Large Volume Inorganic Chemicals - Ammonia ... - ammk-rks.net

Chapter 44.2.3.3 Pyrite roastingFluidised bed roasters are the preferred equipment for pyrite roasting. They are superior to othertypes of equipment in terms of process technology, throughput rates and economy. Whenroasting pyrite to get SO 2 gas, two by-products, iron oxide and energy, are produced. One tonneof acid requires 0.5 tonne pyrite.Due to the heterogeneous character of the raw material pyrite, the SO 2 content in the gases isslightly variable over time (generally 6 – 14 %, O 2 free). The gases are always treated in three tofour cleaning steps with cyclones, bag filters, scrubbers and electrostatic precipitators. Wastewater from the scrubbing is treated before discharge. The clean gas is diluted with air to 6 –10 % and dried before entering the conversion process.4.2.3.4 Spent acid from TiO 2 production and roasting of metal sulphatesSpent acid from TiO 2 production is reconcentrated by applying vacuum and heating, using theheat from the subsequent H 2 SO 4 plant. While the concentrated H 2 SO 4 is recycled to TiO 2production, the precipitated metal sulphates are decomposed.The decomposition of sulphates, e.g. iron sulphate, is carried out in multiple-hearth furnaces,rotary kiln or fluidised bed furnaces at over 700 °C with the addition of elemental sulphur,pyrite, coke, plastic, tar, lignite, hard coal or oil as the fuel compensator. The SO 2 content of thegases obtained is dependent on the type of fuel; after cleaning and drying, the SO 2 content willbe approximately 6 %. The variability of the SO 2 over a period of time is high.The heptahydrate is dehydrated at 130 – 200 °C by flue-gases in spray driers or fluidised beddriers to a monohydrate or mixed hydrate during the first step. In the second step, the material isdecomposed at approximately 900 °C. Gases from this process contain approximately 7 % v/vSO 2 . Today it is common practice for ferrous sulphate to be decomposed in a fluidised bedpyrite roasting furnace at 850 °C or more. Elemental sulphur, coal or fuel oil may be used assupplementary fuels. The gas containing SO 2 leaving the furnace is cooled in a waste heat boilerto approximately 350 – 400 °C and is then passed to the gas cleaning system. The cleaned gasesare fed to the sulphuric acid plant.A mixture of metallic or ammonium sulphates and eventually sulphuric acid resulting from theconcentration of acidic wastes of titanium oxide production or from organic sulphonations canalso be processed in a fluidised bed reactor or a furnace. In individual cases, iron sulphate isalso decomposed in multiple-hearth furnaces with flue-gases from fuel oil or natural gascombustion.Large Volume Inorganic Chemicals – Ammonia, Acids and Fertilisers 157