2.2 Leaching agent
Care should be taken when choosing the right leaching reagent since often several metals are dissolved into the same solvent. Ideally a reagent only dissolves the wanted metals into the solution. This is rarely reached since leaching reagents also take many other metals that are present in the ore into the solution at the same time. There are several aspects to take into consideration when choosing a leaching agent. The desired constituent has to be soluble in the leaching agent, the agent has to be relatively cheap and the metal has to be economically recoverable from the solution. The corrosive nature of the leaching agent has to be taken into consideration as well. The capital cost of the equipment is higher if the leaching agent is corrosive, because tanks and other equipment need to be made of stainless steel, titanium or Hastelloy (Habashi, 1999a). With higher environmental concerns, the regeneration of the leaching agent for recycle is also important (Ray and Ghosh, 1991).
Water is the most ideal leaching reagent because it is cheap and not corrosive.
However, according to Habashi (1999a), using water as a leaching reagent is limited to only a few cases. Naturally occurring salts, flue dusts, calcines and some sulfide concentrates are leached using water. Some sulfides are dissolved and converted to sulfates by water leaching under pressure and at 200 °C in the presence of air. According to Ray and Ghosh (1991) low-grade copper sulfide ores can be leached under atmospheric conditions by water since they slowly form water soluble sulfates. Sulfides are most commonly leached by sulfuric acid (Habashi, 1999a). Commonly used leaching agents, others than water, can be divided into three categories: acids, bases and aqueous salt solutions (Habashi, 1999a). In addition to these categories, aqueous chlorine and hypochlorite are used in minor extents. The most common leaching agents used for different minerals are shown in Table I.
Sulfuric acid is commonly used when leaching metals because of its rather low cost (Noyes, 1994). In addition sulfuric acid only has minor corrosion problems and sulfuric acid can dissolve many metal compounds. Dissolving many metal compounds might however not be wanted since also the worthless metals might be dissolved into the solution.
Ammonia is also used for dissolving nickel and copper (Noyes, 1994; Moore, 1981). Ammonia and ammonium carbonate are however more expensive than for example the commonly used sulfuric acid and hence the recycling of especially ammonia and ammonium carbonate is essential.
TABLE I Summary of commonly used leaching reagents for various minerals. 1Habashi (1999a), 2Moore (1981).
Mineral Leaching reagent Metal sulfates Water2, H2SO42
Metal sulfides H2SO42
, Fe2(SO4)3 solution1,
NH4OH + air (nickel sulfide concentrates)1 Metal oxides H2SO42
Cu/Ni minerals NH32, FeCl32
Gold and silver ores NaCN + air1, Cl2 (aq)1, Aqua regia (noble metals)1
In addition to the importance on the choice of the reagent also the concentration has an effect on the leaching of sulfides. This will be discussed in the following section.
2.2.1 Concentration of leaching agent
Rates of reactions are usually increased by increasing reactant concentration (Habashi, 1999b). Increasing reactant concentration is however not economical beyond stoichiometric concentrations. When designing leaching processes the minimum concentration should be found out in order not to waste any valuable reagent even though leaching agents are reused inside the process.
Sokić et al. (2009) investigated the effect of concentration of sulfuric acid during the leaching of a chalcopyrite flotation concentrate. Sodium nitrate was added as an oxidizing agent. By keeping all the other variables constant and by investigating four different sulfuric acid concentrations (0.6, 1.0, 1.5 and 2.0 mol/dm3) the recovery of copper was determined. After a 240 min leaching time the reaction had not reached equilibrium. At 240 min a copper extraction of 47 % and 75 % was achieved by having acid concentrations of 0.6 mol/dm3 and 2.0 mol/dm3, respectively. This can be explained through the oxidizing agent used.
Increasing leaching acid concentration also increases the concentration of H+ ions which affects the oxidizing potential of NO3-
ions. The higher the concentration of H+ ions, the higher the oxidizing potential of NO3-
ions.
Koleini et al. (2011) concluded in their study that the initial acid concentration has little effect on the recoveries of copper from chalcopyrite. This has been explained by the difficulties in controlling the redox potential. Sulfuric acid concentrations used during the study were 7.5, 15 and 30 g/dm3 (0.075, 0.15 and 0.3 mol/dm3).
Similar recoveries were observed with 7.5 and 30 g/dm3 whereas the highest recovery was obtained with 15 g/dm3. The recovery with an initial acid concentration of 15 g/dm3 was over 80 % whereas with the other acid concentrations the recovery was about 70 %. The changes have been discussed by Koleini et al. (2011) and occur presumably because of the different increases in pH and the difficulty in controlling the redox potential. Oxygen or nitrogen gas was injected continuously to control the redox potential. The increase in pH indicates the use of protons from sulfuric acid when dissolving valuables. At the beginning of the experiments the pH was about 0.8 and at the end a pH of 1.8-1.9 (which was the final pH of 0.15 mol/dm3 sulfuric acid dissolution reaction) was reached. This value of pH is a limit pH when ferric ion might precipitate as jarosite. Jarosite has been discussed to cause passivation of chalcopyrite.
According to Dutrizac et al. (1969) the acid should prevent ferric ions from hydrolyzing. However an acid concentration of 0.1 mol/dm3 should be sufficient to prevent hydrolysis and hence the reason of little change in recovery when increasing the acid concentration presumably lies in the difficulty of controlling the redox potential.
Aydogan et al. (2006) came to similar conclusions as Sokić et al. (2009). They both concluded that copper recoveries were increased when the concentration of sulfuric acid was increased. In the study made by Aydogan et al. (2006) a chalcopyrite sample was leached in sulfuric acid in five different concentrations, 0.1, 0.2, 0.3, 0.4 and 0.5 mol/dm3. Strongly oxidizing conditions were formed by the addition of potassium dichromate. After 150 min leaching a copper extraction of 20 % and 54 % was achieved by having acid concentrations of 0.1 mol/dm3 and 0.4 mol/dm3 respectively. However, there was no significant change in the copper extraction when comparing 0.4 mol/dm3 and 0.5 mol/dm3 acid. The reason for an increase in recovery when increasing the acid concentration occurs of the same reasons as for the study made by Sokić et al. (2009). Increasing the acid
concentration and hence the H+ ion concentration also increases the reduction potential of dichromate ion which was used to obtain strongly oxidizing leaching conditions.
The results of the investigations described above show that increasing acid concentration affects the redox potential through which the recovery of valuables is affected. In addition there should be a sufficient acid concentration in order to prevent the hydrolysis of ferric ions that form products that hinder the dissolution of chalcopyrite.