PROJECT DESCRIPTION
BACKGROUND
Zinc production in the European Union was about 1,907,000 t in 1994, thereof 1,460,000 t by the electrolysis process. The main impurity of the ore concentrates is iron. A Jarosite-residue is formed for the elimination of the iron, which is disposed in a special pond because of its content of minor elements (Jarosite is a by-product of the extraction of iron from zinc concentrates in the electrolysis plant). This residue can contain more than 5 % of the zinc from the raw material. Metaleurop Weser Zink GmbH succeeded, after several years of research, to develop at the laboratory scale an improved process for maximising zinc recovery from raw material. The industrial use of the new process would have important ecological effects (Data from zinc electrolysis Nordenham): - The yield of zinc recovery from concentrate would increase up to 99.5% - The quantity of waste would be up to 5 % less. - In a plant with a production capacity of 120 000 t/y, 1 900 t/y zinc would be additionally recovered - The new technology would be applicable in most electrolytic zinc plants. This project aimed to transfer the process to an industrial scale by setting up a demonstration plant in the zinc electrolysis plant in Nordenham. A computer process simulation would be used to predict the behaviour of minor elements. A running test with a centrifuge would also give the required data for a suitable design. The demonstration plant would facilitate the optimisation of the process parameters and the evaluation of the process.
OBJECTIVES
The objective of the project was to leach the main part of the zinc from the concentrate through the modification of the Jarosite process. In this process the zinc should be completely recovered and therefore a more efficient use of the natural resources could be ensured. Consequently, the quantity of waste (Jarosite) is reduced, in proportion to the zince recovered. The aim of the process was to protect the environment by reducing the quantity of Jarosite waste, and maximising the utilisation of the raw material. The specific objectives were: - To increase the yield of zinc recovery from natural resources - To reduce quantity of waste - To increase recovery of valuable metals
RESULTS
The test phase of the project lasted for nine months and provided the following key results: 1. The centrifuges can be applied to the pre-neutralisation phase 2. Management of the the leaching process requires an additional solid/liquid separation in the second Jarosite step 3. This additional solid/liquid separation is only possible with two parallel centrifuges. This means that the use of five centrifuges (instead of three) is required The operation of the demonstration plant started in July 2000. Technical problems with regard to the driving power of the centrifuges and sealing problems made some further extensive tests, and technical adapation and integration in the leachning process necessary. Due to frequent standstills of the centrifuges, and the fact that simultaneous operation of all five centrifuges was only achieved for a few days, the predicted increase in zinc recovery, from 98.2% to 99.5%, could not be achieved. Therefore, the maximum exploitation rate that has so far been achieved is about 98.8%. This means that, for the moment, the main project objective has only been partly achieved. However, the beneficiary is convinced that the expected increase in zinc recovery of up to 99.5% during leaching is still possible. However, to prove this, more experience in operating the centrifuges is necessary. The test phase of the project lasted for nine months and provided the following key results: 1. The centrifuges can be applied to the pre-neutralisation phase 2. Management of the the leaching process requires an additional solid/liquid separation in the second Jarosite step 3. This additional solid/liquid separation is only possible with two parallel centrifuges. This means that the use of five centrifuges (instead of three) is required The operation of the demonstration plant started in July 2000. Technical problems with regard to the driving power of the centrifuges and sealing problems made some further extensive tests, and technical adapation and integration in the leachning process necessary. Due to frequent standstills of the centrifuges, and the fact that simultaneous operation of all five centrifuges was only achieved for a few days, the predicted increase in zinc recovery, from 98.2% to 99.5%, could not be achieved. Therefore, the maximum exploitation rate that has so far been achieved is about 98.8%. This means that, for the moment, the main project objective has only been partly achieved. However, the beneficiary is convinced that the expected increase in zinc recovery of up to 99.5% during leaching is still possible. However, to prove this, more experience in operating the centrifuges is necessary.
