. Bench Flotation Timetable ... 48

Table 4.1. P80 of Ground Ore Samples ... 50

Table 4.2. Modal Mineral Content of Major Components of Bastnaesite Ore. REE-bearing minerals are in bold. ... 51

Table 4.3. Elemental Composition of Bastnaesite Ore ... 51

Table 4.4. Mass and Bastnaesite Distributions. The mass percentage for each size fraction is given in plain text and the bastnaesite distribution in bold. ... 53

Table 4.5. REE mineral and calcite liberation as cumulative mass recovery. Bolded values represent the combined total of REE minerals (bastnaesite, parisite, monazite, allanite). ... 53

Table 4.6. Calcium, barium, and TREE grade and recovery of WHIMS concentrate products. ... 61

Table 4.7. Stream Concentrations ... 64

Table 4.8. Shaking table mass balance for two similar 500 g tests, 2 and 3. ... 65

Table 4.9. Mass balance for thrice-tabled concentrate. Products are the final concentrate and combined total middlings and tailings. ... 67

(12)

xii

Table 4.10. Signal-to-Noise Ratio for Falcon DOE Responses ... 68

Table 4.11. ANOVA Table for the TREE Grade Model. ... 68

Table 4.12. Grades, recoveries, and concentration ratios of Falcon products. ... 69

Table 4.13. Bench Flotation Results: TREE and Calcium Grade and Recovery ... 70

Table 5.1. Comparison of Plant Annual and Capital Expense Differences (calculated as gravity-flotation plant values less flotation plant values). ... 72

Table 5.2. Concentrator economic model parameters (Operation). ... 74

Table 5.3. Concentrator economic model parameters (Capital Costs). ... 75

Table A.1. Size fraction volume and weight percentages ... 83

Table A.2. Composition (weight percent) of each size fraction as reported by MLA analysis. ... 84

Table A.3. Composition (mass percent) of each size fraction as reported by QEMSCAN. ... 88

Table A.4. Overall calculated mineral abundance of the ground Molycorp bastnaesite ore. ... 89

Table A.5. Composition comparison between QEMSCAN, MLA, XRD, and XRF. ... 89

Table B.1. Microtrac particle size distributions for grind test samples ... 91

Table B.2. Modal mineral content of the bastnaesite ore (wt%). ... 92

Table B.3. REE cumulative mass recovery to each liberation class ... 97

Table C.1. Experimental data for microflotation tests ... 98

Table C.2. Experimental data for WHIMS tests ... 98

Table C.3. Experimental data for bench flotation tests ... 98

Table C.4. Experimental data for Heavy Liquid tests ... 99

Table C.5. Experimental data for Deister table tests ... 99

Table C.6. Experimental data for Falcon tests ... 100

(13)

xiii

(14)

1

CHAPTER 1:

INTRODUCTION

In comparison to the broad spectrum of applications based on the rare earth elements, their supply shows little diversity. Their applications range from polishing media to hard drive magnets to water treatment additives to wind turbine motors. Prior to the 1950s the mineral providing the bulk of the rare earth supply was monazite; a phosphate mineral which was beneficiated primarily from placer deposits. There it could be separated from associated ilmenite, garnet, magnetite, quartz, rutile, and xenotime using a simple physical separation scheme utilizing differences in specific gravity, magnetism, and conductivity. For years this was the source material for cerium, lanthanum, neodymium, and thorium.[1]

In the 1950s the Mountain Pass Mine was developed by the Molybdenum Corporation of America, now Molycorp Inc. This deposit in southern California is the world’s richest source of bastnaesite, a fluorocarbonate containing cerium and

lanthanum, as well as the heavy rare earths neodymium and praseodymium. Separation is much more difficult than with monazite from beach sands. This deposit contains bastnaesite and monazite as the primary valuable minerals (10% of the ore) with calcite and other carbonates (60%), barite (20%), and quartz and other minerals (10%).

Beneficiation of the rare earths from the bastnaesite ore at Mountain Pass

involves concentration by flotation followed by roasting, leaching with hydrochloric acid, and solvent extraction. [1] Traditionally, Molycorp employed a crush, grind, float system to produce a 63% rare earth oxide (REO) concentrate, shown in Figure 1.1. The

flotation conditioning involves alternating additions of steam, soda ash, lignin sulfonate, and tall oil, shown stepwise in Table 1.1. Flotation occurs at temperatures near 82 °C in several roughing, cleaning, and scavenging stages (Figure 1.2). The rougher produces a 30% REO concentrate which is further upgraded to 63% REO through cleaning. This is achieved at a 65-70% recovery.

(15)

2

Figure 1.1. Beneficiation Flowsheet at Mountain Pass (as of 1991) [2]

Table 1.1. Flotation Conditioning Steps at Mountain Pass. [3]

Step Reagent(s) Added

Soda ash Sodium fluosilicate Steam (Bubbled through pulp) 2 Steam (Bubbled) Ammonium Lignin Sulfonate Steam (Bubbled) 4 Steam (Bubbled)

Distilled Tall Oil Steam (Bubbled) 6 Steam (Bubbled) 2.5 - 3.3 0.4 5 0.3 Amount Added (kg/ton) 1 3 2.5 - 3.3

(16)

3

(17)

4

China displaced the United States as the dominant supplier of rare earths in the mid-1980s due to production from the Bayan Obo mine in Inner Mongolia, and now is responsible for more than 90% of the world’s rare earths supply. Little effort has been put forth toward seeking a domestic source of the elements due to the cheap price of rare earths exported from China. Mountain Pass had been unable to compete with Chinese producers; it halted production in 2002 and resumed stockpile processing in 2008.

Hendrick compiled a report on U.S. rare earth commercial activity in 2008. No rare earths were mined, but stockpiled concentrate was processed at the Mountain Pass Mine. Bastnaesite concentrate and monazite prices, respectively compiled from USGS sources and U.S. import values, were $8,000/ton and $480/ton. [4] Again

straining the supply in the US was the 2010 decision by the Chinese government to limit the volume of rare earth exports. This sanction caused a drastic spike in the price of the rare earth elements (shown in Figure 1.3) and brought questions to light of their

availability, considering their importance in strategic applications.

