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

Chapter 5Cross-media effectsTable 5.11 shows an example for emission levels and co-generated output and Table 5.12typical consumption levels for the production of H 3 PO 4 by the thermal process. The mainsources of emissions and waste in the production of phosphorus and phosphoric acid fromelemental phosphorus are:1. Sintering of the phosphate rock pellets and drying of cokes in the sinter furnace.The off-gas from the sinter furnace contains a wide range of pollutants such as dust,fluoride, phosphate, heavy metals, radio nuclides and SO 2 and NO x . The off-gas from theovens is cleaned in two-stage scrubber systems with closed water circuits, before beingemitted to the air. To prevent accumulation, pollutants are removed from the recirculatingwash-water stream through neutralisation followed by flocculation and separation of thesolids. The solids obtained are returned to the process either via the slurry station or via thephosphate rock (after drying).2. Calcination of Cottrell dust, flaring of fuel gas and drainage of liquid slag from thephosphorus furnace.The gas phase produced in the phosphorus furnace contains a considerable amount of dust(so-called Cottrell dust) which is removed through an electrofilter. As a result of closedloops (re-use of waste streams) in the process, the dust is enriched in heavy metals (mainlyzinc) and radionuclides (such as 210-Po and 210-Pb). The dust is mixed with water andrecycled to the slurry station. However, due to the high zinc content in the dust, part of it isremoved to prevent excessive accumulation. The dust is calcined (emission of dust, F andP 2 O 5 to the air) and stored. In the near future, storage will by replaced to a special storagefacility for all kinds of radioactive waste.After the recovery of phosphorus from the gas, the remaining gas mainly consists of carbonmonoxide. This gas is used as fuel gas on the site (e.g. in the sinter furnaces) and is sold tothe nearby power plant. The leftover gas, if any, is flared, which contributes to SO 2 and NO xemissions to the air.Vapours released while draining of the liquid slags from the phosphorus furnace, areremoved, and washed with water in a venturi scrubber before being emitted to the air.Process effluents that have been in contact with phosphorus are sent to the waste waterstation.After treatment (sedimentation followed by neutralisation, flocculation and separation of thesolids formed), 70 to 90 % of the water is recycled to the process. The remaining water istreated with lime for further P 2 O 5 removal and is subsequently treated in an biological wastewater treatment plant before being discharged to the sea. All solids are recycled to theprocess.3. Oxidation of phosphorus in the acid plant and removal of arsenic from the acid.The off-gas from the acid towers is contaminated with traces of P 2 O 5 and phosphoric acid.To minimise emissions, the off-gas is cooled and washed with recirculating acid and water,and subsequently treated in a venturi scrubber (diluted acid) and a demister. The bleed ofthe recovery system is re-used either in the wet phosphoric acid purification plant or theslurry station. The emissions of the acid plant are small compared to the emissions of thesinter plant and the phosphorus plant.Due to the specific fields of application (e.g. food additive and beverages),, traces of arsenicpresent in the phosphoric acid have to be removed. To this end, sodium hydrogen sulphide(NaHS) is added to the acid, upon which arsenic is precipitated as arsenic sulphide (As 2 S 3 ).After separation and further processing, the latter is obtained in concentrated form andstored as hazardous chemical waste.252 Large Volume Inorganic Chemicals – Ammonia, Acids and Fertilisers