Recycling of selenium sludge
Contained in iron pyrite impurities of selenium compounds (0,002-0,02 %) at roasting pass in a gas phase as SeO2, which is caught by sulfuric acid in washing bay. Under influence of SO2, contained in roaster gas, selenium dioxide, contained in a solution of sulfuric acid, is reduced up to metal selenium. Metal selenium is adsorbed on a cinder dust and together with sulfuric acid is in part precipitated in washing bay. There it collects in sediment bowls and collections of acid as weak selenium sludge (up to 5 % Se). At the further cleaning of roaster gas from splashes and fog of sulfuric acid in wet electrofilters there is allocation of rich selenium sludge (up to 50 % Se).
The technology of allocation of selenium is simple and identical for weak and rich sludge. The method consists of the following stages: 1) dilution of sludge by water and heating of solution by live steam for reduction of selenium; 2) filtering of a pulp and washing of it by 50 % solution of soda; 3) drying of sludge at 90-1000С; 4) preparation of average sludge, containing 10-12 % of selenium.
Cleaning of departing gases from so2 and a fog of sulphuric acid
On a global scale the share of SO2, removed in an atmosphere with technological gases of sulphuric acid factories, makes only 0,5 % in comparison with total effluents of SO2. Basic quantity of SO2 is removed in an atmosphere by thermal power stations (50 %), transport (20 %), the enterprises of non-ferrous and ferrous metallurgy (25 %), petrochemical factories (3 %).
Known methods of extraction of SO2 from various technological gases can be subdivided on absorption, adsorption and catalysis.
1. Absorption or wet ways are based on absorption of SO2 by solutions of ammoniac salts (NH4)2SO3, sodium sulphite Na2SO3, caustic soda NaOH, soda ash Na2CO3, suspensions of CaO, MgO, ZnO, other solutions. There is
Fig. 4.8 – The principle scheme of cinder processing by chloridizing roasting
chemical interaction of SO2 with a component with the subsequent separation of the formed compound.
2. Adsorption or dry methods consist in extraction of SO2 with the help of firm sorbents, such as the activated coal, carbonates of alkaline metals, the powder manganese dioxide, specially prepared peat.
3. Catalytical methods enable to receive directly sulphuric acid by oxidation of SO2 on various catalysts.
In sulphuric acid production two processes of recycling of SO2 are most widely used: 1) use for manufacture of sulfuric acid of a method of double contact; 2) ammoniac methods of clearing of departing gases from SO2 (fig. 4.9).
Method of double contact. Feature of a method is the intermediate absorption, which leads to increase of the general degree of oxidation of SO2 into SO3 up to 99,5-99,8 % (hence, increases the general degree of use of sulphur-containing raw material) and to respective reduction of concentration of SO2 in departing gases. The high degree of oxidation of sulphurous anhydride into sulfuric anhydride leads to reduction of emissions of SO2 on 90 % in comparison with the classical circuit. Concentration of SO2 in removed gas does not exceed the maximum permissible concentration (0,02-0,03 volumetric %).
Lacks of process: 1) necessity of the additional equipment; 2) the system processes the gases, containing more of 8 volumetric % of SO2.
Ammoniac ways allow to carry out clearing of gases up to 0,1 volumetric % of SO2. Process includes the following stages:
- Absorption of SO2 by ammoniac water;
- Regeneration of ammonia by decomposition the absorption solution of sulphite-hydrosulphite of ammonium.
The basic lack of process is accumulation in system of ammonium sulfate due to absorption of SO3 and a fog of sulfuric acid and oxidation of sulphite- hydrosulphite of ammonium into sulfate by oxygen, contained in departing gases. 30-50 % of SO2 turn to ammonium sulfate that leads to loss of a plenty of sulphur-containing raw material.