Figure 1.3. REO Export Prices from China. Data retrieved from www.metal-pages.com

In 2011, the US Department of Energy released its Critical Materials Strategy; identifying rare earth materials as “critical” and in need of a more reliable domestic

(18)

5

supply. From this, the Critical Materials Institute was created with Molycorp as a partner in order to investigate their complex beneficiation problem. [5] Gleason’s 2011 article highlighted Molycorp’s strategic plan to resume production of rare earth minerals at Mountain Pass up to a goal of 40,000 metric tons/year by 2013. It also discussed some of the policy proposals laid out to accelerate the United States’ rare earth metals

production. [6]

As of February 2015, Molycorp struggled to upgrade plant operations and compete with low-cost rare earth products from China. Rare earth oxide equivalent production by Molycorp for 2014 was 4,785 mt, which was an increase from 3,473 mt in 2013. [7] Rare earth prices had fallen dramatically from their highs in 2011. Due to this, Molycorp was forced to suspend operations in October of 2015 and operate only for care and maintenance of the operation. [8]

The general forecast is that an increase in rare earth production is needed to meet growing worldwide demand, and it will not be met from Chinese supply alone. More than 90% of the world’s rare earths come from China, but that comes from only 25% of the world’s reserves, accounting for recent estimates containing Canadian and Australian reserves.

Beneficiation of rare earths was recently summarized by Jordens, focusing on bastnaesite at Mountain Pass and Bayan Obo and monazite beach sands elsewhere in the world. Extensive Chinese knowledge in rare earth element (REE) flotation abounds in papers but many of them lack scientific depth and/or accuracy. Gravity concentration of rare earth minerals has been historically difficult due to the influence (and loss) of rare-earth-containing fines and the similar specific gravity of barite. [9] Limited

commercial success has been seen with gravity separation of bastnaesite. The Sichuan Mianning REE Ore deposit uses shaking tables to separate minerals from pre-classified feeds. The mineralogy there is a carbonate containing barite, fluorite, and iron- and manganese-containing minerals with a 3.7% REO grade – most of which is bastnaesite – in coarse (>1mm) and fine powder (80% -325 mesh Tyler ) sizes. An all-gravity

operation classifies and concentrates a 62% passing 200 mesh feed to achieve grades of 30%, 50%, and 60% with an overall recovery of 75%. Gravity and flotation are also

(19)

6

combined (Figure 1.4) to concentrate the feed and produce an overall 30% grade gravity concentrate at 74.5% REO recovery. The concentrate is then reground to 70% -200 mesh and floated with hydroxamic acid, phthalate, sodium carbonate, and sodium silicate to produce a 60% REO grade concentrate with a rare earth recovery of 50-60%. [10] Lab scale beneficiation examples are more abundant. A Mozley Multi-Gravity Separator was used to separate a Turkish bastnaesite ore, producing a 35.5% REO preconcentrate at 48% recovery. [11] Egyptian beach monazite was concentrated using electrostatic, magnetic, and gravitational methods. [12] The beach sands were screened to pass 1mm and deslimed then concentrated successively with a shaking table. The concentrate was subjected to low intensity magnetic separation and the nonmagnetic fraction beneficiated with a shaking table. The shaking table concentrate was dried and processed using high tension electrostatic separation, magnetic separation, and a shaking table again to produce a crude 85% monazite concentrate. More electrostatic and magnetic separation produces a 97% monazite concentrate (Figure 1.5). Humphrey spirals were used to concentrate an Iranian monazite ore. [13] Optimum results were found with an intermediate feed size, high feed rate (1.5 L/s), and low solids density (15%), with the latter parameter showing a less-significant effect. The best total rare earth elements (TREE) grade was reported at 6050x10-6 percent with a 57.06% recovery after gravity separation and leaching.

A developing site, Bear Lodge, owned by Rare Element Resources (RER) has achieved success with purely physical concentration. The deposit is a carbonatite, containing rare earths mostly as bastnaesite, with three regions of mineralization: oxide, sulfide, and transition. Drill core test work showed that the oxide core was successfully treated by scrubbing and sizing. The rare earth oxides were found to concentrate in the passing 500 mesh size. Results of scrubbing tests on the ore are shown in Table 1.2, where it can be seen that a 10-minute scrub yielded the maximized results in relation to recovery, weight rejected, and scrub time. [14]

(20)

7

Figure 1.4. Sichuan Mianning REE Deposit Flowsheet. [10]

Table 1.2. Characteristics of RER scrubbing tests. [14]

Characteristic Washed Scrub

5 min Scrub 10 min Scrub 15 min Scrub 20 min Scrub 60 min Weight % retained @ 90% recovery 52 50 40 37 48 52 REO recovery % @ 50% passing 82 89 91 92.5 88 87 Assay grade @ 90% recovery 13.5 14 16 13.5 17.5 13 Mesh size @ 90% recovery 100 35 35 35 48 150 Head grade % TREO 9.11 7.82 7.15 6.21 9.27 8.62 % < 500 mesh 16.55 14.82 14.42 14.64 22.05 19.47 < 500 mesh TREO grade 19.06 19.64 20.06 20.12 20.39 20.51

(21)

8

Figure 1.5. Egyptian Beach Sand Beneficiation Flowsheet. The first percentage given relates to the original weight and the second represents the assay of monazite. [12]

(22)

9

CHAPTER 2:

LITERATURE REVIEW

Beneficiation of rare earth minerals has been reviewed previously. [15], [16] Special attention is given to the minerals (bastnaesite, monazite, xenotime, calcite, and barite), the collectors (oleic and hydroxamic acids), and the modifiers (soda ash, lignin sulfonate, metal salts, sodium silicate) used in this study. Methods of gravity

concentration have also been surveyed. Prior to the discussion of reagents and their effect in flotation systems, a review of surface chemistry and flotation phenomena, as well as common analysis techniques, will be presented.

2.1 Flotation Surface Chemistry and Analysis Techniques

Flotation is a mineral processing technique that is widely used to concentrate an ore before recovery by pyrometallurgical or hydrometallurgical routes. It involves

bubbling air through a tank of ground and crushed mineral pulp. Reagents are

specifically chosen so as to cause selective adsorption of the desired mineral to the air bubbles. The mineral then rises through the pulp attached to the bubble where it sticks to the other bubbles in the froth. The froth is skimmed off or collected via overflow, and contains the upgraded ore. Often several steps are needed after concentration in the initial cell (“roughing”) to recover value from the concentrate (“cleaning”) or tails (“scavenging”). Several phenomena are at play, including the hydrophobicity of the minerals, the electrical potential of the minerals and solution, dissolution of mineral species, exposed surface area, and adsorption of reagents to mineral surfaces; many of which are affected by the temperature. Laboratory testing of flotation systems occurs in specially designed cells originally developed by A.F. Hallimond.

