4.5. Waste products of sulphuric acid manufacture. Methods of purification, processing and neutralization of solid waste products, sulphurous anhydride and waste water
Properties of sulfuric acid, methods of its production
Sulfuric acid is one of the basic large-scale products of the chemical industry. It is used in different branches of the national economy, because it possesses a complex of specific properties which simplify its technological application. Sulfuric acid does not smoke, has no odor and color, it is a liquid at normal temperature; a concentrated acid does not cause ferrous metal corrosion. At the same time sulfuric acid is one of strong mineral acids; it forms numerous stable salts. It is the cheapest acid. In technology waterless sulfuric acid as well as its water solutions is named as sulfuric acid. These are systems consisting of sulfur oxide (VI) and water with different composition: nSО3·mН2О. Sulfuric acid monohydrate is a colorless, oily liquid with a crystallization temperature of 10,37°C, boiling point of 296,2°C and density of 1,85 t/m3. Monohydrate of sulfuric acid is mixed with water and sulfur oxide (VI) in any ratios forming hydrates (Н2SО4·Н2О, Н2SО4·2Н2О, Н2SО4·4Н2О) and compounds with sulfur trioxide (Н2SО4·SО3 and Н2SО4·2SО3). These hydrates and compounds with sulfur trioxide have different crystallization temperatures and form several eutectics. Some of these eutectics have crystallization temperature below zero or close to zero. These features of sulfuric acid solutions are taken into account at selection of its market grades, which should have low crystallization temperature in accordance with the conditions of manufacture and storage. Sulfuric acid boiling point also depends on its concentration, i.e. on a composition of a system “sulfur oxide (VI) – water”. With increase in concentration of a sulfuric acid solution its boiling temperature increases and reaches a maximum value of 336,50C at 98,3% concentration, which corresponds to an azeotropic composition, and then it decreases. At increase in free sulfur oxide (VI) content the oleum boiling point decreases from 296,20C (monohydrate boiling point) to 44,70C corresponding to the boiling temperature of 100% sulfur oxide (VI). At heating sulfuric acid vapors above 4000C it is subjected to thermal dissociation according to the reaction:
2Н2SО4 ↔ 2Н2О + 2SО3 ↔ 2Н2О + 2SО2 + О2 ()
Sulfuric acid occupies the first place among mineral acids according to output and consumption. Its world production has grown more than in three times over the past 25 years and now makes more than 160 million tonnes per year. Applications of sulfuric acid and oleum are various. Its significant part is used for mineral fertilizer production (30-60%), and also for manufacturing of dyes (2-16%), chemical fibers (5-15%), and in metallurgical industry (2-3%). It is applied for various technological purposes in other mineral acids, salts and explosive substances production, in organic synthesis, metal pickling, in textile, food and other branches of industry as well as a dehydrating and desiccant agent.
Sulfuric acid can be produced from different kinds of raw materials. These are elemental natural sulfur and sulfur-containing compounds. The sulfur is found in nature most often in the form of metal sulfides and metal sulfates, as well as a part of oil, coal, natural gas and associated gas. Significant amounts of sulfur are contained as sulfur dioxide in flue gases, in waste gases of non-ferrous metal production as well as hydrogen sulfide emitted at cleaning combustible gases. Thus raw material sources for sulfuric acid production are enough varied, although up to now predominantly elemental sulfur and iron pyrite are used as raw materials. Limited application of flue gases of thermal power stations is explained by low concentration of sulfur oxide (VI) in them. At that the iron pyrite share decreases in raw material balance and the sulfur share increases. In a general scheme of sulfuric acid production the first and the second stages – raw material preparation and it’s burning or roasting – are most essential ones. Their contents and instrumentation essentially depend on the raw material nature, which largely determines complexity of the sulfuric acid production.
