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Ammonium Sulphate – A By-product of Coal Coking Process


Ammonium Sulphate – A By-product of Coal Coking Process

Ammonium sulphate is produced as a by-product during removal of ammonia (NH3) from the raw coke oven gas generated during the coking of the metallurgical coal in by-product coke ovens. Its industrial production began over a century ago, as by-product in gas cleaning in coke and coal gasification plants. Ammo­nium sulphate is produced as crystals. It is an organic, white odorless solid and crystalline salt with a number of commercial uses. It contains around 21% nitrogen (N2) and 24% sulphur (S).

The Chemical Abstracts Service (CAS) registry number for ammonium sulphate is 7783–20–2. Its EC number is 231-984-1. Its IUPAC ID is Diazanium sulfate. It is currently classifiable under Harmonized Tariff Schedule of the United States (HTSUS) subheading 3102.21.0000.

This process by which ammonium sulphate is produced consists of absorption of ammonia in the coke oven gas in a solution of ammonium sulphate and sulphuric acid. The absorption reaction is 2NH3 + H2SO4 = (NH4)2SO4. The ammonium sulphate produced by the reaction of NH3 with H2SO4 is recovered by crystallization. The crystals are then centrifuged, washed and dried.

Since ammonium sulphate is produced as a by-product during the recovery of ammonia from coke oven gas, there are normally a lot of impurities from different sources. These are organic impurities as well as the inorganic impurities. The combined effects of these impurities are complex and influence the purity and the crystal shape and size.

Ammonium sulphate is available in a variety of grades, including granular and standard grades. Granular grade accounts for the vast majority of the market.  Its relatively large particle size (typically 2.5 mm) makes it well suited for mixing with other fertilizers and application by spreading machines.  Standard grade has a smaller particle size (less than 2 mm) and is well suited to applications which do not require mechanical spreading, including in plantations or lower technology farms.

Ammonium sulphate was once the leading form of nitrogen (N2) fertilizer, but it now supplies a relatively small percentage of the world total N2 fertilizer because of the rapid growth in the use of urea, and ammonium nitrate. The main advantages of ammonium sulphate are its low hygroscopicity, good physical properties (when properly prepared), chemical stability and good agronomic efficiency. Its reaction in the soil is strongly acid forming, which is an advantage on alkaline soils and for some crops such as tea, while in some other situations its acid forming character is of disadvantage. Its main disadvantages is its lower analysis (around 21 % N2), which increases packaging, storage and transportation costs. As a result, the delivered cost at the farm level is usually higher per unit of N2 than that of urea or ammonium nitrate. However, in some cases, ammonium sulphate may be the most economic source of N2 when the transportation at low cost, or when a credit can be taken for its content. Hence, the production of ammonium sulphate is normally controlled primarily by the needs of the industrial process, rather than by fertilizer market demands.

Properties of ammonium sulphate

Fertilizer grade ammonium sulphate specifications normally indicate a minimal of N2 content, which is usually not lower than 20.5 %. Usually limitations on free acidity and free moisture are also demanded. Typical figures for free acidity and free moisture contents are 0.05 % maximum for free sulphuric acid and 0.2 % maximum for free moisture. Occasionally, maximum values for certain organic or inorganic impurities may also be specified for by-product ammonium sulphate material. Typical properties of ammonium sulphate are given in Tab 1.

Tab 1 Properties of ammonium sulphate
Sl.no. Property Unit Value
1 Formula (Fig 1) (NH4)2SO4
2 Molar mass g/mol 132.14
3 Appearance White hygroscopic crystal
4 Nitrogen content % 21.2
5 Sulphur content % 24
6 Colour White
7 Density of solid g/cc 1.769 at 20 deg C
8 Specific gravity of saturated solutions 1.2414 at 20 deg C
1.2502 at 93 deg C
9 Loose bulk density kg/cum 962
10 Angle of repose degrees 28
11 Specific heat of solid cal/g-deg C 0.345 at 91 deg C
12 Specific heat of saturated solutions cal/g-deg C 0.67 at 20 deg C
0.63 at 100 deg C
13 Heat of crystallization kcal/kg 11.6 from 42 % solution
14 Heat of dilution kcal/kg 6.35 from 42 % solution to 1.8 % solution
15 Melting point deg C 512.2
16 Thermal stability Decomposes above 280 deg C
17 pH 5-6
18 Solubility in water at 0 deg C g/100 g 70.6
19 Solubility in water at 20 deg C g/100 g 74.4
20 Solubility in water at 100 deg C g/100 g 103.8
21 Critical relative humidity at 20 deg C % 81.1
22 Critical relative humidity at 30 deg C % 81
23 Magnetic susceptibility cc/mol -0.000067