Fuerstenau collected data on contact angles, adsorption density, zeta potential, and flotation rate for a quartz-dodecylamine system over the full pH range. By plotting them together on sensible scales, their correlation was shown. That work refuted the claim that solid-liquid interface phenomena (adsorption density, zeta potential) cannot be connected to solid-liquid-gas interface phenomena (contact angle, flotation rate). [17] Hence many of these techniques can be used to predict overall flotation behavior. The

(23)

10

benefits and shortcomings of some of the industry’s common floatability tests have been previously detailed. There is no one industry standard, because all of the tests seem to have some sort of bias, whether it is operator expertise, non-industrial chemistry, unrealistic conditions, intensive time requirements, or some combination thereof. [18]

2.1.1 Hydrophobicity and Contact Angle

Whether or not water attaches to a surface (of a mineral in this case) is qualified by its hydrophobicity, it’s “fear of water”. Water will attach to and wet a hydrophilic surface, but not attach to a hydrophobic substance. Sulfur and graphite are very

hydrophobic, while calcite, quartz, and gypsum are hydrophilic materials. The source of this attraction is in the interfacial energies of the solid/air, solid/liquid, and liquid/air interfaces, related by Young’s Equation, represented visually in Figure 2.1:

Figure 2.1. Visual Representation of Young's Equation, showing the contact angle of a hydrophobic (left) and hydrophilic (right) surface. [19]

The contact angle can be experimentally determined using the sessile drop technique, which involves taking high-framerate consecutive images of a water drop as it is placed on a mineral surface. The geometry used to determine the contact angle is set up Figure 2.2. At the moment it hits the mineral surface, the water droplet forms a dome with a base diameter of d. If this dome were to be extended below the surface to form a sphere, that sphere’s radius would be R. The height of the dome above the mineral surface is given as a, and b is the difference in length between the sphere’s radius and the dome’s height (R minus a). The contact angle is then found at the tangent point T on the mineral surface using the triangle OBT.

(24)

11

Figure 2.2. Illustration of the geometry used to determine the contact angle of water on carbon. [19]

Studies on bastnaesite, monazite, calcite and barite have shown that, like most minerals, they have hydrophilic surfaces. Flotation is a matter of selectively making mineral surfaces hydrophobic so they will become aerophilic, bind to the air bubbles, and float.

2.1.2 Hydrolysis Reactions

When an ionic compound is placed in water, it will dissolve until equilibrium is reached. When sodium chloride does this, it leaves effectively no solid behind as the sodium and chlorine ions diffuse away from one another surrounded by shells of protons or electron pairs from water molecules. The result is a neutral solution. Some compounds, such as weak acids, equilibrate with a large concentration of the neutral species. The family of weak acids includes fatty acid collectors, like oleic acid. As it dissolves, a proton is taken away from the neutral acid and what remains is a charged molecule that can adsorb to an oppositely-charged mineral surface. Oleic and alkyl-hydroxamic acids can form salts with sodium and potassium to become sodium oleate and potassium octyl hydroxamate (when the alkyl chain is an octyl group). Collector structures and features will be detailed later. Another hydrolysis phenomenon takes place at the mineral surface, where exposed ions can attract charge-balancing H+ or OH- ions. The hydrolysis of cerium is particularly relevant in rare earth flotation: the

O T B

(25)

12

dissolution, hydrolysis, and readsorption of cerium is the primary attachment mechanism for hydroxamate collectors. [20]

The solubility product for the dissociation reaction HOl = H+ + Ol- at 20°C is 10 -10.9

. Calcium and Barium oleate solubility products are 10-12.4 and 10-11.8. [21] Solubility products for hydroxamic acid and calcium and cerium hydroxamates are 109.35, 102.4, and 105.45. The equilibrium constant for the hydroxylation of trivalent cerium ions, Ce(OH)3 = Ce3+ + 3OH-, is 1.5x10-20. [22]

The aqueous equilibria of cerium species is given in Figure 2.3. As the pH is increased, the species in equilibrium become less-positively charged. At a pH of around 10, the dominant species is the uncharged Ce(OH)3. At lower pH, the charged species Ce3+, Ce(OH)2+, and Ce(OH)2+ are dominant.

(26)

13

2.1.3 Zeta Potential and Point of Zero Charge

For salt-type minerals, unequal dissolution can occur because the ionic

components of the salt have different sizes and charges, which do not enter solution at the same rate. Because of this mineral surfaces are charged in solution. That surface attracts oppositely-charged ions from the bulk solution and creates an electrical

potential. The ions manipulating the surface charge are called the potential-determining ions, and can consist of H+, OH-, CO32-, SO42-, and other ions (particularly those

dissolving from the mineral). The Stern plane, illustrated in Figure 2.4, is the name given to the plane at which bound counter ions cannot come closer to the surface. The shear plane is the plane at which ions are capable of motion in the solution when forced. The potential at the Stern plane cannot be determined experimentally, but the potential at the shear plane can be, and is known as the zeta potential. At a specific pH this

potential becomes zero. That pH is referred to as the PZC, the point of zero charge, for that mineral.

(27)

14

The PZC is used as a marker for which kind of surfactant can be used with a mineral. At pH higher than the PZC, a surface appears positively charged (and the solution negative) as positive counter ions bind to the mineral surface, but the reverse is true below it. For example, oleate chemically adsorbs onto the surface of calcite above its point of zero charge, where the oleate ions bind with the calcium ions. Although the PZC is a critically important characteristic of a mineral, it often varies from sample to sample based on composition, crystallinity, and which plane of the mineral lattice is exposed (so which ions are free to dissociate). Table 2.1 demonstrates this variance for bastnaesite, Table 2.2 for monazite, and Table 2.3 for barite, calcite, and other

semisoluble salt-type minerals.

Table 2.1. Bastnaesite Points of Zero Charge (PZC). [24]

(28)

15

Figure 2.5. Zeta Potential of bastnaesite and monazite. (Luo Jiake 1984, from [16])

(29)

16

Figure 2.6. Zeta potential of barite, bastnaesite, and calcite in pure water. (Smith 1986, from [16]).

