University of Saskatchewan

Geological EngineeringGEOE 498.3

Introduction to Mineral Engineering

Lecture 11 Mineral Processing 4

Mineral Processing OverviewMineral Processing Terms, EconomicsComminution and ClassificationPhysical processing methodsChemical processing methodsWaste products treatment and disposalProcess plant flow sheets: uranium and potash

These course notes are a compilation of work conducted by many people.Notes have been taken from the following Edumine courses:The Mill Operating Resource 1&2Extractive Metallurgy 1&2Hydrometallurgy 1,2,3,4

Lecture 11Chemical ProcessingHydrometallurgyBasic CircuitsLeachingSolvent ExtractionPrecipitationDrying

PyrometallurgySmelting

ElectrometallurgyElectrowinning

Chemical Processing Hydro- Versus Pyrometallurgy:Techniques have competed over the years

Pyrometallurgy: Very ancient technologywas most successful with high-grade, simple ores, large scaleHigh temperature, fast reactionProblems can include pollution of the environment, high energy consumption, and excessive dust formation

Hydrometallurgy: works better with low grade, complex ores, smaller scaleLower temperature, slower reactionFirst hydrometallurgical process: alumina from bauxite, at start of 20th century

Hydrometallurgy TerminologyHydrometallurgy: aqueous methods of extracting metals from their ores hydrometallurgical plant: large amounts of water are needed, and a water balance must be maintained Generally involves two distinct steps:leaching = Selective dissolution of the metal values from an oreprecipitation= Selective recovery of the metal values from the solution

Sometimes includes purification/concentration

HydrometallurgyPurposes of hydrometallurgy:Recovery of salts directly from their deposits. examples: common salt, sodium carbonate, potash, borax, etc.Production of pure solutions - high purity metals can then be produced by electrolysis, examples: zinc, cadmium, nickel, copper, gold, and silver.Production of pure compounds - can be subsequently used for producing the pure metals by other methods. examples: aluminum, magnesium, uranium, and beryllium Chemical beneficiation- undesirable components of the raw material are leached away and the remaining solids are the valuable product that has to be processed further. Examples: desulfurization of coalDirect production of pure metals - suitable for the market after a subsequent minor treatment. Examples: precipitation of cobalt, nickel, and copper from solution by hydrogen under pressure

Leaching Before leaching:usually crushed and groundsometimes beneficiated by physical methods.In some cases treated by thermal methods, such as oxidation, reduction, before being leached:render the material more amenable to leaching, orexclude an undesirable component. Leaching is usually followed by:filtration, washing, and solution purification steps.

Leaching The choice of a leaching agent depends on the following factors:

Solubility. Large and rapid solubility of the material to be leached in the leaching agent.Cost. An expensive reagent is undesirable because any traces lost during handling will represent a large economic loss.Materials of construction. If the leaching agent is corrosive and has to be handled in tanks made of stainless steel, titanium, or Hastalloy, the capital cost will be high, and therefore its use will be less desirable.Selectivity. An ideal reagent will extract only the desired component.Regeneration. Ability of regenerating the reagent for recycle is also an important criteria.

Leaching - Solubility Water molecules - high polarity But, Hydrogen bonds are weak Solubility of any substance in water is related to the polarity of water and the association of water molecules together by forming hydrogen bonds. Ionic crystals: attractive forces between the anions and cations in a salt are reduced by a factor of 80 when water is the medium between them non-ionic crystals such as metals (metallic bond) or covalent crystals like quartz (SiO2) are insoluble Slightly soluble electrolytes are more soluble in foreign salts than in pure water

Leaching

Water is an ideal leaching agent because it is cheap and noncorrosive, but its action is only limited to few minerals. Leaching agents commonly used other than pure water are: Acids, bases, and aqueous salt solutionsLeaching agents may be used either alone or in combination with oxidizing agents. An oxidizing or a reducing agent is sometimes needed during leaching to solubilize certain minerals which do not dissolve otherwise. Commonly used oxidizing agents are: oxygen (or air), ozone, hydrogen peroxide, ferric ion, manganese dioxide, sodium nitrate, and sodium chloratecommonly used reducing agents are ferrous ion, hydrogen and sulfur dioxide.

