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

Chapter 5The operating conditions are generally selected so that the calcium sulphate will be precipitatedas the di or hemihydrate form, i.e. 26 – 32 % P 2 O 5 at 70 – 80 °C for dihydrate precipitation and40 – 52 % P 2 O 5 at 90 – 110 °C for hemihydrate precipitation. Circulation of the reactor contentsprovides the necessary mixing. The reaction system consists of a series of separate agitatedreactors. The multiple vessel reaction system is replaced by a single tank in some processes foreconomy of materials and space. Some of these single tanks may be divided into compartments,effectively creating separate reactors. The slurry temperature is controlled by the use of a flashcooler. This also de-gases the slurry making pumping easier. The temperature may also becontrolled, for instance, by using an air circulation cooler.The filtration stage separates the phosphoric acid from the calcium sulphate. About four to fivetonnes of gypsum are generated for every tonne (P 2 O 5 equivalent) of acid produced. The mostcommon types of filtration equipment are the tilting pan, rotary table or travelling belt.The filter medium is moved in sequence through the various stages for continuous operation.The initial separation is followed by at least two-stages of washing, which ensures that all thesoluble P 2 O 5 is recovered. It is only possible to achieve the required separation rate if thefiltration is either pressure or vacuum assisted. In practice, vacuum is invariably used. At theend of the washing sequence, the remaining liquid is taken out of the filter cake. The cake isthen discharged and the cloth efficiently washed to avoid the filter blinding. The vacuum isreleased during the discharge of the cake. It is also beneficial at this point to blow air through inthe reverse direction to assist in the solids dislodgement.The filtrate and washings are then separated. They then have to be further treated under vacuumconditions to de-aerate the product so that it can be transported under atmospheric pressure, as aproduct or for recycling to the process. The pressure difference is usually maintained by diplegsinto barometric tanks situated at a level below the separators to be adequate for thehydrostatic head to maintain a balanced vacuum.Phosphate rockH 2OOptional:GrindingReaction andcrystallisationwith temperaturecontrolH 2OFiltrationOptional:Re-crystallisationRepulpingGypsumCaSO . 4nH 2OH 2SO 4Weak acidOptional:EvaporationFigure 5.1: Overview of the production of H 3 PO 4 (wet process with H 2 SO 4 )This figure is based on [29, RIZA, 2000, 31, EFMA, 2000].Phosphoric acidH 3PO 4Large Volume Inorganic Chemicals – Ammonia, Acids and Fertilisers 215