Added-value chemicals from secondary and low-grade primary sources
A number of metals have been recovered from a variety of secondary and low-grade primary sources. Processing the leach solution directly to recover metals has the advantage over the conventional metal salt recovery system which usually requires the expensive chemical purification steps to get high added-value chemicals. Metals were recovered from dilute solutions using fluidised bed electrolysis in an easily dissolved form to produce added value-chemicals. A detailed study on the separation of cobalt and nickel from cobalt-nickel cake by oxidative precipitation using Caro's acid as an oxidant is also reported. Following encouraging laboratory studies the process was successfully tested on pilot scale at a copper nickel smelter. The cobalt level was brought down from 8.6 g/l to 50 ppm with the pH kept above 4. Studies have also been carried out for the recovery of gold along with other metals from computer scrap. Following nitric acid pretreatment the gold was recovered using acidic thiourea leaching method. The manufacture of arsenic acid from arsenic(IH) oxide by nitric acid was also studied. Two reported methods are, (1) standard method (addition of solid arsenic oxide to hot acid) and (2) slurry method (addition of acid to slurried arsenic oxide). The problems associated with the standard method are, (a) Excessive foaming and (b) Evolution of large volume of gases leads to uncontrollable reaction. These problems were avoided by changing the standard route to slurry method. The slurry method was tested successfully on a plant scale production at William Blythe & Co. Ltd., U.K. The company is now producing arsenic acid routinely using slurry method. The production of aluminium sulphate from clay samples and the purification of quartz for glass industry is also reported. The use of 50% sulphuric acid was found to be the best to extract all the aluminium in the clay samples. The excess of sulphuric acid (23% of stochiometry) is required to leach all the aluminum. The product was found to contain trace level impurities of iron, calcium, potassium, and sodium. The quartz purification step includes the use of sulphide and non-sulphide collectors to remove the undesirable impurities. The final stage involves the acid washing step to dissolve the acid soluble impurities. The acid washed product was pure and suitable for glass industry.