It has been determined that in the pH window for flotation, calcite is positively charged, bastnaesite is negatively charged, and the barite surface is undergoing transformation to a carbonate surface. [22]

Reagents are responsible for changes in the PZC of minerals during flotation by adhering to their surfaces and thus changing their charge. Bastnaesite, barite, and calcite surface chemistry has been analyzed in response to changes in soda ash concentration (used to modify the pH). The carbonate ion had the most pronounced effect on the bastnaesite zeta potential, and the least on the calcite potential. The barite zeta potential changed dramatically from positive to negative at 8x10-4 M carbonate due to the formation of barium carbonate on the mineral surface. [25] Initially, the zeta potential of the bastnaesite is more negative with respect to barite, but the barite

becomes more negative after an addition of 1x10-4 kmol/m3 ammonium lignin sulfonate, because of the stronger adsorption onto the barite surface. [26]

Cheng’s PZC of monazite was reported as a pH of 5.3. This, combined with the negative zeta potential at high pH, leads to the conclusion of oleate ions being

chemisorbed onto the surface. [27] The monazite zeta potential curves are shifted to the left (the PZC becomes lower) and made steeper in the presence of hydroxamate

(30)

17

basis for the wide range of reported PZC values (pH 3-7) for monazite and xenotime, respectively. An alternative cause for variation was given: impurities from other ore bodies and crystallinity, which determines which ions are exposed. [29]

2.1.4 Adsorption Density

Surfactants can bind to mineral surfaces in different configurations. The

mechanisms for surface attachment can be classified as low- or high-energy processes.

Physical adsorption involves van der Waals bonding and hydrogen bonding, while the

higher energy chemical adsorption relies upon covalent bonding. Molecules can attach themselves horizontally, leading to lower adsorption densities; or in vertical configurations, leading to higher adsorption densities. Multiple layers can form as the molecules match hydrophobic ends or hydrogen bond from one chain to another. As many as six hydroxamate layers have been reported to form at the interface. [22] Adsorption densities on barite (as a horizontal monolayer) and on calcite (as a horizontal layer, then a vertical layer) are much lower than on bastnaesite. Calcite exhibits the curious characteristic of a linear increase in adsorption perhaps, but unconfirmed, as the result of calcium hydroxamate precipitation.

Figure 2.7. Oleate adsorption onto bastnaesite as a function of pH with and without pre-boiling. (Smith 1986 from [16]

(31)

18

Figure 2.8. Adsorption isotherm of oleate on calcite at pH 9.3. [21]

Figure 2.9. Adsorption Density of hydroxamate on barite, calcite, and bastnaesite at 21°C and pH 9.3. [22]

Adsorptions of hydroxamate were found to be endothermic. [22] The free energy of adsorption was estimated as -26, -28, and -57 kJ/mol for barite, calcite, and

(32)

19

bastnaesite. Bastnaesite’s trivalent state (as opposed to the divalent state of the alkaline-earths) is proposed as a factor in the increased adsorption. [20]

FTIR has been used to distinguish whether physical adsorption or chemisorption occurs between monazite and bastnaesite and sodium oleate and potassium octyl hydroxamate. Sodium oleate physically adsorbs onto bastnaesite and monazite at pH 9 and pH 3 and 8, respectively. Potassium octyl hydroxamate adsorbs chemically onto both minerals at pH 9.3 and 9. FTIR was unable to distinguish whether or not

chemisorption at pH 8 occurs for monazite and sodium oleate. [30]

2.1.5 Hallimond Tube Flotation

Fuerstenau modified the design of the original Hallimond Tube to a version similar to that used in this study. [31] It is one of the most common devices used for microflotation tests, although due to its simplified design, it is not considered

representative of industrial flotation. [18] Several reasons for this are incomplete chemistry (a frother is not necessary, difficulty in generating reliable grade-recovery curves, and unrealistic flow conditions.

Although operation of a Hallimond tube is simple, the parameters are not

standardized. Gas composition and flow rate, stirring speed, specific water volume and temperature, reagent additions, and flotation time are all determined by the researcher. The tube is filled with a very dilute slurry (one gram of mineral per 100 mL of water), and gas is bubbled through a frit at the bottom. A magnetic stir bar disperses the bubbles, which attach to particles in the slurry. As the bubbles rise to the top of the water, they break, and the mineral falls into the concentrate stem.

2.1.6 Temperature Effects

The literature shows that for all three minerals, collection increases with increased temperature (Figure 2.11). Bastnaesite shows a more pronounced effect. This is the impetus for Molycorp’s steam conditioning: increasing selectivity. The endothermic nature of adsorption revealed that it is a chemical adsorption process and temperature was proposed as a driver of the increased adsorption. [32] Flotation with elevated conditioning temperatures has shown (Figure 2.12) that between hydroxamate

(33)

20

and fatty acids, the selectivity for bastnaesite increases with temperature, but more so for hydroxamate. [22], [32]

Figure 2.10. Schematic Drawing of a Modified Hallimond Tube. [31]

(34)

21

Figure 2.12. The flotation recoveries of Mountain Pass ore as a function of conditioning temperature for fatty acids (left) and hydroxamate (right) collectors. [22]

2.2 Minerals

Hanna and Somasundaran discussed many aspects of the flotation behavior of salt-type minerals, including mineral crystal structure, dissociation, and surface hydroxylation. [21] Surface charging and zeta potential were discussed with regard to calcite and other minerals. Interactions of several depressants, including metal salts and sodium silicate, were included. Surface characteristics of rare earth

semisoluble salt minerals were discussed as well. [24] Electrokinetic properties, surface wetting, and collector adsorption behavior are detailed in later technical sections.

2.2.1 Bastnaesite

Bastnaesite is a rare-earth fluorocarbonate mineral with a formula of (Ce,La)FCO3. Cerium occurs in the mineral more often than lanthanum. It has been found in numerous deposits around the world; the two most notable of which are Bayan Obo in China and Mountain Pass in California. [1] It has a specific gravity of 5.0.

(35)

22

As the predominant supplying mineral of rare earth elements in the world its beneficiation has been extensively studied and reviewed. [3], [9], [22] Pradip performed electrokinetic, Hallimond tube and Denver cell flotation, adsorption, and x-ray studies in his thesis. The surface chemistry of bastnaesite, barite, and calcite has been analyzed. Flotation experiments have been conducted with fatty acids and hydroxamic acids, along with additions of inorganic salts, organic ions and molecules, soda ash, and lignin sulfonate.