Several kinds of sulfuric acid are produced in chemical industry. They differ by concentration and impurity content. These are reactive (especially pure), accumulator and technical sulfuric acid. Technical sulfuric acid (SS 2184-77) is produced as a tower acid containing minimum 75% of sulfuric acid and a lot of impurities such as nitrogen oxides, which decrease the product quality; a contact sulfuric acid containing minimum 92,5% of H2SO4 without nitrogen oxides admixtures. Oleum represents a sulfur trioxide solution in waterless sulfuric acid. Mainly oleum containing minimum 20% of free SO3 is produced. These products are not combustible and are rated as a second toxicity class.
Quality of different kinds of sulfuric acid is determined by features of its production; it mainly depends on conditions of sulfur dioxide oxidation. This reaction has very high activation energy, therefore to accelerate the process it is necessary to apply catalysts.
Nitrous sulfuric acid process
Processing sulphur dioxide into sulfuric acid by this method is performed in cylindrical towers with height more than 15 m filled with ceramic rings. Nitrogen oxides are used as a catalyst for SO2 oxidation:
SO2 + N2O3 + H2O = H2SO4 + 2NO ()
where N2O3 is a mixture of NO and NO2 gases.
Diluted sulfuric acid containing nitrosyl-sulfuric acid is prepared according to the reaction:
N2O3 + 2H2SO4 = 2NOOSO3H + H2O ()
Sulphur dioxide oxidation with nitrogen oxides takes place in a solution after its absorption with nitrous vitriol. The latter is hydrolysed:
NOOSO3H + H2O = H2SO4 + HNO2 ()
This acid is sprayed from the tower top towards the gas flow. The sulfur dioxide interacts with water forming sulfurous acid:
SO2 + H2O = H2SO3 ()
Interaction between HNO2 and H2SO3 leads to obtaining sulfuric acid:
2HNO2 + H2SO3 = H2SO4 + 2NO + H2O ()
The NO liberated is converted into N2O3 (as a mixture of NO+NO2) in the oxidation tower. Then the gases are fed in absorption towers, where the nitrous vitriol is formed. Thus nitrogen oxides’ recycling is provided. Unavoidable N2O3 losses with the waste gases are compensated with HNO3 addition. Tower sulfuric acid has low concentration (75% of H2SO4) and contains harmful impurities (for example, As).
Nowadays this production method is not used because of ecological problems concerning NO and NO2 gases emission outside and environmental pollution.
Physicochemical bases and technology of sulphuric acid production by a contact method
A contact method allows obtaining concentrated sulfuric acid and oleum of very high purity. Now the contact method is most applied where sulfur dioxide oxidation takes place on solid catalysts [10, 11]. Thermal resistant vanadium oxide (V) contact mass in the form of granules and rings with low ignition temperature is used as a catalyst. The sulphur dioxide oxidation degree on the catalyst reaches to 99,8%, that allows excluding the necessity of additional waste gas purification.
Chemism of the process includes three basic stages:
1. Sulfur dioxide obtaining.
In dependence on used raw materials the following reactions take place:
Elementary sulfur burning:
S + O2 = SO2 + Q
Iron pyrite roasting:
4FeS2 + 11O2 = 2Fe2O3 + 8SO2 + Q
3FeS2 + 8O2 = Fe3O4 + 6SO2
Zinc blende roasting:
2ZnS + 3O2 = 2ZnO + 2SO2
Hydrogen sulfide burning:
2H2S + 3O2 = 2SO2 + 2H2O
Gypsum thermal treatment:
СаSО4·2Н2О = СаSО4 + 2Н2О
СаSО4 = СаО + SO2 + 0.5О2
2. Sulfur dioxide contact oxidation according to the reaction:
2SO2 + O2 = 2SO3 + Q
3. Absorption of sulfur oxide (VI) by a sulfuric acid solution up to required concentration:
SO3 + H2O = H2SO4
The basic stages of the contact silphuric acid process with using iron pyrite as a raw material are given on fig. 4.7; the waste products formed in the process are specified.
Fig. 4.7 – Basic stages of sulphuric acid production from pyrite (classic scheme)
The basic waste products of manufacture of H2SO4: a pyrite cinder and selenium sludge (at work of system on pyrite), the departing gases, containing SO2 and the fog of H2SO4, and also sour drains.
