This research project will investigate a combination of hydrometallurgy for iron precipitation and pyrometallurgy for treatment of the precipitate. The aim of this project is to improve the quality of the iron residues coming from the zinc hydrometallurgy, and which could be used by other industries such as the cement or steel industry.
By combining an optimized hydrometallurgical route with a thermal treatment, a cleaner solution has been developed for iron residue. The iron slag produced can be considered as inert material.
Goethite requires 15% less energy to be smelted than jarosite.
Complete sulphur removal by calcination cannot be reached because gypsum decomposes at temperature higher than the melting. Therefore, a 1 step thermal treatment, avoiding the multiplication of sulphur dioxide gas cleaning systems has been developed.
To manage a bleed for the hydrolizable impurities a flowsheet composed of 4 stages (reduction, neutralization to pH = 2, partial goethite precipitation at pH about 3.5 main 'clean' goethite at pH = 3) was developed for treating the acid leach overflow.
Incorporation of some impurities in the 'clean' goethite from the neutralizing agent (calcine) is unavoidable.
To avoid high consumption of coke at the fuming step of the thermal route, it is important to keep the zinc content of the goethite as low as possible by appropriate choice of the pH (determined at 3). Zinc content will be in the range of 7%.
Several batches of industrial goethite were smelted in an electric furnace. The goethite is smelted into a slag mainly composed of iron silicate. By adding coke, the zinc, lead arsenic and cadmium are volatilized in the off gases producing very clean slags.
Preliminary sizing of the main process equipment items was performed and a factored capital cost estimate has been prepared.
Comparing operating costs to the 200000 t/year zinc production, means that this additional treatment cost cannot be afforded by any zinc producer with the current zinc price.
If a thermal route is needed to put iron into unleachable form, it should be far simplier and cheaper. Also, it will replace a potential environmental risk of soil pollution (dumping) by a high energy consumption with the corresponding carbon dioxide emission.
This programme has 2 parts:
the hydrometallurgical part, where the conditions of iron precipitation will be reviewed to get a goethite free of one or more of the following groups of impurities, zinc, lead, minor impurities (arsenic, selenium, antimony, cerium and fluorine) and sulphate.
and pyrometallurgical treatment in electrical furnace of conventional goethite and also of the improved goethite obtained above.
Funding SchemeCSC - Cost-sharing contracts
6024 AA Budel