2.2.2 Monazite

The initial source of rare earth elements, monazite, is a rare-earth phosphate (La,Ce)PO4. It was originally beneficiated as a nuclear reactor material from placer deposits using gravity, magnetic, and electronic separation techniques. When

bastnaesite containing much lower amounts of thorium was discovered, monazite fell out of fashion. It is found in deposits containing bastnaesite, and thus necessitates a separation step for those ores. Monazite’s specific gravity varies from 5.0 to 5.4.

Pavez and Peres tested species of monazite, zircon, and rutile using three different collectors (sodium oleate, potassium octyl hydroxamate, and a commercial hydroxamate) and a depressant (sodium metasilicate). [28]

2.2.3 Calcite and Barite

Calcite is a carbonate mineral of calcium, CaCO3. It is a semisoluble salt-type mineral, and has been the subject of numerous investigations. [33], [34], [21], [35], [25] It is present in Mountain Pass ore as one of the primary gangue constituents. It has a specific gravity of 2.7.

The other major gangue mineral is the sulfate barite, BaSO4. It has also been studied, often in attempt to depress it from bastnaesite. [21], [22], [25], [26] Barite has a specific gravity of 4.5.

(36)

23

2.3 Reagents

Industrial flotation requires the use of surface-modifying chemicals in order to efficiently separate the desired minerals from the gangue. Collectors are those used to target the desired minerals, while modifiers and depressants are used either to

promote flotation of the desired mineral or to inhibit gangue flotation.

2.3.1 Collectors

Mineral flotation relies on collectors to attach to desired mineral surfaces and bubbles. In sulfide flotation, the dominant collector type are xanthates. In rare earth flotation, the traditional molecules used are fatty acids, while hydroxamates are a promising group undergoing laboratory study, with limited industrial application.

2.3.1.1 Fatty Acids

A fatty acid is an organic acid consisting of a chain of singly or doubly bonded carbon atoms and a functional group capable of donating a proton. As fatty acid chain length increases from 8 to 12 carbons, collector concentration required for flotation decreases (Figure 2.13). [34] As shown in Figure 2.14, at a concentration of 10-3 mol/L fatty acid, chain lengths of 11 and 12 carbons float well from pH 6-12.5. Smaller chains show minimal recovery above pH 10. The proposed mechanism for the collector

adsorption to surface calcium and carbonate is by the reaction CaCO3 + 2(RCOO-)  Ca(RCOO)2 + CO32-.

The structure of oleic acid is shown in Figure 2.15. It is a monounsaturated 18-carbon chain ending in a carboxyl group (a double-bonded oxygen and a hydroxyl group are bonded to the end carbon). Flotation of barite, calcite, and fluorite was studied using oleic acid and sodium oleate (the sodium salt formed by oleic acid and sodium ions). Electrokinetic, Hallimond tube, and abstraction tests suggest that a layer of calcium oleate forms around the minerals, which prevents further dissolution to equilibrium. Due to their similar characteristics, selectivity between the three minerals was proposed as unlikely to be obtained. [35]

(37)

24

Figure 2.13. Calcite recovery as a function of fatty acid addition for various fatty acids at pH 9.7. [34]

Figure 2.14. Calcite recovery as a function of pH for various fatty acids at a constant collector addition of 10-3 mol/liter. [34]

(38)

25

Figure 2.16. Bastnaesite recovery by sodium oleate as a function of pH. [26]

Recovery of bastnaesite with and without additions of a depressant is shown in Figure 2.16. The maximum is in the slightly-alkaline pH region. Bastnaesite, barite, and calcite all float best around pH of 9.5 but bastnaesite flotation requires a lower

concentration of sodium oleate (3x10-4). By combining that fact with the calcite minimum around pH 8.5, conditions for effective separation have been determined, although plant practice dictates the use of depressants. [22]

The maximum floatability of monazite was shown to coincide with the maximum concentration of Ce(OH)2+ and La(OH)2+ (from thermodynamic modeling), the greatest particle-bubble adhesion, and a pH of 8.5 – 9. [27]

Sodium oleate floatability experienced a minimum between pH of 4-5. Maxima exist on either side for monazite (3 and 7), zircon (3, 6-8), and rutile (3, 7-8).

Hydroxamate floatability occurs between 3 and 7, 2.5- 9, and 3-8, for monazite, zircon, and rutile. [28] Shown in Figure 2.17, the percentage of calcite floated was 20% at 3x10 -6

(39)

26

Figure 2.17. Oleate concentration required to float salt-type minerals.[21]

2.3.1.2 Hydroxamates

Hydroxamate collectors are chelating molecules that contain several active sites to which an ion can bond and a sufficiently long chain to provide the necessary

hydrophobicity. That chain (represented by R in Figure 23) can be an alkyl group with 7-14 carbons, a naphthalene ring, or other organic group. The number of active sites varies from specific molecule to molecule, but they all function by forming coordinating bonds with metal ions. [36] The specific mechanism of attachment can also vary but an example is given in Figure 2.18, where the hydroxyl group is deprotonated and the two oxygens coordinate to bind the metal ion. Different layer formations (either monolayer or multilayer) are the result of horizontally versus vertically oriented molecules. These layers form due to chemisorption with surface cations: cations form hydroxy complexes, readsorb, then bond with the hydroxamate. [20]

Figure 2.18. A mechanism of hydroxamate adsorption to an ion on a mineral surface. [20]

(40)

27

For his thesis Pradip detailed the preparation of potassium octyl hydroxamate (structure, Figure 2.19) and characterized the reagent used as being composed of 50% potassium octyl hydroxamate and 50% octyl hydroxamic acid. [22]

Figure 2.19. The structure of potassium octyl hydroxamate. [22]

Modified hydroxamic acid (R-OHCOONH2, where R is a naphthalene ring) chelates the cerium on the bastnaesite surface, yielding best flotation between pH 8 – 9.5 (Figure 2.20). [37] The primary factor limiting the use of these more selective reagents has been their high cost. [36]

Figure 2.20. Modified hydroxamic acid chelating surface cerium(III). [37]

Figure 2.21 demonstrates the floatability of bastnaesite with hydroxamates between pH 6 and pH 9. Denver cell flotation based on the Hallimond tube results showed the increased selectivity (at both room temperature and elevated temperature) of the hydroxamate as compared to the fatty acid. [22]

(41)

28

Figure 2.21. Hallimond tube flotation results on bastnaesite with K-octyl hydroxamate as collector. [22]

Figure 2.22. Bastnaesite ore recovery (as total weight, including gangue, recovered) by K-octyl hydroxamate as a function of pH. [22]

(42)

29

2.3.2 Modifiers and Depressants

Collectors alone are rarely sufficient to achieve desired selectivity in flotation. A variety of compounds exist to modify the surface and/or solution chemistry of a mineral flotation system. The pH must be regulated to create the proper electrokinetic

environment for the collectors to attach to the minerals. As bastnaesite flotation occurs in alkaline environments, a base (hydroxides and soda ash are common) must be added to raise the pH. Other chemicals are used to inhibit flotation of a specific mineral; these are depressants.

