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

Chapter 55.4.2 Hemihydrate process (HH)DescriptionThe process operating conditions are set to enable calcium sulphate to be precipitated as thehemihydrate. In this way it is possible to produce 40 – 46 % P 2 O 5 acid directly, enablingvaluable savings to be made in energy usage. Figure 5.4 gives an overview of the hemihydrateprocess.Acid from the hemihydrate process tends to contain much less free sulphate and suspendedsolids and lower levels of aluminium and fluorine than evaporated dihydrate process acids ofthe same strength. Additionally, a satisfactory rate of reaction can be achieved from muchcoarser rock than in the dihydrate process, because of the more severe reaction conditions in thehemihydrate process.CondenserVacuumWaste gasPhosphate rockVenturiscrubberFlashcoolerH 2OAttack tanksAgingtankH 2SO 4H 2OGypsumHemihydrate CaSO . 4½H 2OfiltrationH 3PO 4Weak acid40 – 46 % P 2O 5Figure 5.4: Overview of the hemihydrate process[31, EFMA, 2000]Hemihydrate crystals tend to be small and less well formed than dihydrate crystals sohemihydrate slurries tend to be more difficult to filter than dihydrate slurries, unless crystalform modifiers are used to suppress excessive nucleation. With a good hemihydrate processhowever, there is no need to use modifiers.The amount of wash-water that can be used is restricted by water balance needs. Additionally asa result of the higher P 2 O 5 concentration of the slurry being filtered, the amounts of both solubleand insoluble P 2 O 5 remaining in the filter cake are greater. However, despite these losses thesimplicity of the hemihydrate plant and the absence of silicofluoride and chukhrovite (acrystallised mineral compound) scaling in the hemihydrate filter may compensate for the higherinsoluble P 2 O 5 losses through the hemihydrate cake.Hemihydrate is not a stable form of calcium sulphate so there is a tendency for it to revert togypsum even before the acid has been filtered off. The conditions are even more favourable forrehydration during washing. In a well run hemihydrate plant, there is no conversion in thereactor train. A small quantity of anti-scaling agent may be required in a single-stagehemihydrate plant filter to minimise scaling.228 Large Volume Inorganic Chemicals – Ammonia, Acids and Fertilisers