Leaching Sulfuric acid is the most common leaching agent. Dilute: used for leaching copper oxide ores, zinc oxide, phosphate rock, and a variety of other ores. In combination with an oxidizing agent: used for leaching uranium ores and sulfides.Concentrated: used for treating more resistant minerals such as sulfide concentrates, laterites, monazite, and titanium slag.

Leaching

Addition of oxygen to an atom, ion, or molecule is known as oxidation reaction. Removal of oxygen is known as reduction. Electrode (oxidation) potential - The tendency of a substance to be oxidized or reduced, measured in voltsFor reactions involving hydrogen ions, the electrode potential is dependent on pH.

LeachingLeach general principles:

percent recovery is a major concern rate of a leaching process - the percent recovery as a function of time Related factors: particle size, concentration of leaching agent, temperature, pulp density, agitation intensitycompromise is always made between the increased rate of leaching and the negative effect of any of the factors influencing this increase

LeachingLeach general principles:After leaching, slurries obtained are usually filtered/thickened to recover the leach solution, then washed to remove entrained solution from residues CCD circuitResidues likely also need pH adjustment and heavy metal removal

LeachingAgitated pulp (tank) leaching:

leaching agent added to finely ground raw material, forms a pulpagitated continuously to prevent the solids from settling, and to terminate the leaching process in the shortest possible time.Generally used under the following conditions:The metal values are of fine grain size and disseminated in the host rock - extensive crushing and grinding to liberate.Raw material is of moderate to high grade.The metal values are difficult to dissolve and that is why intensive agitation is needed to increase the rate.

Agitated LeachingAgitation may be accomplished in two ways:Mechanical: motor-driven impellers are used. More expensive capital and maintenance cost.Pneumatic (pachucas): compressed air or high-pressure steam is used. This has the advantage of low initial cost and low maintenance cost because there are no moving parts. Steam is used instead of air when heating is desired.

Leaching High-pressure leaching: uses pressure reactors (or autoclaves).Closed vesseltemperature higher than the boiling point Without oxidizing agent: pressure generated is the result of the vapor pressure of the solution With oxidizing agent: oxygen partial pressure is the controlling factor on leaching rate

Leaching High-pressure leaching: shape may be vertical or horizontal cylinders, spherical, or a long horizontal tube Agitation: steam, mechanical impellers, or rotating the whole autoclave media is hot and corrosive: constructed of special steel alloys, titanium, and other high-grade materials. Interior may be lined with rubber or ceramic. usually connected in series for continuous operation.

Leaching High-pressure leaching:

Horizontal autoclave oxygen addition, cascading flow. Fill to 65-70% to allow space for the exhaust gases

Rotating autoclave contains grinding media to expose mineral surface (titanium ores)

Leaching High-pressure leaching:Tube autoclave: slurry is pumped through High pressure diaphragm-piston pumps (10,000-20,000 kPa) made this design possible.Used for bauxiteCharacterized by extremely short residence time, high thermal efficiency, and low capital cost

In-Situ Uranium Leaching Used in USA carbonate leach In Kazakhstan acid ISLThe ore is simply leached in place over long periods of time because it is usually too low in grade to justify mining and transportation expenses

In Situ Leaching Two basic criteria required for an underground deposit to be considered suitable for leaching in place are:The ore body must be enclosed between impermeable strata that will prevent the loss of solution.It must be permeable to the leaching solution.

Heap/Dump Leaching clear vegetation then level at a slight inclination cover with layer of asphalt or flexible plastic sheetcrushed ore transport from the mine to the prepared site by dump trucks to a level of 10-15 m high The leaching agent is sprayed at the top of the dump through which it percolates and the leach solution is collected at the bottom.When the material is fully leached, the dump is either abandoned or re-used for leaching another batch.

Heap/Dump LeachingMaterial handling and stock-piling have become enormous engineering operations. Problems include plugging with fine materials, evaporation losses, leakage at the bottom, and channeling.Bio-heap leach using bacteria is commercial option

Bio-Heap Leaching example Talvivaara, Finland Is Takes

Hydrometallurgy Terminologypurification/concentration operation:After leaching.Prior to precipitation. Goals: 1) purification and 2) increase solution concentration, from which the metal values can subsequently be precipitated effectively. methods used are: adsorption on activated charcoal,sorption on ion exchange resinsextraction by organic solvents. Common operation scheme: loading, washing, and unloading (elution or stripping) is used in all three operations. After the elution step, the material is ready for another cycle.