2.3.2.1 Soda Ash

Soda ash (Na2CO3) is used to regulate pH. The carbonate ion is a potential-determining ion for bastnaesite and calcite. In the flotation window of 8 < pH < 10, HCO -3- and CO32- are present in solution. At high enough concentrations, the surface of barite is converted to a barium carbonate surface. Excess soda ash can lead to depression of bastnaesite. [22]

2.3.2.2 Lignin Sulfonate

Lignin sulfonate is a complex compound derived from the sulfonation of lignin, a cellulose binder, in wood pulp. It is known to flotation as a barite depressant. [21], [22], [26] Its selectivity for barite at high pH is likely due to the highly positive surface of the barite compared to that of bastnaesite and calcite. It has also been proposed that the molecule fits better on the barium sulfate structure as compared to the carbonate structures of bastnaesite and calcite.

Bastnaesite recovery as a function of ammonium lignin sulfonate concentration is given in Figure 2.23. Barite flotation with 1x10-5 kmol/m3 sodium oleate is virtually

eliminated by 1x10-5 kmol/m3 ammonium lignin sulfonate.

2.3.2.3 Metal Salts

Inorganic salts can increase flotation by binding and providing new adsorption sites or decrease it by competing with collector ions. Salts composed of

(43)

30

barite and CO32- for calcite and bastnaesite. [22] Gaudin studied calcite flotation with various metal salts, showing that many of them depress calcite flotation by causing precipitation of the oleic and undecylic acid collector. The order of their effectiveness was found to be Cu(NO3)2•3H2O > Na2SiO3 > CuSO4•5H2O > FeSO4•7H2O. [33]

Figure 2.23: Bastnaesite recovery as a function of ammonium lignin sulfonate concentration. [26]

Figure 2.24. Recovery of calcite as a function of increased concentration of metal salts. [33]

(44)

31

2.3.2.4 Sodium Silicate

Sodium silicate is well known as an inorganic modifier. In rare earth mineral flotation, sodium metasilicate has been found to be an effective zircon and rutile

depressant while not affecting monazite. [28] It was shown to be an effective depressant of zircon and rutile (Figure 2.25), more so than sodium sulfide. A temperature increase slightly increased selectivity. [38] The increase in sodium silicate adsorption and

decrease in oleate adsorption are shown to reduce flotation recovery in

Figure 2.26. It increases flotation of barite and calcite from quartz, but can depress calcite with respect to fluorite, especially when aluminum salts are used. [21] Figure 2.27 shows depression of barite with respect to rare earth minerals and fluorite.

Figure 2.25: Effect of sodium metasilicate for oleate (top) and hydroxamate (bottom) flotation. [38]

(45)

32

Figure 2.26. Adsorption and Floatability of sodium oleate and sodium silicate on calcite and fluorite as a function of sodium silicate concentration. [21]

(46)

33

2.4 Gravity Concentration

Often before flotation is considered, gravity separation is investigated as a means of concentrating an ore. In the case of liberated, similarly-sized beach sands or free gold deposits where the desired mineral is much denser than the undesired

minerals, gravity can quickly sort the valuable material from the gangue. This approach relies on complete liberation – distinct particles of the valuable component must exist without contact with gangue particles. These separations generally show better results with a near-size deslimed feed. When a disparity exists between the specific gravities of the two components, gravity may be used to sort them.

The Concentration Criterion is used as a first estimate of the relative success of gravity separation. It compares the specific gravities of the heavy particles Dh, light

particles Dl, and fluid (usually water) Df.

Concentration criteria greater than 2.5 indicate that gravity separation is viable, while those below 1.25 mean it is practically impossible. Values between those suggest that using the right equipment and a carefully controlled feed, a separation could be made. [39] Table 2.4 shows the concentration criterion for major components of the bastnaesite ore used in this study.

Not every type of gravity equipment is suitable for a given application, which is why so many exist. There are vibratory motion-based devices, such as jigs and shaking tables, centrifugal units like Falcon and Knelson concentrators, and other devices: spirals, multi-gravity separators, and heavy media separators. Characteristics of the equipment such as allowable feed size (Figure 2.28), throughput, water requirement, plant footprint, and power requirement dictate their applicability to a given separation. Following the Concentration Criterion calculation, a float sink analysis may be done to determine the efficiency of the separation as a function of specific gravity.

(47)

34

Table 2.4. Specific Gravity (SG) and Concentration Criterion (CC) for major components of Molycorp ore.

These tools, along with preliminary testwork, can be used to assess the type of equipment suitable to beneficiate an ore. In the case of rare earth ores, the recent development of centrifugal-type concentrators has pushed the boundary of allowable separation into the domain at which these minerals concentrate. As the minerals require liberation, they must be ground finely. Excessive fines can be problematic in flotation due to entrainment and agglomeration issues, although dispersants can be used to mediate that effect. The slimes can also reduce the sharpness of the separation, as seen in Figure 34. While potentially a problem for some types of equipment, spinning-bowl concentrators are able to process feeds as fine as tens of microns. They are joined by hydrocyclones, tilting frames, Mozley tables, and froth flotation as the only

non-magnetic unit operations, according to Figure 2.28, capable of handling a feed of that size. Wet tables reach into the top end of this range, along with many other operations. Two carbonatite operations similar to the Mountain Pass deposit that use physical beneficiation methods are Sichuan Mianning (shaking tables and flotation) [10] and Bear Lodge (scrubbing and sizing) [14].

