How to Mining – Recycling – Refining of Zinc
Primary non-sulphide zinc deposits formed the initial targets for the company. Most deposits of zinc are formed of zinc sulphide, sphalerite (ZnS2) and the zinc grade of the ore is generally between 5% and 8%. Following crushing and milling individual mineral grains are liberated and those containing zinc are separated from the other minerals and a concentrate of the zinc bearing minerals, running from 53% to 58%, is obtained. The zinc concentrate is then shipped to an electro-refinery where zinc metal is produced.
When zinc sulphide is near the surface, it is unstable and will over millions of years react with air and water to form a variety of minerals, including: oxides, carbonates, silicates and aluminium silicates. Most commonly a mixture of these minerals is formed. However, as oxidation is generally the prime reaction, they are collectively referred to as “oxides”.
These “oxide” minerals each have different properties so that a process designed to concentrate one mineral may not work for another. The mixed mineralogy of oxide deposits frequently requires complex processes to recover a high proportion of the zinc, and as each process has its own costs, such an approach is rarely economically viable. A process that recovers all the zinc, irrespective of its mineralogy is likely to be more attractive. This may be achieved either by hydrometallurgical techniques that dissolve out the zinc or by pyrometallurgical techniques that “boil” off the zinc, the latter, however, tend to be very energy intensive.
The hydrometallurgical approach enables the zinc to be recovered either as metal or some other refined product. However such processing requires substantial capital expenditure, and to justify this only larger deposits are likely to be economic.
ZincOx undertook a global literature search for oxide deposits. This found that zinc deposits frequently consist of several small high grade pods of mineralization. The extent of those pods near the surface is generally small and therefore the potential to generate large tonnages of oxide zinc minerals is very limited. The study concluded that higher grade deposits had generally been processed by pyrometallurgical methods at a time when energy was relatively cheap, and so these deposits are generally exhausted. The lower grade deposits may remain unexploited but are generally too small to justify hydrometallurgical treatment.
To date the company has successfully obtained interests in two major zinc oxide deposits: Jabali, in
There are, in addition, a small number of other substantial zinc deposits in which the company would be interested to participate. While attempts to date to secure an interest in these projects has not been successful, they remain potential targets.
The variable nature of primary zinc oxide deposits has required the evaluation and modification of numerous metallurgical techniques. These methods have enabled ZincOx to consider a very broad spectrum of materials including industrial wastes. Most of these wastes are generated in small quantities that are insufficiently large to be of commercial interest. The one notable exception is electric arc furnace dust (EAFD): a waste generated by the recycling of steel.
Steel scrap is recycled in electric arc furnaces by a simple remelting process. Steel production by this method accounts for about one third of global steel production annually. Steel objects are increasingly galvanized to protect them from corrosion. Galvanising involves the coating of steel by a thin layer of zinc metal. As more steel is galvanized so too scrap contains an increasing amount of galvanized material. When the scrap is recycled, the zinc from galvanising, together with other base metals, alkalis and halides, are driven as a dust and caught up in the flue gasses. These fine particles need to be filtered out before the furnace gasses can be returned to the atmosphere. The filters are periodically cleaned and the resulting dusty material is known as electric arc furnace dust or EAFD.
For every tonne of steel produced by recycling there are typically between 12 and 20 kg of EAFD produced. EAFD generally contains between 18% and 25% zinc, which is about four times richer than the average zinc deposit found in nature. In addition to zinc the EAFD contains 20% to 30% of iron and 1% to 3% of lead.
Generally speaking at historical zinc prices, the recovery of zinc from EAFD has not been possible without a subsidy, i.e. it has no value and is therefore considered to be a waste. Since it also contains traces of toxic elements such as cadmium, arsenic and mercury, it is classified as a hazardous waste. EAFD has been processed for many years using Waelz kiln technology but this does not recover the iron and due to the addition of fluxes the residue created by the process is greater than the tonnage of EAFD treated. As a hazardous waste, international transportation of EAFD is subject to legal restrictions.
The ZincOx approach recovers:
A superior zinc oxide concentrate, that can be sold to zinc smelters
Pig iron, the basic intermediate feed for the steel industry
Building aggregate that can be sold to the construction industry.
There is no waste from the process. As such it represents a major advance in waste and metals recycling.
ZincOx Strategy - Closing the "Zinc
The ZincOx approach uses the following basic steps:
Feed Preparation- EAFD blending, mixing with pulverized coal, and briquetting.
Metal Reduction - Briquettes are treated in a rotary hearth furnace at a temperature of 1,350oC for 16 minutes and the metals are reduced by reaction with coal. Zinc and lead are driven off and are collected in the gas cleaning system as oxides Iron is transformed to its metallic state but remains mixed with slag as a solid in the briquette.
Melting - Hot iron bearing briquettes (DRI) are fed to an electric furnace. The briquettes are melted, the iron sinks to the base of the furnace and is tapped off and cast as small crude ingots. The other elements combine to form a semi-molten vitreous surface layer, slag, which is periodically discharged.
Since liquid steel is being produced the steel structure of the plant and equipment needs to be insulated from the very high temperatures required by the process. These insulating bricks result in the requirement for very heavy equipment that requires economies of scale for economic development. ZincOx believes that a 200,000 tonnes per annum EAFD capacity is a logical minimum size for a plant.
The rotary hearth furnace is the key equipment for the process. It has been used for the production of pig iron from zinc poor steel industry waste for several years.
Refining operations provide the opportunity to add value to the concentrates produced by mining and recycling. Due to the very high quality of the concentrate produced by the recycling operations, the production of metal, compared to conventional zinc smelters, can be significantly simplified. Furthermore, after the removal of zinc by dissolution, the high purity of the concentrate results in a leach residue that is so high in lead that it can be sold as a concentrate, or further converted into saleable lead chemicals.
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