RECYCLING

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Introduction
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.
 
Galvanised Scrap
 
Waste Generation
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.
 
EAFD Dump
 
Current Practice
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 Loop"

The ZincOx approach uses the following basic steps:

 

  1. Feed Preparation- EAFD blending, mixing with pulverized coal, and briquetting.
  2. 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.
  3. 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.

  

ZincOx Process  [click image to enlarge]

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.

Concentrate Upgrading
The zinc oxide concentrate produced by the rotary hearth furnace is generally high grade but it contains halides (chlorides and fluorides) that are poisonous to conventional smelters. The halides will be removed from the zinc concentrate by washing. The halides are highly soluble, so that they may be removed by water alone and provided the solution is under alkali conditions, zinc and lead are not mobilized. Wash water can be purified but the halides cannot be removed so that the final effluent is essentially a brine solution that can be discharged into the sea. The washed oxide concentrate will be of exceptional purity and is referred to as EZO (Electrolytic Zinc Oxide) and has the following advantages:

  1. Very low residue produced
  2. It can be fed into the leach circuits of conventional zinc electro-refineries without prior roasting and this has the potential to increase production without significant additional capital expenditure.
  3. It will act as a neutralization agent

These advantages will combine to create a more valuable product that will command a premium price.

Environmental Superiority
The process is environmentally superior in three respects:

  1. No waste creation
  2. High recovery - almost 100% recycling of zinc and lead and over 90% recovery of iron.
  3. Produces less carbon emissions than traditional technologies.

In recognition of the superiority of ZincOx’s approach, in 2007 the company was awarded the Mining Journal’s Outstanding Achievement Award for Sustainable Development.