Hydrometallurgy

 

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LEACHING     Ion Exchange    Solvent Extraction    Electrowining   Electrorefining 

Hydrometallurgy is the field of extractive metallurgy involving the use of aqueous chemistry for the recovery of metals from ores, concentrates, and recycled or residual materials. 

    Hydrometallurgy is typically divided into three general areas:

 

  LEACHING :  Leaching involves the use of aqueous solutions containing a lixiviant which is brought into contact with a material containing a valuable metal. The lixiviant in solution may be acidic or basic in nature. The type and concentration of the lixiviant is normally controlled to allow       some degree of selectivity for the metal or metals that are to be recovered. In the leaching process, oxidation potential, temperature, and pH of the solution are important parameters, and are often manipulated to optimize dissolution of the desired metal component into the aqueous phase.

The three basic leaching techniques are 

In-situ leaching is also called "solution mining." The process initially involves drilling of holes into the ore deposit. Explosives or hydraulic fracturing are used to create open pathways within the deposit for solution to penetrate into. Leaching solution is pumped into the deposit where it makes contact with the ore. The solution is then collected and processed

In heap leaching processes, crushed (and sometimes agglomerated) ore is piled in a heap which is lined with an impervious layer. Leach solution is sprayed over the top of the heap, and allowed to percolate downward through the heap. The heap design usually incorporates collection sumps which allow the "pregnant" leach solution (i.e. solution with dissolved valuable metals) to be pumped for further processing

Vat leaching involves contacting material, which has usually undergone size reduction and classification, with leach solution in large tanks or vats. Often the vats are equipped with agitators to keep the solids in suspension in the vats and improve the solid to liquid contact. After vat leaching, the leached solids and pregnant solution are usually separated prior to further processing

Solution concentration and purification

After leaching, the leach liquor must normally undergo concentration of the metal ions that are to be recovered. Additionally, some undesirable metals may have also been taken into solution during the leach process. The solution is often purified to eliminate the undesirable components. The processes employed for solution concentration and purification include:

   Precipitation.  Precipitation in hydrometallurgy involves the chemical precipitation of either metals and their compounds or of the contaminants from aqueous solutions. Precipitation will proceed when, through reagent addition, evaporation, pH change or temperature manipulation, any given species exceeds its limit of solubility. In order to improve efficiency in downstream processes, seeding to initiate crystallization is often used

Metal Recovery

Metal recovery is the final step in a hydrometallurgical process. Metals suitable for sale as raw materials are often directly produced in the metal recovery step. Sometimes, however, further refining is required if ultra-high purity metals are to be produced. The primary types of metal recovery processes are 

       1. Electrowining                                 2.  Electrorefining

 

 Ion Exchange

   Ion exchange is a reversible chemical reaction wherein an ion (an atom or molecule that has lost or gained an electron and thus acquired an electrical charge) from solution is exchanged for a similarly charged ion attached to an immobile solid particle. These solid ion exchange particles are either naturally occurring inorganic zeolites or synthetically produced organic resins. An organic ion exchange resin is composed of high-molecular-weight polyelectrolytes that can exchange their mobile ions for ions of similar charge from the surrounding medium. Ion exchange reactions are stoichiometric and reversible, and in that way they are similar to other solution phase reactions. For example:  a resin with hydrogen ions available for exchange will exchange those ions for nickel ions from solution. The reaction can be written as follows:

       2(R-SO3H)+ NiSO4 = (R-SO3)2Ni+ H2SO4

 R indicates the organic portion of the resin and SO3 is the immobile portion of the ion active group.The degree the reaction proceeds to the right will depend on the resins preference. or selectivity, for nickel ions compared with its preference for hydrogen ions. The selectivity of a resin for a given ion is measured by the selectivity coefficient. K. which in its simplest form for the reaction

R-A++B+=R--B++A+ 

is expressed as: K = (concentration of B+ in resin/concentration of A+ in resin) X (concentration of A+ in solution/concentration of B+ in solution). The selectivity coefficient expresses the relative distribution of the ions when a resin in the A+ form is placed in a solution containing B+ ions.

Resin Types

Ion exchange resins are classified as cation exchangers, which have positively charged mobile ions available for exchange, and anion exchangers, whose exchangeable ions are negatively charged. Both anion and cation resins are produced from the same basic organic polymers. They differ in the ionizable group attached to the hydrocarbon network. It is this functional group that determines the chemical behavior of the resin. Resins can be broadly classified as strong or weak acid cation exchangers or strong or weak base anion exchangers.

Strong acid resins are highly ionized in both the acid (R-SO3H) and salt (R-SO3Na) form. They can convert a metal salt to the corresponding acid by the reaction:

2(R-SO3H)+ NiCl2 --> (R-SO4),Ni+ 2HCI 
Weak acid resins exhibit a much higher affinity for hydrogen ions than do strong acid resins. This characteristic allows for regeneration to the hydrogen form with significantly less acid than is required for strong acid resins.

Weak Acid Cation Basins. In a weak acid resin. the ionizable group is a carboxylic acid (COOH) as opposed to the sulfonic acid group (SO3H) used in strong acid resins. These resins behave similarly to weak organic acids that are weakly dissociated.

Strong Base Anion Resins. Like strong acid resins. strong base resins are highly ionized and can be used over the entire pH range. These resins are used in the hydroxide (OH) form for water deionization. They will react with anions in solution and can convert an acid solution to pure water:

R--NH3OH+ HCl -> R-NH3Cl + HOH

Solvent extraction

Solvent extraction and partitioning, is a method to separate compounds based on their solution preferences for two different immiscible liquids, usually water and an organic solve . It is an extraction of a substance from one liquid phase into another liquid phase. In solvent extraction, a distribution ratio is often quoted as a measure of how well-extracted a species is. The distribution ratio (D) is equal to the concentration of a solute in the organic phase divided by its concentration in the aqueous phase. Depending on the system, the distribution ratio can be a function of temperature, the concentration of chemical species in the system, and a large number of other parameters.