Molycorp bastnaesite ore has seen prior attempts at shaking table concentration in a lab setting. [40] The bastnaesite ore was pulverized and split into four fractions: 20-38 μm, 38-53 μm, 53-75 μm, and 75-106 μm then purified with a Frantz Isodynamic

Mineral Formula SG CC

Monazite (Ce,La)PO4 5.2 2.47

Bastnaesite (Ce,La)FCO3 5.0 2.35

Barite BaSO4 4.5 2.06

Parisite Ca(Ce,La)2(CO3)3F2 4.4 2.00

Rutile/Anatase TiO2 4.1 1.81

Strontianite SrCO3 3.8 1.65

Ankerite Ca(Fe,Mg)(CO3)2 3.0 1.18

Chlorite (Mg,Al,Fe)12[(Si,Al)8O20](OH)18 2.9 1.12

Calcite CaCO3 2.7 1.00

Quartz SiO2 2.7 1.00

(48)

35

Separator. That was then processed on a Mozley shaking table at 90 rpm with a 3.5” stroke and 3L/min of wash water. These methods yielded a relatively pure bastnaesite sample in the 38-53 μm and 53-75 μm fractions. Figure 35 shows the XRD patterns of those relatively pure samples. Outside of those size fractions, considerable amounts of calcite, barite, and quartz were observed.

Figure 2.28. Applicability of Beneficiation Equipment to a given feed size. [41]

Heavy liquid separation will be elucidated due to its importance in characterizing the gravity response of an ore. Shaking tables are a tested and proven method of separating an ore, with a slightly finer size range than jigs and spirals. Here, centrifugal concentrators were used to demonstrate their ability to effectively separate a feed of such fine size.

(49)

36

Figure 2.29. Effect of contaminating slimes and suspension viscosity on the cleaning of Roslyn coal. [42]

Figure 2.30. XRD Patterns of Molycorp Bastnaesite size fractions upgraded with a Frantz Isodynamic Separator and reference minerals. All diffraction peaks have been

(50)

37

2.4.1 Heavy Liquid Separation

To quote Chris Mills, “The first step at the laboratory level should always be heavy liquid analysis of the ore to be fed to the gravity separation plant.” The

information gained from such tests can dictate whether gravity separation will be easy or difficult and which types of equipment are available to make the separation. [41] While many fluids exist in the specific gravity range of 1.2 - 2.0, options available for gravity separation of ores are both more limited and more expensive. [42] Historically, potentially hazardous halogenated hydrocarbons have been used for this type of work. Sodium polytungstate, a newer, nontoxic reagent with a maximum s.g. of 3.1

(adjustable by means of the water-to-powder ratio) was used in float/sink analysis. [43]

Figure 2.31. Density of aqueous sodium polytungstate solution as a function of mass percent. [43]

2.4.2 Shaking Tables

Shaking tables are rectangular-shaped tables with riffled decks across which a film of water flows. (Figure 2.32 and Figure 2.34) The mechanical drive imparts motion along the long axis of the table, perpendicular to the flow of the water. [44] The water carries the particles of the feed in slurry across the riffles in a fluid film. This causes the fine, high density particles to fall into beds behind the riffles as the coarse, low-density particles are carried in the quickly-moving film. (Figure 2.33) The action of the table is such that particles move with the bed towards the discharge end until the end of the

(51)

38

table stroke, at which point the table rapidly moves backwards and the particles’ momentum propels them still forward.

Figure 2.32. Deister shaking table deck. Dark circles represent low-s.g. particles. White circles represent high-s.g. particles. The diameter of the circles corresponds of the

particle size. [45]

(52)

39

The feed characteristics, feed rate, riffle pattern, and motion of the table should all be tailored to fit the desired application. Feed characteristics are generally set by the comminution circuit, but classifying, either with screens or cyclones, can influence the separation on the table. Riffle pattern is most easily controlled by changing decks: often a sands deck is used for coarse feeds and a slimes deck is used for fine feeds. The drive of the table can be manipulated in both stroke length and frequency. A longer stroke will require more water but moves heavies to the concentrate end more quickly. Deister decks are set up at an incline from the drive to discharge end to allow migration of heavies to the concentrate end and allow light particles to fall to the tails easier. The tilt from the dressing side to the tailings side is generally maintained to allow a wide spread of material at the concentrate end. While the tails-middlings cut point is dictated by collection bins around the table, the concentrate-middlings cut point is made by the operator at some point along the discharge end.

Figure 2.34. Wilfley shaking table

2.4.3 Knelson and Falcon Concentrators

Development of centrifugal concentrators was pioneered by those searching to separate free gravity recoverable gold (GRG). In the 1970s, the Knelson Bowl (Figure 2.35) and later the Falcon Concentrator (Figure 2.37) were developed to use centrifugal force to amplify the force of gravity for the purpose of separating constituents of an ore.

(53)

40

A rotor spins the bowl, which throws the feed coming into the center against the walls of the bowl. Light and fine particles are carried out of the bowl with the tailings while heavy and coarse particles are collected and removed from the bowl. Industrially they can operate as batch or continuous units, with pores opening intermittently to collect concentrate.

Figure 2.35. Knelson Concentrator schematic. [46]

The Knelson Bowl is the more widely-accepted centrifugal unit. The difference between a Falcon and Knelson lies in the proprietary design of the bowl. Factors such as bowl height, wall angle, radius of the bowl sections, and size and placement of collection ports vary between the two brands. In particular, the Knelson utilizes concentrate collection all along the height of the bowl, while a Falcon collects the concentrate in several rings at the top of the bowl. Laplante’s Standardized GRG test used a Knelson to separate and classify feeds and determine the amount of GRG in

(54)

41

each size fraction, as well as the overall susceptibility to gravity recovery. [47] Exemplary curves for a poor, intermediate, and exceptional response are shown in Figure 2.36 as lines a, b, and c.

Figure 2.36. Cumulative gold retained as a function of particle size. [47]

Falcons have been shown to have dependence on slurry density only to a slight degree as the particles are quickly forced from the fluid and into the bed. There is a positive relationship between flow rate and recovery up to a plateau around 20 L/min. At low slurry feed rates and densities (10 L/min and 10% solids) the Falcon recovery

suffered. [48] Behavior of slimes in a Falcon has been modeled previously and shown to correlate with industrial experimental data. [49]

(55)

42

(56)

43

CHAPTER 3:

EXPERIMENTAL METHODS

Once the literature survey was completed, an experimental campaign was developed and carried out to determine the effectiveness of gravity separation before froth flotation.

