4. Purification, neutralization and recycling industrial waste products in technology of inorganic substances

4.1. Environmental problems of soda ash production

Soda ash is a cheap large-tonnage product, which is wide used in the national economy. It is produced from non-deficient raw material. About 30 % of the soda ash produced is applied in the chemical industry for manufacture of sulfates and fluorides, phosphoric fertilizers, synthetic detergents, household chemical goods, chromic and baric salts, fiber glass and foam plastics.

There is natural and artificial soda. Sources of natural soda are insignificant; they are mainly situated in the USA. Artificial soda ash Na₂CO₃ is generally produced by an ammonia method (Solve method). Raw materials are sodium chloride as a water solution (brine) and carbonate raw material – chalk or limestone. A reaction occurs at the presence of ammonia:

NaCl + NH₃ + CO₂ + H₂O = NaHCO₃ + NH₄Cl (1)

Sodium bicarbonate as a weak soluble salt drops out in a sediment, and ammonium chloride remains in a solution.

The process of soda ash production includes the following stages (fig.4.1.):

1) Purification of crude brine from ions Ca²⁺ and Mg²⁺;

2) Preparation of limy milk;

3) Preparation of ammoniated brine (absorption);

4) Carbonization of the ammoniated brine;

5) Filtering a sodium bicarbonate suspension; crude sodium bicarbonate is fed on calcination, and a mother liqueur (called also a filter liquid) is pumped on distillation;

6) Calcination of NaHCO₃ (thermal decomposition with formation of a ready product – soda ash, and also carbon dioxide and water steam):

2 NaHCO₃ = Na₂CO₃ + CO₂ + H₂O (2)

7) Distillation of the filter liquid with the purpose of ammonia and carbon dioxide regeneration; ammonium carbonates and bicarbonates are decomposed at heating, and for decomposition of ammonium chloride the liquid is processed with a limy milk solution and fed in a distiller for distillation of ammonia with sharp steam and returning it in the cycle:

2NH₄Cl + Ca (OH)₂ = 2NH₃ + CaCl₂ + 2H₂O (3)

Fig. 4.1 – A schematic diagram of soda ash production by an ammonia method

At the ammonia regeneration a distiller liquid is formed. This liquid represents a suspension of insoluble products (such as CaCO₃, CaSO₄, Ca(OH)₂, sand) in a solution of calcium and sodium chlorides. For burial this liquid is pumped in sludge settling tanks (so-called “white seas”).

Besides the distiller liquid waste products of soda ash manufacture are:

- a sludge formed at the brine purification, which contains CaCO₃ and Mg(OH)₂;

- waste gases of carbonization and washing stages;

- air of filters, containing ammonia;

- a waste gas of roasting-limy furnaces, containing carbon oxide;

- limy fines (less 40 mm) – a by-product formed during the CaCO₃ preparation for roasting, unsuitable for use in roasting furnaces.

Reduction ways of the waste amount:

- Putting into operation of recycling water supply systems (use of treated waste water for NaCl leaching);

- replacement of a limy milk suspension on dry lime that leads to reduction of the distiller liquid volume on 16-18 %;

- application of air cooling instead of water cooling;

- increase of NaCl efficiency;

- replacement the steam used at the distillation on the steam formed at the evaporation of the distiller liquid;

- pulsating air feed in a roasting zone. In the process the after-burning degree of carbon monoxide CO increases, that leads to the reduction of its concentration in the waste gas on 40-60 %.

Ways of recycling the distiller liquid. The distiller liquid can be used for recovery of CaCl₂ and NaCl. The table salt obtained can be applied as a dietary salt or returned in the manufacture. Sale of calcium chloride is a more serious problem failing a heavy consumer of this product. Calcium chloride can be used in the chemical industry as a drier of gases and liquids, as an additive to concrete, for melioration of saline soils, etc.

The distiller liquid is clarified, purified from sulphate-ions and evaporated. At first NaCl is precipitated. It is separated, washed and wrung out on centrifuges. The remained 32-38 % СаCl₂ solution in tanks is dispatched to a consumer. If a consumer needs a solid calcium chloride, the solution is in addition evaporated; again the NaСl precipitated is filtered, and a melt formed is cooled and granulated. The product represents calcium chloride dihydrate containing 76-80 % of CaCl₂.

It is necessary to note, that CaCl₂ has the limited application, and the evaporation requires a plenty of energy.

Sludge from the brine sediment bowls is diluted with water up to a ratio of 1:6 and pumped by a pump in stores (“white seas”) intended for cleaning the waste liquid from suspended impurities. The clarified liquid is discharged in reservoirs or fed on subsequent processing.

The filter liquid can be applied for manufacture of technical NH₄Cl and NaCl ("Extra" dietary salt). Use of this manufacturing scheme allows completely eliminate the waste water formation; it is economic, however it demands an additional amount of ammonia.

The filter liquid composition, g/l: chlorides - 160-165

ammonia - up to 20

ammonium salts - 50-55

suspended substances - up to 10

tiosulphates - 0,8-2,0

sulfides - 0,01-0,1.

The technical ammonium chloride production process includes the following stages:

1) Saturation of the filter liquid with ammonia;

2) Saturation of the filter liquid with table salt;

3) Clarification of the solution formed;

4) Cooling the solution and crystallization of ammonium chloride;

5) Separation of ammonium chloride crystals on a centrifuge and their washing;

6) Drying and packing of ammonium chloride.

Waste products formed in this process (a mother liqueur, washing water and sludge formed at the salt dissolution) are again fed in a soda ash production shop for recycling by means of distillation.