Potash Production

Froth Flotation

• Applied surface science• Oil sands• Wastewater• Mineral benefaction.

Mineral Annual Production (MTonnes)

Copper 8 Zinc 6 Lead 4 Nickel 0.7 Potash 35 Coal 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

Cleaned Water

Oil Phase

Collectors

• Selectively render surface of desired mineral hydrophobic.• Surfactants

Solid Surface

Polar

Non-polar

Polar

Non-Polar

Collector Type Structure Applications Xanthates

S R-O-C

S Na+

Pb, Cu, Zn, Ag, Co …

Dithiophosphates

R O S P R O S M+

Cu, Zn, Ag, Cu

Carboxylic Acids

O R-C O - H

Iron ore, chromite

Amines

R1 + H N R2 H

Potash, SiO2

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+ Si O + H+ Equ. (33) H

• pzc of quartz is 1.9

Mineral Formula pzc Anthracite 8.1 Biotite K(Mg Fe)3(AlSi3O10)(OH)2 2.1 Chrysotile Mg3Si2O5(OH)8 10.5 Fayerite Fe2SiO4 4 - 5 Feldspar, orthoclase K(AlSi3O8) 4.8 Molybdenite MoS2 1.0 Muscovite KAl2(AlSi3O10)2(OH)2 4.0 Quartz SiO2 1.9 Pyrophyllite Al2(OH)2Si4O10 1.7 Sulfur S 2.0

• Various pzc values can lead to specificity

Chemisorption

• Bond formation between collector and mineral.

S S PbS + 2O2 + R O C Pb S C OR + SO4

= Equ. (34)

S

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 MaDevonian Period, -350 Ma

• Seas covered continent

• Mountain uplift, then cutoff

• Solar evaporation concentrated brine

• Sea water concentrated

• Chloride salts crystallized in sequence

• Now buried by 1000 m of sediments

Ore MineralogyOre Mineralogy

• Composition:– NaCl: 55%– KCl: 35-40%

– KMgCl3.6H2O: 1-5%

– Insolubles: 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 slurry is mixed to provide good coverage of reagents. • Frother added just prior to flotation.

Ore Conditioning

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.

Vapour Vapour Vapour Vapour

External Coolant

Cool Liquor1 2 3 4Hot

Feed

Product