Lecture Presentation CONCENTRATION & PURIFICATION

Lecture Presentation

CONCENTRATION & PURIFICATIONBy:Cut Aja Fauziah, S.Si., M.Eng.Sc (Metallurgy)20 May 2015

Department of Mining and Metallurgical EngineeringWestern Australian School of Mines

ReviewPretreatment Crushing, grinding and separation of minerals by physical or flotation methods.i.e. getting the ore or concentrate into a form suitable for the leaching process.Leaching Use of appropriate reagents and conditions (T, P) to selectively dissolve the desired mineral into solution at a reasonable ratei.e getting the desired mineral from the ore or concentrate into solution by simple dissolution or by chemical reaction to convert it into a soluble form.Purification The leach liquor is purified and/or concentrated by selective precipitation, ion exchange, solvent extraction or other processes.i.e. purification of the leach solution to separate the desired mineral from other soluble and/or insoluble impurities.Recovery (the next encounter)

Review (2)The four basic steps are called hydrometallurgical process

Hydrometallurgy is the term used to describe the processes involved in the production of metals or metal compounds from ores or concentrates by wet chemical methods

Hydrometallurgical is a relatively recent technique compared to the ancient science of PyrometallurgyPurificationIon Exchange and Solvent ExtractionI. Ion ExchangeIon exchange is the reversible interchange of ions between an insoluble solid phase (the ion-exchanger) and solution phase

A represented equation: (cation exchange)M-A+ + B+ M-B+ + A+Solid Solution Solid Solution

The process is used to purify solutions and/or obtain a more concentrated solution for subsequent processing.Ion Exchange MaterialThe materials need to possess two basic properties:Should have a very low solubility in waterShould possess a significant quantity of mobile (or exhangeable ions)

The first ion exchange materials used were natural inorganic zeolites and later synthetic zeolites

Zeolites are sodium or potassium alumino-silicates that generally comprise the clay fraction of many soils e.g. kaolinite, halloysite, montmorillonite. Example: Kaolinite, Talc, Saponite, Quartz (Silicates Mineral)Analcite, Chabazite, Mordenite (Zeolites)Ion Exchange Material (2)Zeolites, unfortunately, tend to decompose irreversibly in acid solution

Most modern ion exchange resins are formed by the process of addition polymerisation to produce a cross-linked hydro- carbon matrix

This polymer provides an insoluble framework called a "matrix"Ion Exchange Material (3)A cross-linked hydro- carbon matrix:

Ion Exchange Material (4)The ion exchange group is then added to the surface of the resin by reacting the copolymer with a suitable reagent

For example, reaction with hot sulphuric acid introduces a sulphuric acid functional group to each benzene nucleus

RH + H2SO4 R - SO3H + H2OOther acidic groups such as - CO2H (carboxylic acid), -PO3H2 (phosphonic acid) and -AsO3H2 (arsonic acid) can be similarly added to the matrix

Ion Exchange Material (5)These acidic ionogenic groups (as they are called) will all produce cation exchange resins

Anionic exchange resins can be similarly prepared by incorporating basic functional groups, usually substituted ammonium groups, as the ionogenic substituents

The strength of the acidic or basic substituent is one of the most important properties of a resin

Types of Ion Exchange Resins

Type Functional Group PropertiesStrong Acid Sulphonic acid Ionised at all pH's ConstantCation -SO3-H+ capacity. Requires excess strong acid for complete regeneration.

Weak Acid Carboxylic acid Negligible capacity belowCation -COOH pH 5. Regenerated by dilute or weak acid.

Strong Base Quartenary Requires strong base forAnion ammonium e.g. regeneration -N+(CH3)3OH-

Weak Base Alkyl ammonium Regenerated by weak baseAnion - N+HXR3-X e.g. NH4OH, Na2CO3

CH2-COOH Special affinity forChelating -N polyvalent metal ions. CH2-COOH Iminodiacetate

Characteristics of Ion Exchange Resins

They are the Selectivity Coefficient, Separation Factor, Ion Exchange Isotherm, Ion Exchange Capacity, and Breakthrough Capacity1. Selectivity CoefficientThe exchange reaction between cations A+ attached to a resin R and cations B+ in a solution can be written as below: R - A(s) + B+(aq) R - B(s) + A+(aq)The thermodynamic equilibrium constant, in terms of activities, a, can be written as:

1. Selectivity Coefficient (2)Since activities in the solid phase are very difficult to define and determine, a selectivity coefficient KBA is defined as:

where [A], [B] are the solution concentrations of A+, B+ in mol L-1.[AR], [BR] are the resin concentrations of A+, B+ in mol L-1.A, B, R-A , R-B are the respective activity coefficients

2. Separation FactorBecause of the difficulty in defining the concentration of a component in the solid phase, we define the term equivalent ionic fraction

The equivalent ionic fraction of A in solution (XA) and in the resin ( ) are defined as:

XA = ZAmA/(ZAmA + ZBmB) and = ZA A / (ZA A + ZB B )

where ZA, ZB are the ionic valencies of the species A and B mA, mB are the molalities (moles/kg of solvent) of A, B in solution A , B are the molalites of A, B in the resin.

