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We are an important link in the chain completing
substance cycles for major
and small and medium-sized
companies of the basic substance industry and
for
galvanizers of every kind. Our international team
with specialists from
Germany, Kazakhstan, Hungary,
Poland and Finland makes us one of
Europe's most
efficient producers.
Our name obliges us to maintain a tradition:
we are RuhrZink GmbH.
Leaching Department
Materials arriving from the roasting department are stored in five silos possessing a total capacity of 2400 tonnes. Screw conveyors transport the material from the silos to leaching tanks were cell acid is used to extract the zinc (electrolyte from the hall of baths from which zinc has been removed). Steam from the roasting works is used to heat the leach to between 80 and 85°C in order to ensure that the material is dissolved as completely as possible. Four stainless-steel tanks, each possessing a working capacity of 130 m³, are available for acid leaching. The tanks are arranged in a cascade and are continuously operated. After acid leaching, the solution is neutralized with roasted material in three additional tanks. At the same time, oxygen is admitted to the tanks to convert the dissolved iron into an indissoluble form. Due to the fact that the roasted material is not completely dissolved, the solids (iron-zinc concentrate) must be removed. The concentrate is removed in a thickener possessing a diameter of 26 m and a capacity of more than two million litres. The thickener's clear overflow, the raw leach, is heated with steam in two heat exchangers to 92°C and then conveyed to the leach-purification section. The underflow is then cleaned in two drum filters and subsequently washed and drained in six centrifuges and two chamber filter presses. It’s advantageous to remove as much water from the zinc-iron concentrate in the centrifuges as possible because better washing results and lower residual moistures may be achieved there. The maximum leach throughput is 220 m³/h whereby more than 93% of the zinc is removed from the roasted material.
The employment of Waelz oxide
One-third of the manufactured zinc is not produced from primary ores but from secondary raw materials, so-called Waelz oxides. Ruhr-Zink GmbH thus considerably contributes to the creation of material flows and therefore to the conservation of natural resources.
Waelz oxide is created by processing steelworks dust using coal in Waelz pipes. In this stage, zinc is vaporized and separated from the fumes as zinc oxide.
The Waelz oxide also contains large quantities of lead, chloride and fluoride. While the lead does not affect the hydrometallurgical reclamation of zinc because it may be separated as insoluble lead sulphate, it is absolutely mandatory that no chloride or fluoride is admitted. The chloride and fluoride would become enriched during the dissolution cycle and could cause the electrodes and other components to corrode. That's why the chloride and fluoride must be with removed from the Waelz oxide, which is achieved through the reaction of Waelz oxide with soda. The chloride and fluoride compounds cannot be dissolved in water and are converted during this process into sodium chloride (salt) and sodium fluoride which may be dissolved in water and then separated with the washing filtrate. A high temperature of around 120 to 130°C is required for almost total separation. That's why autoclaves are used (pressure reactors that are similar to giant pressure cookers).
After washing, the Waelz oxide is filtered on two filter presses and leached similarly to the roasted material with cell acid in two tanks. A thickener and filter presses are employed to separate the insoluble lead sulphate from the still somewhat acid leach. The lead sulphate is then sold as raw material for lead production. The clear zinc solution is transported to the leaching section for roasted materials.
Laugenreinigung
Consistent high zinc quality and the optimum utilization of the electrolysis flow make it necessary to remove all elements that are nobler than zinc (copper, cobalt, nickel, cadmium, antimony, arsenic, thallium, germanium, etc.) from the leach. Therefore the leach is cleaned in three stages.
In the first stage, copper, cobalt and nickel are separated through the addition of zinc dust and arsenic at temperatures of between 85 and 92°C. The process is continuously monitored by staff who carry out analyses in order to check that the specified residual concentrations are adhered to. The solids thus created (copper cement) is filtered off, washed and sold to the copper production industry.
Subsequently, the elements of cadmium and thallium are separated in five in-line fluidized-bed reactors. This creates a cadmium cake containing more than 90% of cadmium. This has to be dumped. Final cleaning before the leach is filtered and conveyed to the storage and mixing stations takes place in a third stage were zinc dust is also used.
Due to the fact that the residues mainly consist of unused zinc dust, the dust is recirculated in the first cleaning stage. The natural leach now contains around 140g/l of zinc. Around 4 to 6% of the produced zinc is required in the shape of zinc dust in leach purification.
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