Ways of recycling of pyrite cinder. The basic mineral, included in pyrite ores, - iron sulfide. Except for it, pyrite ores contain also the compounds of copper as CuFeS2, Cu2S, CuS, zinc as ZnS, arsenic as FeAsS, and also in small amounts of compounds of Se, Co, Ni, Mn, Ag, Au, etc. After flotation of such polymetallic ores a flotation concentrate and a flotation pyrite are received. The flotation pyrite contains from 40 up to 45 % of sulphur and from 35 up to 39 % of iron. The compounds of non-ferrous and precious metals, contained in initial ore, are also in part turned into flotation pyrite. At high-temperature roasting of flotation pyrite (750-8000С) about 700 kg of the pyrite cinder on 1 t of pyrite is formed.
The most rational way of pyrite cinder processing is the low-temperature chloridizing roasting (fig. 4.8), allowing in a complex to extract rare and non-ferrous metals and to process the ferriferous residue on pig-iron, and extraction of one only copper from pyrite cinder is economically justified due to its realization.
Surplus of sodium chloride is used in quality of chloridizing agent. First the pyrite cinder is crushed and sifted. Granules in the size of 4 mm are mixed with sodium chloride (8-20 weight %). Received charge is fired in furnaces at 550-6000С. From departing gases HCl and SO3 are caught in the absorption towers, irrigated by water, with production of hydrochloric and sulfuric acid. The firm deposit, containing chlorides of metals, is processed by a solution of sulfuric acid (acid leaching), thus copper, zinc, cobalt, thallium, cadmium and silver pass in solution. From this solution copper is precipitated together with silver and gold by cementation of it by scrap. Further the cement copper is exposed by remelting, clearing, casting of anodes and electrolytic refining. After separation of copper the Glauber’s salt (Na2SO4 . 10H2O) is precipitated from a solution by vacuum-crystallization, and then by roasting - sodium sulfate.
Mother liquor after vacuum-crystallization contains cobalt, zinc, iron and manganese.
In the beginning the mother liquor is oxidized by chlorine and processed СаСО3; iron is allocated as a iron-gypsum deposit.
Cobalt is allocated by oxidation of its compounds by chlorine with subsequent sedimentation by Са(OH)2. Formation of Co(OH)3 occurs at 500С and рН=4. Together with cobalt also manganese and zinc hydroxides are precipitated. For production of metallic cobalt the received deposit is dissolved in a sulfuric acid with the subsequent re-precipitation of hydroxides, calcinated at 11000С and subjected to reduction smelting.
Extraction of cadmium, thallium and indium is carried out by a amalgam method from solutions before sedimentation of zinc.
At complex processing of pyrite cinders by the low-temperature chloridizing roasting the following degree of extraction of metals (in %) is achieved: iron into cake and agglomerate - 80, sulphur into sodium sulphate - 40, copper - 80, zinc into ZnO - 80, lead - 40, silver - 65, cadmium - 40, cobalt - 50, thallium - 17.
There are other ways of processing of pyrite cinder:
1) High-temperature chloridizing roasting of cinder or the method of sublimation (10000С). In this case chlorides of metals are sublimated during roasting and then caught and dissolved in water with the subsequent separation.
2) Sulphatizing roasting of pyrite raw material with the subsequent hydrometallurgical processing of the cinder.
3) The pyrite cinder can be used as initial raw material for manufacture of mineral pigments (iron minium, mummy, ochre). For preparation of mineral pigments of high quality the pyrite cinder is cleared from compounds of sulphur, which cause corrosion of the equipment, and grinded. Water-soluble compounds of sulphur are taken from the pyrite cinder by water washing, and the full removal of sulphur - by roasting at 800-9000С. The obtaining after roasting product contain 90-95 % of Fe2O3, it is grinded and mixed with filler. In quality of fillers chalk, alabaster, clay are used.
4) The large consumer of the pyrite cinder is the cement industry. The pyrite cinder is used as a ferriferous flux.