3.1 Characterization and Mineralogy Procedures

Ore was provided in two lots by Molycorp Inc. The first was crushed in a jaw and roll crusher, then ground batch-wise in a laboratory ball mill until it was 100% passing 100 mesh. This ore was then blended and split in a Jones Riffle, and was used for the microflotation tests. The second was crushed in a roll crusher until it passed 12 mesh then was blended and split into representative samples in a Jones Riffle. Those samples were wet ground in a rod mill for the required length of time.

Samples of the ore were sent to the Center for Advanced Mineral and

Metallurgical Processing (CAMP) at Montana Tech and to the Colorado School of Mines Geology Department for MLA and QEMSCAN analysis, respectively. Several samples, ground in a rod mill for specific lengths of time (zero, 10, 30, 60, and 90 minutes), were characterized by CAMP to determine the liberation behavior of the ore components as a function of grinding time. Elemental composition was determined by the automated mineralogy software (as part of MLA and QEMSCAN), x-ray fluorescence, and x-ray diffraction. The microflotation samples were analyzed with the Kroll Institute XRF, while all of the gravity and magnetic test samples were analyzed by Hazen Research, Inc. Size analysis was performed with a Microtrac Particle Size Analyzer and several Tyler sieves.

3.2 Microflotation

Sodium oleate, octanohydroxamic acid, soda ash, ammonium lignin sulfonate, sodium silicate, and copper (II) nitrate hemipentahydrate were procured as solids and dissolved in de-ionized water to create stock reagent solutions. Concentrations (listed in

(57)

44

Table 3.1) were chosen based on previous studies on single-mineral systems.

Research purity nitrogen was chosen to minimize the gas species present in the system and was obtained from General Air.

Ore grind size, the use of soda ash for pH control, oleic acid as a collector, a sulfonate depressant, and flotation at elevated temperatures were based off of Mountain Pass plant practice (Figure 2.12 and Figure 2.16). [3], [22] Octyl hydroxamate was chosen based on reports of increased selectivity as compared to oleate (Figure 2.12). [20], [22], [28], [32], [36] Ammonium lignin sulfonate was used as a barite depressant (Figure 2.23). [26] Copper nitrate was chosen due to previous depression of calcite (Figure 2.24). [33] Sodium silicate was used to depress gangue, particularly from monazite (Figure 2.25). [38] Microflotation tests were performed in a Hallimond Tube (similar to that in Figure 2.10).

Table 3.1. Microflotation Reagents

mg/L Test

Number Test Name

Sodium Oleate Octano-hydroxamic Acid Ammounium Lignin Sulfonate Copper (II) Nitrate Sodium Silicate 1 NaOl 11 1.00E-05 2 NaOl 3 5.00E-05 3 NaOl 14 1.00E-04 4 H9 1.00E-04 5 H5 3.00E-04 6 H1 5.00E-04

7 NaOl 2 5.00E-05 5.00E-06

8 NaOl 13 5.00E-05 5.00E-06

9 NaOl 4 5.00E-05 1.00E-05

10 NaOl 7 5.00E-05 2.00E-05

11 H10 3.00E-04 5.00E-06 12 H6 3.00E-04 1.00E-05 13 H13 3.00E-04 2.00E-05 14 NaOl 8 5.00E-05 100 15 NaOl 5 5.00E-05 300 16 NaOl 10 5.00E-05 500 17 H11 3.00E-04 100 18 H7 3.00E-04 300 19 H14 3.00E-04 500

20 NaOl 12 5.00E-05 1.00E-07

21 NaOl 6 5.00E-05 5.00E-07

22 NaOl 9 5.00E-05 1.00E-06

23 H12 3.00E-04 1.00E-07

24 H8 3.00E-04 5.00E-07

25 H15 3.00E-04 1.00E-06

(58)

45

One-gram samples were taken in batches from a bag (approximately 15-20 grams per batch) and the grade of those batches was recorded. Conditioning was performed in two separate 150-ml beakers. This was done to limit the amount of slurry entering the collecting arm of the tube, therefore artificially inflating the recovery

numbers. The volumes were selected based on the volume of the cell. At around 40 ml, solution began to pour into the collecting arm. As such, 25 ml of water was targeted for the slurry beaker (ore, water, and depressant), although this number varied slightly as more or less stock depressant solution was used to achieve the desired concentration. The solutions were heated on a hot plate as the slurry was stirred with a stir bar. After ten minutes (at a temperature of 80±10°C), the collector was added to the slurry and pH was adjusted to 9.0±0.1 using drops of soda ash solution to follow plant practice. After fifteen minutes, the slurry was removed from the hot plate and poured into the

Hallimond tube. The remaining solution was then poured in.

Flotation lasted for two minutes at a flow rate of 60 cc/min nitrogen gas. The slurries were stirred with a stir bar to ensure mixing. The apparatus used for the tests is shown in Figure 3.1. After that time, the concentrate was gathered by removing the stopper from the collecting arm and flushing the froth from the tube. The remaining tails solution was collected as well. Both the concentrate and tails were filtered through Whatman 40 (8-μm pore size) filter paper and dried for 18 hours.

(59)

46

3.3 Magnetic Separation

Fifty grams of ore ground for zero, 30, and 90 minutes were charged at 10% solids through the WHIMS at different field strengths, based on percentages of

maximum amperage. The two-factor DOE matrix (designed with Stat Ease) is given in Table 3.2.

Table 3.2. WHIMS DOE Parameters

3.4 Gravity Concentration

Sodium polytungstate heavy liquid medium was used to investigate the possibility of beneficiation by gravity separation. Five grams of ore (ground for zero, 30, and 90 minutes) were centrifuged in 10 mL of fluid of specific gravity ranging from 2.70 to 2.95. The floats were poured and skimmed off. The middling solution was poured off, and the fines were flushed. Each product (floats, middlings, and sinks) was washed and filtered several times and dried. The composition of the products was determined with x-ray fluorescence.

A Deister table was used to develop a qualitative response to gravity

concentration. The process variables were adjusted based on visual observation. An initial 500 g feed was tabled, yielding a concentrate, middling, and tailing product. That concentrate was tabled again to represent a cleaning step. Two more 500 g batches were tabled under the same conditions. From one of those tests, the concentrate, middlings, and tailings were tabled again. A 300 g charge of 100 x 325 mesh ore was also processed to investigate the effect of a classified feed. Fluidization water was set at

DOE

Standard WHIMS Run P80, µm

Field Strength, Gauss 5 1 762 5000 4 2 50 5000 6 3 762 10000 1 4 144 7500 3 5 50 10000 2 6 144 7500