Purification and ConcentrationActivated charcoal and ion exchange processes are often conducted in columnsTwo main steps: loading of the desired metal and elution (unloading)Water-washing between these steps to remove the entrained solution. After the elution step, the column is ready again for loading

Purification and Concentration Adsorption on activated charcoal: Used for concentrating gold and silver from cyanide leach solutionCan be used for turbid solutions or pulps thus saving an expensive filtration step.Low adsorption of metal ion by activated charcoal compared to ion exchange; however, activated charcoal is a much cheaper material Charcoals heated at 400-800 C produce a highly porous material called "activated charcoal", usually in as pellets of 2 mm diameter

Purification and ConcentrationCarbon adsorption and elution are slow processes: Typically it takes about 24 hours to adsorb gold from a solution containing about 10 ppm gold and 50 hours to elute.

One ton activated carbon adsorbs about 10 kg gold.

Three variations of the process are used: columns, carbon-in-pulp and carbon-in-leach.

Two factors contribute to the choice of the adsorption process:filtration properties of the pulppresence of organic matter in the ore

Granular Activated Carboncolumns are used when the ore can be filtered easily and clear solutions can be obtained Carbon-in-pulp used to treat ores containing clay particles which are difficult to filter. Pulp is agitated in tanks with the charcoal pellets, then screened to collect gold-laden pelletsCarbon-in-leach used to treat ores containing organic matter - gold cyanide complex susceptible of being lost in the residue. Granular activated carbon is added in the leaching tanks so that it can adsorb the gold cyanide complex ASAP.

Example - GoldGold Cyanidation Pulp flows through a series of agitated tanks.Oxidative dissolution by hydrogen peroxide or air (or both).NaCN (0.32 gm l-1)pH is maintained at 10.0 by lime addition. 4Au + 8NaCN + O2 + 2H2O 4Na+[Au(CN)2]- + 4NaOH

Gold Cyanidation

Gold LeachingPulp from cyanidation is sent to a second series of agitated tanks.Carbon adsorption:[Au(CN)2]- complex is adsorbed onto the surface of activated carbon granules.washing with water to remove the entrained solutiondesorption, usually with a solution of 0.2% NaCN and 1% NaOH at 90 CLoaded charcoal is removed and acid washing to remove CaCO3 precipitateDewatering, regeneration by heating in a kiln for 30 minutes at 700 C in absence of air, then quenching and recycling.Carbon is transferred through the series of tanks, counter-current to the flow of pulp.Waste rock is disposed.Strip solution is plated onto stainless steel electrodes.Plates out at 65% gold.

Gold Cyanidation

Ion Exchange (IX)Natural (zeolite) or synthetic (polymer resin) materialUranium was the first metal to be recovered commercially using IX paved the way for other metalsEspecially useful in the treatment of very dilute solutions with metal ion concentration of the order of 10 ppm or lessAn ion exchanger is a framework or a matrix (sponge) which carries a positive or a negative electric charge. Counter-ions (holes) can be replaced by other ions of the same sign, while the fixed ions (matrix) are not mobile.

Ion Exchange

Types of IX systems:Columns fixed resin bed. Batchwise, carousel operation

Ion Exchange EquipmentTypes of IX systems:

Resin-in-pulp - unfiltered leach liquor fed through tanks with wire-mesh baskets containing coarse-grade resin

Continuous pump the resin between loading and elution

Ion ExchangeTypes of IX resins:

Strong acid: cation exchangers, containing -SO3H groups Weak acid: cation exchangers, containing -COOH groups

Strong base: anion exchangers. Strength of the resin can be increased by using substituted amines Weak base: anion exchangers, containing amino groups Anion exchange.. The extracted species is a negatively charged ion and the extractant is a base, e.g., an amine:

Solvent Extractionleach solution is mixed with an immiscible organic solvent so the desired metal ion in aqueous phase is transferred to organic phase The two phases are then allowed to separate.The process is then reversed by contacting the loaded organic phase with an aqueous (strip) solution that transfers the desired metal ion back out of the organic. The aqueous phase obtained is a pure and concentrated solution suitable for metal recovery while the stripped organic phase is suitable for recycle