2. Separation Factor (2)The separation , factor AB is defined as follows:

This ratio is constant, irrespective of the units used

A value of AB greater than unity means that the ion A is preferred by the exchanger. A value less than unity means that the ion B is preferred.

3. Ion Exchange Capacity: (Q)

Ion exchange capacity of a resin is the number of moles of exchangeable ions per unit weight of the resin. It has the units of moles per kilogram of dry resinTo obtain the ion exchange capacity, for a cation exchange resin, the resin must be first fully converted to the hydrogen form by washing with acid solution and then washed free of excess acid by rinsing in waterThe resin is then titrated with NaOH in the presence of an inert salt and a pH titration curve obtainedAn anion exchange resin is similarly titrated with HCl3. Ion Exchange Capacity: (Q) (2)The titration curves take the form shown below:

Ion Exchange Capacity (Q) = vol. Titrant x Molarity Mass of dry resin

e.g. For 1.0 g dry resin consuming 25.00 mL of 0.10 M NaOHQ = 0.025 L x 0.10M 0.001 kg = 0.25 mol kg-1

4. Breakthrough CapacityIt is obtained by passing a solution of the ion of interest through the column.In the initial concentration of the ion in the feed solution is Co, then the breakthrough capacity is defined by point a on the curve below:

4. Breakthrough Capacity (2)At point A, the concentration of the ion in the column effluent C, divided by the initial concentration, Co, becomes finite.

This indicates leakage of the ion of interest. It is no longer being effectively adsorbed onto the column.

Breakthrough capacity = vol. solution at A x Co Mass of dry resin

4. Breakthrough Capacity (3)The breakthrough capacity depends upon the dynamic behaviour of the resin and will hence depend on:(i) the ion undergoing exchange(ii) the type of resin(iii) the resin particle size(iv) the liquid flow rate(v) temperature.

II. Solvent ExtractionSolvent extraction of metals is based on the use of an organic extracting agent that forms a compound with the metal ion, the metal-organic compound being soluble in an organic diluent such as kerosene and insoluble in aqueous solution

Once the metal has been extracted into the organic phase and thereby separated from all the "gangue" minerals, it must be able to be "stripped" from this phase back into an aqueous phase by a suitable stripping agentExtracting AgentsThey can be broadly classified into three main groups:

(i) Acidic - extract metals in cationic form.

(ii) Basic - extract metals in anionic form.

(iii) Neutral - extract metals as solvates or addition compounds.

(i) Acidic Extracting AgentsThese compounds contain acidic hydrogen atoms which can be ionised and replaced by metal ionsTypical of this class of agents are the dialkyl phosphoric acids (e.g. Di-(2-ethylhexyl) phosphoric acid, D2EHPA)

e.g. 2 R2HPO4 + M2+ M(R2PO4)2 + 2H+ (1)

(ii) Basic Extracting AgentsBasic reagents extract anions only, hence the metal ion to be extracted must exist at least partially in anionic form

The most common basic reagents are the primary, secondary and tertiary amines or quarternary amines

A typical mechanism is shown below for the extraction of uranium from a uranyl sulphate solution using a tertiary amine extraction agent:(ii) Basic Extracting Agents (2)The first step is the formation of the "-onium ion" (the solution must be acidic for this to occur):2R3N + H2SO4 2R3NH+ + SO42- (2)

In sulphuric acid, the uranyl sulphate forms an oxo- sulphato complex: UO2SO4(s) + 2H2SO4 UO2(SO4)34- + 4H+ (3)

A metal-organic compound, soluble in the organic phase, is formed between the -onium ion and the complex uranium ion:4R3NH+ + UO2(SO4)34- (R3NH)4UO2(SO4)3(organic) (4)

(iii) Neutral Extracting Agents

These are non-ionic. Metal extraction is effected by the transfer of complete metal salts into the organic phase in the form of solvates or addition compounds

The extraction agent in these compounds is normally covalently bonded to the metal ion in the salt.

e.g. UO22+ + 2NO3 + 2(BuO)3PO kerosene UO2(NO3)2.(BuO)3PO (5) tributyl phosphateOrganic SolventsThe organic solvent is required to selectively dissolve one component of a solution in preference to othersIt should therefore have a large distribution coefficient (ie. ratio of desired metal complex concentration in the solvent phase to the raffinate phase)A desirable organic solvent should also have a large number of other desirable properties, such as low water solubility, low viscosity, vapour pressure and flammabilityIt should also have a considerable difference in density to water (to allow easy separation of phases) and a low chemical reactivityReferencesAustralian Mineral Foundation. "Hydrometallurgy for Design and Operating Engineers, Adelaide, 1974

Browner, Richard. Lecture Notes of Hydrometallurgy Unit, Western Australian School of Mines, Curtin University. 2013.

R.W. Grimshaw & C.E. Harland "Ion Exchange: Introduction to Theory and Practice, Chemical Society (London). 1975.