Fig 1 Formula for ammonium sulphate

Ammonium sulphate decomposes upon heating above 250 deg °C, first forming ammonium bi-sulphate. Ammonium sulphate when heated at higher temperatures results into its decomposition into ammonia, N2, sulphur dioxide, and water. As a salt of a strong acid (sulphuric acid) and weak base (ammonia), its solution is acidic with a pH value ranging from 5 to 6. In aqueous solution the reactions are those of NH4+ and SO4-2 ions. For example, addition of barium chloride precipitates out barium sulphate. The filtrate on evaporation yields ammonium chloride.

Ammonium sulphate forms many double salts (ammonium metal sulphates) when its solution is mixed with equi-molar solutions of metal sulphates and the solution is slowly evaporated. With trivalent metal ions, alums such as ferric ammonium sulphate are formed. Double metal sulphates include ammonium cobaltous sulphate, ferrous diammonium sulphate, ammonium nickel sulphate which are known as Tutton’s salts and ammonium ceric sulphate. Anhydrous double sulphates of ammonium also occur in the Langbeinites family.

Ammonium sulfate becomes ferroelectric at temperatures below -49.5 deg C. At room temperature, it crystallizes in the orthorhombic system, with cell sizes of a = 7.729 angstroms, b = 10.560 angstroms, c = 5.951 angstroms.

Important aspects of ammonium sulphate

Several factors contribute to trouble free storage of ammonium sulphate. First, the product is to be of uniform crystal size and is to contain a low percentage of fines. It is to be dry and preferably have below 0.2 % of free moisture and practically with no free acidity. It is to be cooled with dry air under controlled condition, particularly when the ambient temperature and humidity are sufficient high to cause subsequent moisture condensation after cooling in a bulk storage pile or in sealed bags. Ammonium sulphate is usually dispatched only in polyethylene or paper bags.

The price of ammonium sulphate normally varies with the various types of product quality available. The largest disparity is in connection to particle size where the price differential between the price of less than 1 mm crystals and that of granular (2 mm to 3 mm) crystals is quite substantial. This price differential usually is a strong incentive to produce large crystals.

Consumer applications of ammonium sulphate are such that consumer exposure is typically infrequent, for brief periods and to small quantities or low concentrations. Ammonium Sulphate is not known to cause adverse human health or environmental effects and it is not classified as a dangerous product. The usual general precautionary measures to prevent worker and environmental exposure to chemicals are to be taken when handling ammonium sulphate in industrial uses. No other specific risk management measures are required.

Ammonium sulphate is not classified as a hazardous substance. Its acute toxicity is low via all exposure routes. Studies have not shown any long term effects. Ammonium Sulphate has neither irritant nor sensitizing properties. If it is taken up in the body via ingestion or inhalation, then it dissociates in the ammonium and sulphate ions (NH4+, SO4 2-). Ammonium is an endogenous substance that serves a major role in the maintenance of the acid-base balance. Sulphate is a normal intermediate in the metabolism of endogenous sulphur compounds and is excreted unchanged or in conjugated form in urine. Inhalation of ammonium in dust form should be avoided.

Uses of ammonium sulphate

Ammonium sulphate is used in a variety of applications. It is used as a pH corrective agent and as chemical intermediate in a range of chemical manufacturing processes. It finds also application in insecticides, herbicides and fungicides, as an ingredient in pharmaceutical and cosmetic products, as well as in flame retardants and as laboratory chemicals. Although the substance may be contained in certain products for professional or consumer use, ammonium sulphate is mainly applied in pure or more concentrated form in products for industrial use.

Ammonium sulphate is a solid, crystalline salt, primarily used as a fertilizer.  Like other fertilizers, ammonium sulphate is a source of nitrogen for soil, but unlike other nitrogenous fertilizers, it is also a source of sulphur, used by plants to make amino acids and chlorophyll.