Solvent ExtractionPregnant loaded with the metal of interestBarren metal of interest has been removedPregnant aqueous the feed solution to SX that contains the components to be separated.Solutes minor components in the feed (or other) solutions = dissolved metals.Solvent the immiscible liquid added to a process for the purpose of extracting a solute or solutes from the feed.Organic the light phase, used for extraction from feed.Raffinate the liquid phase left from the feed after extraction = barren aqueous.Strip solution acts as the solvent to remove metal of interest from the organic phase

Separatory funnel - emulsification

Hydrometallurgy Equipment Solvent extraction mixer-settlersmixing chamber: aqueous and organic phases are mixed together by a rotating impeller settling chamber: mixed phases are given enough time to separate

Solvent ExtractionOnly clear filtered solutions can be extracted by organic solvents

Usually many stages are used (3 to 5) and are operated in counter-current in the extraction as well as in the stripping steps.

Sometimes, a washing step is inserted between extraction and stripping to remove loosely bound metal ions

Solvent Extraction EquipmentKrebs mixer-settler:

Interphase regulatorMixer and conical pumpTop launder for initial phase disengagement

Solvent Extraction Equipment

Column cell:Discs and doughnutsNo exposure to airGentle mixing

Solvent Extraction Equipment

Outotec Spirok mixersLow shear

Solvent ExtractionOrganic typically has the following components:Carrier main volume of organic, eg. keroseneExtractant active in collecting metal. Eg. amineDiluent Reduces surface tension, aids phase separation, eg. Isodecanol (alcohol)

Example: Uranium Uranium Solvent Extraction:

Aqueous leach solution (3 - 13 g/l uranium) is separated from the waste rock and is sent to solvent extraction as the feed solution.Aqueous feed is mixed with an organic extraction solvent consisting of:kerosene (91%)isodecanol (3%)tertiary amine (6%)Uranium is selectively transferred to the organic phase.Barren aqueous phase (raffinate) is recycled to CCD or discarded.

Solvent ExtractionUranium SX - Complexation Reactions 2R3N + H2SO4 2 R3NH+ + SO42Extraction:4 R3NH+ + UO22+ + 3 SO42(R3NH)4UO2(SO4)3Extraction is selective for uraniumStripping: (R3NH)4UO2(SO4)3 + 4NH4OH 3R3N + UO22+ + 3SO42- + 4NH4+ + 4H2OPrecipitation:2UO22+ + 2SO42- + 6NH4OH (NH4)2U2O7 + 4 NH4+ + 2 SO42 + 3H2O

Uranium Solvent Extraction and Precipitation

Uranium Solvent ExtractionKey Lake SX circuit

HydrometallurgyPrecipitation is the final step in many hydrometallurgical processes. It is also used as a purification step to separate impurities Can be physical or chemical

Precipitation EquipmentSolar Crystallizers:Used for evaporating sea water or brines from wells for the bulk recovery of sodium chloride or other salts (magnesium chloride, lithium chloride)Large evaporation ponds are constructed adjacent to the source.Climate in the region must show high yearly evaporation and low rainfall.

Precipitation EquipmentVacuum crystallizer:no reagents are added, but the concentration and temperature adjustedconcentrate a solution such that crystallize solids by evaporation Evaporation is conducted under vacuum to decrease the boiling point of the solution and thus economize in heat requirement common procedure for obtaining pure salts, e.g., sodium chloride, ammonium sulphateHowever, cooling will also effectively lead to crystallization of a salt provided its solubility is largely dependent on temperature.

Precipitation Equipmentmultiple effect evaporators - steam generated in the first evaporator is used to heat the charge in the second evaporator, and that from the second is used to heat the charge in the third.

