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Sep 23, 2019

Outotec FPS technology – treatment of fine zinc concentrates and sulfur residue in zinc smelters

A considerable part of the global zinc production originates from direct leaching plants. The unavoidable byproduct is sulfur cake, which is predominantly still dumped nowadays. Typical sulfur cake contains 50-70 % elemental sulfur, up to 10 % sulfidic sulfur and varying contents of gangue minerals, metal compounds and impurities.
Smelter at Dundee Precious Metals

Due to its rheologic characteristic sulfur cake requires extensive dumping areas. Attempts to upgrade it into a saleable product have not been successful so far. Co-combustion of sulfur cake with zinc concentrate in fluid bed roasting furnaces frequently causes operation difficulties in downstream plant equipment. Sulfur cake lumps evaporate partially in the furnace and condense in colder plant sections forming sticky coatings in gas cleaning equipment.

Outotec confirmed in pilot test work that sulfur residues can be completely combusted in the roasting furnace. It is essential that the fine-grained material is completely disintegrated and homogenously blended with other feed materials.

Our engineering team consisting of Jochen Güntner, Maciej Wrobel and Alexandros Charitos designed a solution to solve this problem – the Outotec Feed Preparation System (FPS), which is currently under construction on the American continent. It is integrated into the complex feed area of an existing zinc smelter. Feed materials are sulfur residue and fine zinc concentrates. FPS is suitable for different fine and ultrafine feed materials like sulfur cake, waelz oxide, dust and fine zinc concentrates.

Jochen Güntner, Senior Roasting Product Manager, told us more about the system.

How does the Feed Preparation System work?

The Outotec Feed Preparation System (FPS) ensures optimum homogeneity of the feed blend. In addition, fine constituents of other feed materials are captured in microgranules of 0.1 – 0.7 mm size. This artificial coarsening of the feed materials permits roasting of feed materials that otherwise are too fine for fluid bed roasting.

Picture 1 shows a typical feed material after homogenization and microgranulation. The corresponding furnace discharge is shown in picture 2.

Feed material and furnace discharge
Picture 1 (left): Typical feed material after homogrnization and microgranulation. Picture 2 (right): Typical calcine from roasting furnace.

Fine feed materials are typically entrained in the process gas and combust in the upper furnace section instead of the fluidized bed. This displacement of the combustion causes high temperatures at the furnace top. The temperature spread between the default roasting temperature in the bed and the furnace top easily reaches 100 °C. Most smelters reduce the feed rate to protect the furnace dome against overheating. The Outotec FPS technology solves this problem. Most of the fines are kept in the fluidized bed and the full furnace capacity can be used.

FPS technology

When did the idea of the FPS appear?

We developed this process over the past 10-20 years, when it became more and more obvious that smelters are using ultra fine feed materials to improve the economics of their operation. Such fine feed materials are problematic not only because of their fineness but often also due to their chemical and mineralogical composition.

What differentiates the Outotec FPS from the competitive alternatives?

The microgranules are stable and their size is well controlled (majority 0.1-0.7 mm), while competitive alternatives produce random size pellets, which contain unoxidized core due to their size.

What are the next steps in the product development?

We want to apply this technology in different fields, roasting of zinc, gold, copper and pyrite ores as well as sulfation of rare earths and decomposition of sulfate residues. Our further objective is increased use of residues in smelter operation. Such residues contain valuable metals beside problematic impurities.

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