Transcript Slide 1
Potash Production Froth Flotation • • • • Applied surface science Oil sands Wastewater Mineral benefaction. Mineral Copper Zinc Lead Nickel Potash Coal Annual Production (MTonnes) 8 6 4 0.7 35 2 500 Waxing My Car (a hypothetical experiment) Interactions of a submersed solid with an air bubble: Flotation of Hydrophobic Materials • • • • • Talc sulfur bitumen oil graphite Dispersed Air Flotation Oil Phase Cleaned Water Collectors • Selectively render surface of desired mineral hydrophobic. • Surfactants Non-polar Non-Polar Polar Polar Solid Surface Collector Type Structure Applications S Xanthates R-O-C Pb, Cu, Zn, Ag, Co … S Na RO Dithiophosphates + S P RO S Cu, Zn, Ag, Cu + M O Carboxylic Acids R-C Iron ore, chromite O-H R1 Amines + H N R2 Potash, SiO2 H Mechanisms of Collector Adsorption • Poor basic understanding • Mechanisms: – – – – Physical Adsorption Chemisorption Weak Electrostatic Interaction Neutral Molecule Adsorption. Physical Adsorption • dipole-dipole interactions • hydrogen bonding • electrostatic attraction Point of Zero Charge (PZC) • Surface charge often a function of pH + H Si O Si O H + H H • pzc of quartz is 1.9 + Si O + H + Equ. (33) Mineral Anthracite Biotite Chrysotile Fayerite Feldspar, orthoclase Molybdenite Muscovite Quartz Pyrophyllite Sulfur Formula K(Mg Fe)3(AlSi3O10)(OH)2 Mg3Si2O5(OH)8 Fe2SiO4 K(AlSi3O8) MoS2 KAl2(AlSi3O10)2(OH)2 SiO2 Al2(OH)2Si4O10 S • Various pzc values can lead to specificity pzc 8.1 2.1 10.5 4-5 4.8 1.0 4.0 1.9 1.7 2.0 Chemisorption • Bond formation between collector and mineral. S PbS + 2O2 + ROC S S = Pb S C OR + SO4 Equ. (34) Other Flotation Chemicals • Depressants • Frothers • Extenders Depressants • Inhibit flotation of undesired minerals • Starch, guar depress flotation of clays in potash • depression of copper with Na2S or NaHS for molybdenum recovery Frothers • • • • • Stabilize air bubbles so they don’t burst formation of a foam layer small chain alcohols coat air bubbles - lower air/water C6 - C10 alcohols used in potash Extenders • • • • • Improve recovery of desired mineral non-polar liquids in potash, use heavy oil (fused ring aromatics) extenders improve recovery of larger particles mechanism unknown Potash Processing • • • • Saskatchewan has immense reserves of potash nameplate capacity of 20 M tonnes/year 10 plants 7 plants use flotation Rocanville, Sask. Sussex, N.B Devonian Period, -350 Ma • Seas covered continent • Sea water concentrated • Mountain uplift, then cutoff • Chloride salts crystallized in sequence • Solar evaporation concentrated brine • Now buried by 1000 m of sediments Ore Mineralogy • Composition: – NaCl: – KCl: – KMgCl3.6H2O: – Insolubles: 55% 35-40% 1-5% 1-8% • Size Range of Natural Crystals in Ore: – 1-10 mm 3 mm Discrete KCl/NaCl/insoluble particles Crushing & Grinding • • • • • • • • Dry crushed to - 8 mm crushing with hammer mills crushed ore passed through screens wet grinding in rod mills or cage mills ground ore classified by screen or cyclone Ground to < 4 mm Ground to the “liberation size” closed circuits Desliming • • • • series of agitated tanks (scrubbers) clays scrubbed from ore surface insolubles separated by cyclone or flotation dirty brine sent to thickener – clean brine recycled – U/F discarded Ore conditioning • Depressants – inhibit flotation of residual clays. – Starch, guar, carboxymethylcellulose (CMC) – conserves amine • Collector – CH3(CH2)17NH3+ – selectively coats KCl particles – exact mechanism unknown Ore Conditioning • Extender – heavy oil (fused aromatics) – added with the collector • Frother – C6-C10 aliphatic alcohols – stabilizes the product froth Ore Conditioning • Ore slurry is mixed to provide good coverage of reagents. • Frother added just prior to flotation. Flotation Equipment • • • • • Conditioned feed, 20-40% solids 3 to 6 cells in series forms one bank each cell has an agitator, air injection froth collected from top by paddles gangue flows through cells Bubble-solid interactions agitation zone: - bottom of the cell - bubbles dispersed by agitator - bubbles adhere to surface of desired mineral. quiescent zone: - intermediate depths - agitation is less vigorous. - air bubbles carry the desired mineral up - unfloated material flows on to the next cell Bubble-solid interactions concentration zone: - top of the cell - froth enriched in the desired mineral - product collected by paddles - flows into a collection launder Mineral Flotation Cell Flotation • Amine-treated KCl collected from top of cells • product is 95% KCl • waste salt flows through cells and is discarded. Product Treatment • Product slurry debrined by centrifuge (3-5% moisture) • dried in rotary kiln or fluidized bed driers • sized by screen Drying and Screening • Potash from flotation is dried and sized to produce fertilizer-grade products. • Finished product is screened into Granular (SGN 285) and Standard (SGN 95). • Undersize sent to crystallization or compaction. Compaction • Granular product is produced by compacting the dust from screening. • Compaction fuses (sinters) the dust into a flake in compactors under moderate temperature and high pressure. • Flake is then crushed and screened to produce more Granular product. Toshiba 2000 HP VFD Drive Crystallization • Widely used: – – – – – potash salt ammonium sulfate sugar chemicals • Can be thermal or evaporative Crystallization • Potash dust is dissolved in hot brine. Purified KCl is then produced as it cools. • KCl is much more soluble in a hot brine than a cool brine. • Dust from screening is dissolved in hot (100°C) brine. Crystallization (cont’d) • The hot, saturated brine is sent through a thickener to remove colored impurities. • Clear brine is pumped to a series of crystallizers. • In the crystallizers, the brine cools to 40°C and precipitates purified KCl. • The grade of the product is regulated by controlling the sodium chloride content of the circuit. • 600-1200 micron particles – rounded by abrasion – composites • 300-425 micron particles – complex microstructures – agglomerates • 150-212 micron particles – primary KCl cubes – some agglomerates Crystallization (cont’d) • Solid product is debrined by centrifuge and dried in rotary kiln dryers. • Sold for industrial and pharmaceutical uses. External Coolant Vapour Hot Feed 1 Vapour 2 Vapour 3 Vapour 4 Product Cool Liquor