Hydrometallurgy EquipmentChemical precipitation methods:hydrolysis just add water! Precipitation of oxides, hydrated oxides, hydroxides, or hydrated salts Ionic - ions formed are neutralized by a base, example:Reduction - a reducing agent is added which results in the precipitation of a metal and the agent is oxidized:

An important sub-group is hydrogen reductionSubstitution precipitate metal ions from organic solvents

PrecipitationParticle size Particle size and form of a precipitate depend upon the conditions of formation. Freshly formed precipitate is sometimes described as amorphous or gelatinous and is difficult to separate by filtration. Precipitates undergo continuous recrystallization as they age ... accelerated by heating

PrecipitationPrecipitation involves two steps: nucleation crystal growth Rate of nucleation is influenced by: concentration agitation nucleating agents

Change in valency by adding an oxidizing or reducing agent may be used to effect selective precipitations.

Extractive Metallurgy Terminology Electrometallurgy use of electrical energy to induce a chemical transformation

Electrowinning to precipitate a metal from solution using electric potentialElectrorefining to purify a metal by dissolving it, then re-precipitating it

Electrometallurgy EquipmentElectrolytic process: precipitation of a metal from its aqueous solution is affected by imposing an outside electromotive force from a direct current source. This can be represented by:

Electrometallurgy EquipmentAlternating anodes and cathodes in a tankhouse for electrowinningFor example, copper, zinc, cadmium, and nickel are recovered industrially from leach solutions by electrolytic methods Example: Gold and silver are recovered from the eluate by electrolysis using steel wool cathodes The aqueous solutions are electrolyzed using inert electrodes; the pure metal is deposited on the cathode.

Extractive Metallurgy Terminology Pyrometallurgy use of heat to induce a chemical transformationRoasting convert to oxide form. Often first step preceding smelting for Cu, Ni, Pb Example: 2 CuS2 + 5 O2 2 CuO + 4 SO2Equipment fluidized bed roaster

Extractive Metallurgy Terminology Smelting - uses reducing substances that will combine with those oxidized elements to free the metal. Example: 2 Fe2O3 + 3 C 4 Fe + 3 CO2 Converter add back a bit of oxygen to purify, example: blister copper

Pyrometallurgy EquipmentSmelter:Add flux (silica or lime) to remove impurities - waste becomes slagDust and off gas control are big issues

PyrometallurgyThe molten components coalesce, each forming an individual molten layer Slag: top layer with specific gravity 3.6, and is composed of silicates.Matte: next layer with specific gravity 5.2, and is composed of sulfides.Speiss: next layer with specific gravity 6.0, and is composed of arsenides.Bullion: bottom layer with specific gravity > 6, and is composed of metals.

PyrometallurgyCalcination Chemical decomposition but not oxidation or reduction. Example: CaCO3 = CaO + CO2(g)

PyrometallurgyMulti-hearth calciner/roaster:

Pyrometallurgy Gold SmeltingGold-bearing sludge is mixed with NaNO3 flux.Heated to melting.Impurities transfer to the slag.Final gold product (Dore bar) is then poured, then to refinery.Dore product composition:90% Au5-6% Ag

Pyrometallurgy SteelmakingScrap steel is segregated into piles according to composition.Each batch (heat) has proportions of the various scraps added to yield approximately the correct steel composition.One heat is ~ 135 tonnesAlloys addedLime added as flux, leads to the formation of a slag layer above the molten iron.Impurities in the steel form compounds soluble in the slag:2 Mn + O2 2 MnOSi + O2 CaSiO34P + 5 O2 + 6 CaO Ca3(PO4)2Mn + S + CaO CaS + MnO4 Al + 3 O2 2 Al2O3

Pyrometallurgy SteelmakingCurrent applied through three large carbon electrodes.Arc generates heat up to 5500 F which melts the steelFurther heat provided by injecting oxygen, which reacts with carbon to form CO

Pyrometallurgy SteelmakingAfter slag has been removed, the steel is poured into a ladleA sample of the ladle is taken and assayedBased on the results of the assay and the desired composition, additional alloys are added: Cr, Ti, Ni, V, MoMixed by injecting argon until homogeneousPoured into a continuous caster to make steel plate

Assignment / Tutorial #11 Tutorial / AssignmentComplete selected EduMine sections:

Hydrometallurgy 1: Review #3Extractive Metallurgy 2: Review #2

Flowsheet examplesAluminum: from bauxiteCopper: from chalcopyriteIron: from hematiteGold Placer, sulphide and oxide Dead Sea Israel

Aluminum

Copper

Iron

Gold

Dead Sea brine

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