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

Chapter 6low boilers leaving the column are gathered with the residual gases from the condensers andpassed through a final H 2 SO 4 scrubber, where most of the HF is recovered. Thereafter residualgases pass through the SiF 4 absorber, where H 2 SiF 6 is produced as an aqueous solution. Inplants without a final H 2 SO 4 scrubber, H 2 SiF 6 acid with a relatively high HF content isproduced.Cold H 2 SO 4 from the final H 2 SO 4 scrubber, which contains dissolved HF, passes to the firstH 2 SO 4 scrubber (pre-purification column), where it is heated in contact with the process gasesof the kiln. Acid from the scrubber is mixed in a vessel with fresh H 2 SO 4 and oleum to yield thefeed H 2 SO 4 for the reaction process.Depending on the quality of the HF after the low boilers distillation and on the end-use, asecond distillation may be required to remove any high boilers, mainly H 2 O and H 2 SO 4 . As allthe HF has to be evaporated, this distillation requires a significant amount of energy, estimatedat 350 kg steam per tonne of HF (0.6 GJ/tonne HF). The corresponding amount of refrigerationenergy required to condense the HF also needs to be taken into account. The liquid HF leavingthe head condenser is pure HF. Part of the HF is mixed with water to produce HF of varyingconcentrations: 85, 73, 60 and 40 %. Mixing is strongly exothermic and sufficient coolingsystems are necessary to avoid HF losses by evaporation. The production of high purityelectronic grade HF out of technical grade HF requires special plants and sophisticatedanalytical techniques.6.2.5 Tail gas treatmentThe quantity of tail gases needing treatment depends on the composition of the fluorspar. Awater and/or caustic absorption treatment of tail gases produces a neutralised aqueous effluentcontaining sulphates and fluorides, e.g. silica impurities in the fluorspar will be contained in theanhydrite or released as SiF 4 in the tail gases. In order to reduce SO 4 2- and F - concentrations,precipitation with calcium compounds and solid/liquid separation techniques are applied. Thisresults in a solid waste (CaSO 4 , CaF 2 ) which may be combined with the main flow of anhydriteco-product. The water absorption of the SiF 4 produces aqueous H 2 SiF 6 , which in some casesmay be sold or used as a feed material for the manufacture of fluorides or silicofluorides. H 2 SiF 6can also be chemically combined to produce CaF 2 and silica.6.2.6 Co-product anhydriteThe process generates anhydrite, CaSO 4 . This can, after further treatment of varying complexityand cross-media effects (i.e. after neutralisation), be further used as a market product in cementindustries or as binders for floor screed. This reduces the amount of waste for disposal [6,German UBA, 2000]. If not marketable, anhydrite is disposed of.6.2.7 Product storage and handlingAnhydrous HF is a liquid, which boils at 19.5 °C. It is possible to keep liquid HF at a lowtemperature, preferably below 15 °C, by cooling or by the installation of condensers in thestorage vent pipelines which condenses the evaporating HF. Liquid HF is normally stored atatmospheric pressure in carbon steel tanks. A thin protective layer of FeF 2 is quickly formedwhich prevents further corrosion. Liquid velocity in the pipelines should be below 1 m/s toavoid erosion of the FeF 2 layer.HF acid with a concentration of at least 70 % is stored in carbon steel tanks. Acid withconcentrations of less than 70 % is stored in lined steel tanks or alternatively in polyethylenetanks.262 Large Volume Inorganic Chemicals – Ammonia, Acids and Fertilisers