Ammonium sulphate is a complex fertilizer. Ammonium sulphate was one of the first and most widely used N2 fertilizers for crop production. It is now less commonly used, since it contains only 21 % N2 and there are other fertilizer sources which are more concentrated and economical to handle and transport. The main reason for this is the relatively low N2 content of ammonium sulphate, as compared to that of for instance urea (45 % N2). However, ammonium sulphate is an excellent source for two nutrients namely N2 and S. Hence, it is used primarily where there is a need for supplemental N2 and S to meet the nutritional requirement of growing plants. S has numerous essential functions in plants which include protein synthesis. Therefore, ammonium sulphate is a valuable fertilizer where both N2 and S are required.

In ammonium sulphate, N2 fraction is present in the ammonium form, and because of it, ammonium sulphate is frequently used in flooded soils for rice production, where nitrate-based fertilizers are a poor choice due to de-nitrification losses.

A solution containing dissolved ammonium sulphate is often added to post-emergence herbicide sprays to improve their effectiveness at weed control. This practice of increasing herbicide efficacy with ammonium sulphate is particularly effective when the water supply contains significant concentrations of calcium, magnesium, or sodium. A high-purity grade of ammonium sulphate is often used for this purpose to avoid plugging spray nozzles.

High solubility of ammonium sulphate provides versatility for a number of agricultural applications. After addition to soil, the ammonium sulphate rapidly dissolves into its ammonium and sulphate components. If it remains on the soil surface, the ammonium may be susceptible to gaseous loss in alkaline conditions. In these situations, incorporation of the material into the soil as soon as possible, or application before an irrigation event or a predicted rainfall, is advisable. Most plants are able to utilize both ammonium and nitrate forms of N2 for growth. In warm soils, microbes rapidly begin to convert ammonium to nitrate in the process of nitrification [(2(NH4+) + 3O2 =2(NO3-) + 2H2O + 4(H+)]]. During this microbial reaction, acidity (H+) is released, which ultimately decreases soil pH after repeated use. Ammonium sulphate has an acidifying effect on soil due to the nitrification process, not from the presence of sulphate, which has a negligible effect on pH. The acid-producing potential of ammonium sulphate is greater than the same N2 application from ammonium nitrate, for example, since all the N2 in ammonium sulphate is get to converted to nitrate, while only half of the N2 from ammonium nitrate is converted to nitrate.

In case of non-agricultural uses ammonium sulphate, it is generally added to bread products as a dough conditioner. It is also a component in fire extinguisher powder and flame-proofing agents. It is used for many applications in the chemical, wood pulp, textile, and pharmaceutical industries.

In case of laboratory uses, ammonium sulphate precipitation is a common method for protein purification by precipitation. As the ionic strength of a solution increases, the solubility of proteins in that solution decreases. Ammonium sulphate is extremely soluble in water due to its ionic nature, therefore it can ‘salt out’ proteins by precipitation.

In the analysis of rubber lattices, volatile fatty acids are analyzed by precipitating rubber with a 35 % ammonium sulphate solution, which leaves a clear liquid from which volatile fatty acids are regenerated with sulphuric acid and then distilled with steam. Selective precipitation with ammonium sulphate, opposite to the usual precipitation technique which uses acetic acid, does not interfere with the determination of volatile fatty acids.

As a food additive, ammonium sulphate is considered generally recognized as safe (GRAS) by the U.S. Food and Drug Administration and in the European Union it is designated by the E number E517. It is used as an acidity regulator in flours and breads. There are some other uses of ammonium sulphate which are described below.

Ammonium sulphate is used on a small scale in the preparation of other ammonium salts, especially ammonium per sulphate. A saturated solution of ammonium sulphate in heavy water (D2O) is used as an external standard in sulphur in NMR spectroscopy with shift value of 0 ppm. Ammonium sulphate has also been used in flame retardant compositions acting much like diammonium phosphate. As a flame retardant, it increases the combustion temperature of the material, decreases maximum weight loss rates, and causes an increase in the production of residue or char. Its flame retardant efficacy can be enhanced by blending it with ammonium sulphamate. It has been used in aerial firefighting. Ammonium sulphate has been used as a wood preservative, but due to its hygroscopic nature, this use has been largely discontinued because of associated problems with metal fastener corrosion, dimensional instability, and finish failures.

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