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Roasting and sulphanic acid plant.

The individual types of zinc concentrate and the oxidized feedstock possess very different chemical compositions and are therefore prepared on the basis of a mixing plan drawn up in advance. A bridge crane is used for the mixing process.

The crane places the different types of material on heaps. The mixture is then transported when 150 tonnes have been reached via screens and conveyor belts to the storage bunker for the fluidized-bed reactors. On the way to the reactor, the material is moistened specifically with canal water via a control. The daily concentrate throughput is around 500 tonnes when only one reactor is taken into operation but may be increased to around 1000 tonnes per day with the second fluidized-bed reactor. Shovel dozers may also be used to increase flexibility when feeding the fluidized-bed reactor.

It is important for smooth operation that the various types are selected properly by the grippers and that the material is stored tidily in the hall. Stocks are measured each day, which may result in mixtures being adjusted. Contamination in the concentrates, for example, wood and plastics, etc., are separated through sieves and treated separately. The hall is equipped with automatically closing gates to prevent the air from being contaminated with dust when they have been opened and closed. The permitted dust emissions are measured and processed at various points in the factory. Special attention is paid to sealing the hall to prevent such dust emissions.

The blended zinc concentrates are processed first. The sulphuric feedstock is conveyed from the charging bunker by plate and thrower belts to the fluidized-bed reactor. The reaction to subsequently takes place in the furnaces is called roasting. Here, the zinc sulphides are converted into zinc oxides at temperatures of around 950°C. The processing heat, which is released during this stage is removed indirectly through cooling elements and subsequent steam production. Due to the fact that the concentrates contain around 30% of sulphur, no additional heat is required. Oil heaters are only necessary when the system needs restarting. The gas required for roasting contains between 10 and 12% sulphur dioxide; it is cooled in the waste-heat boiler with horizontal gas throughput to around 350°C and initially dedusted here by four centrifugal separators.

The heat in the waste gas is also used to generate steam. One tonne of concentrate may be used to generate one tonne of high-pressure steam at 45 bars to heat the solutions in the leaching department. The temperature in the furnace is controlled exclusively through the addition of the feedstock and the admission of water through nozzles into the reactor bed. The active roasting area within the furnace totals 55 m² and contains 5000 individual jets. A fan forces air through these jets to create turbulence in the fluidized bed. Each furnace possesses a natural overflow through which the red-glowing roasted materials are discharged. The plant's performance is not only measured by material throughput, it is also measured by the quality of the roasted material. This quality is indicated by the sulphide-sulphur figure and may not exceed 0.5%. The roasted material in the furnace, in the gas coolers and in the cyclones is cooled, ground and transported by pneumatic equipment along with the dust discharged from the hot-air filters to the leaching sections intermediate silos. Here, around 3000 tonnes of roasted material may be stored so that any stoppages in the fluidized-bed plant may be absorbed.

The processes are monitored at measuring stations and directly at site. Careful boiler cleaning and maintenance is particularly important. All conveyor systems and containers are either vacuum cleaned and de-dusted with the roasting gas or directly with filters and fans. Emission sources are continuously measured and monitored.

Roasting Gas Cleaning

More dust is removed from the pre-cleaned but still hot roasting gas in three hot-gas electric filters from which the gas is then conveyed through six parallel Venturi washers where the wet-gas purification process takes place. The washed and moist gases are then demisted in a two-stage wet electric-filter system. Six vertical star pipe coolers and –  in front of this stage –  six gas coolers are located between these two groups of filters. This is where the gas is cooled to a saturation temperature of around 60°C. The gas leaves the roasting-gas-purification system without dust and is visually clear. The purity of the waste gas is checked by visual measuring devices when it exits the plant –  this is absolutely essential for the next stage – mercury removal.

Careful rinsing of the wet electric filters where lasting contamination is removed from the filters' interior fittings maintains the plant's performance capacity. The dirt contained in the concentrate and in the rinsing water is removed in a filter press through the addition of sodium sulphide. The residue thus produced its transported as sludge containing mercury, selenium and sulphide to an underground dump. The cleaned filtrates are then transported for further processing to the waste-water-treatment plant.

Video: Röstgasreinigung

Before the SO2-containing roasting gas is conveyed to the sulphuric-acid plant, it passes through a gas-purification stage, where the elementary mercury is removed, i.e. absorbed by a watery II-chloride solution. This reaction takes place in the plant's washing tower which possesses tower packings. The calomel created here is very difficult to dissolve and therefore precipitates into a container from the calm solution and thus removed from the process.

Ongoing chemical analyses monitoring the process are necessary to maintain the reaction. The quality of the work at this cleaning stage greatly influences of the quality of the sulphuric acid produced in the following dual catalyst system..

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