GELTUNGSBEREICH
2 SODA ASH
2.3 Present consumption and emission levels
2.3.6 Co-products
The manufacturing of soda ash by the Solvay process enables two main co-products (calcium chloride and refined sodium bicarbonate) to be produced.
Calcium chloride can be produced either within an integrated soda ash complex or via other process routes, starting from different raw materials. In turn, the refined sodium bicarbonate process is almost always fully integrated within the soda ash complex and, therefore, it is described below in Section 2.3.6.2.
2.3.6.1 Calcium chloride
Outlet liquor from the distillation unit contains primarily CaCl2 in solution in quantities corresponding to production of sodium carbonate. By treating this liquid to remove suspended solids and dissolved sodium chloride, a pure solution of calcium chloride can be obtained. By successive concentration steps, the solution is concentrated up to around 78 % CaCl2, to produce a white solid hydrated flake or prill (see Section 7.11).
2.3.6.2 Refined sodium bicarbonate 2.3.6.2.1 Background information
Refined sodium bicarbonate is produced in all major regions of the world and the current worldwide capacity is estimated to be around 2.4 million t/year (2002) – see Table 2.18. The predicted growth of the market for the coming years is 3 – 5 % per year.
Production capacity
EU-25 US Central and Eastern Europe
Africa and Middle East
Japan Others Total
kt/year 650 800 210 100 105 500 2365
% 28 34 9 4 4 21 100 %
Table 2.18: Worldwide refined sodium bicarbonate annual capacities (reference year: 2002) [33, CEFIC-ESAPA, 2004]
The capacities and location of production sites in Europe are given in Table 2.19.
Producers Country - location Capacity (kt/year)
Producer A France – Dombasle 120
Germany – Rheinberg 50
Spain – Torrelavega 120
Italy – Rosignano 125
Portugal – Povoa 10
Austria – Ebensee 15
Producer B Bulgaria – Devnya 10
Producer C United Kingdom – Northwich 90
Producer D France – La Madeleine 20
Producer E Germany – Stassfurt 45
Producer F Poland – Inowroclaw 50
Producer G Turkey – Mersin 70
Producer I Romania – Ocna Mures 24
Producer L (1) Czech Rep. – Usti nad Labem 5
Producer J Bosnia – Tusla 15
(1) production not coupled with a soda ash plant
Table 2.19: European refined sodium bicarbonate capacity and producers (reference year: 2002) [33, CEFIC-ESAPA, 2004]
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Anorganische Grundchemikalien – Feststoffe und andere - August 2007 65 Refined sodium bicarbonate has a wide range of uses (see Section 2.1.3). The consumption breakdown in the EU-15 is presented in Table 2.20.
Application Per cent (%) Application Per cent (%)
Animal feed 43 Detergents/Cleaning Agents 7
Personal care/pharmaceuticals 18 Fire Extinguishers 2
Flue-gas treatment 11 Textiles/leather 1
Baking/foodstuffs 10 Others 8
Total 100 %
Table 2.20: Consumption of refined sodium bicarbonate in the EU-15 (reference year: 2002) [33, CEFIC-ESAPA, 2004]
2.3.6.2.2 Process description
The refined sodium bicarbonate is mostly produced from a solution of sodium carbonate that may also contain small amounts of dissolved sodium bicarbonate. This solution can be prepared following two ways. The first consists of dissolving soda ash in water. In the second, the solution is the result of the thermal decomposition of crude sodium bicarbonate from the filtration step of the soda ash process, after being suspended in a soda ash solution (Figure 2.3).
In the latter case, the thermal decomposition reaction is as follows:
2 NaHCO3 Na2CO3 + CO2 + H2O
The CO2 produced by this reaction is totally recovered in the soda ash process by mixing it with the outlet gas from the calcination of the crude sodium bicarbonate (Figure 2.2). The prepared in the sodium carbonate solution is sent to a bicarbonation tower where CO2 is blown until precipitation of sodium bicarbonate occurs. Soda ash dissolution followed by the crystallisation (precipitation) of sodium bicarbonate ensures the purity of the refined NaHCO3 product:
Na2CO3 + CO2 + H2O 2 NaHCO3
If the refined sodium bicarbonate unit is linked to a soda ash plant, the CO2 gas is sourced from the lime kiln gas excess.
The bicarbonate is separated from the mother liquor by filtration/centrifugation. The mother liquor is recycled upstream for the preparation of the sodium carbonate solution. The purge from the recycle loop may be used as a carbonate source (e.g. it may be sent to the brine purification area in a soda ash plant – see Figure 2.2).
The bicarbonate cake from the separation unit (filtration/centrifugation) is dried (e.g. by hot gas), sieved, graded, stored and sold as a bagged or bulk product.
The process diagram for the manufacture of refined sodium bicarbonate is given in Figure 2.3.
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66 August 2007 Anorganische Grundchemikalien – Feststoffe und andere - Figure 2.3: Process flow diagram for the manufacture of refined sodium bicarbonate
[33, CEFIC-ESAPA, 2004]
2.3.6.2.3 Major environmental impact
Gaseous effluents
The gaseous effluent from the bicarbonation columns (GO100) contains some unconverted CO2
due to the equilibrium limit of the reaction. When kiln gas is used to feed the carbonation columns, the CO already present in the gas stream (essentially inert, as in the soda ash process), flows through the bicarbonation step and is dispersed with the vent gas into the atmosphere, where it is converted to CO2 (see Section 2.3.3.2). Typical quantities of CO2 and CO in the gas released after bicarbonation are given in Table 2.21.
Component Quantity (1)
(kg/t sodium bicarbonate)
CO2 130 – 290
CO 3 – 25
(1) figures in this Table are indicative ranges of annual averages based on various measurement or estimation techniques
Table 2.21: Vent gas from bicarbonation columns blown with lime kiln gas [33, CEFIC-ESAPA, 2004]
The gas from the dryers is cleaned before being released (GO101) or recycled, depending upon the drying technique used.
Liquid effluents
If the bicarbonate plant is located near a soda ash plant, any purge from the recycle loop (LO100) is recovered, for instance in the brine purification area, such that the bicarbonate process has no liquid waste water process stream.
(1)
Purge
Storage and bagging
REFINED SODIUM BICARBONATE GRADES
Grading of sodium bicarbonate
Drying of bicarbonate cake
Filtration of bicarbonate slurry
Bicarbonation of sodium carbonate
solution Preparation of sodium carbonate
solution
SODA ASH SODIUM CARBONATE SOLUTION
coming from soda ash plant or
Preparation of the CO2gas
(filtration,…) CO2gas
Drying gas
Preparation of the drying gas
Cleaning of vent gas
Vent gas
Air Gas cleaning
GO100
Mother liquor
L100
Air dedusting
Outlet gas
GO102 (1)
(1) depending upon the drying technique which is used
GO101
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Anorganische Grundchemikalien – Feststoffe und andere - August 2007 67 Integration with the soda ash plant
Refined sodium bicarbonate plants are typically integrated with the soda ash plants (refer to Figure 1.15 and Figure 2.3). The degree of integration in the EU-25 is quite high, with 10 sites currently producing refined sodium bicarbonate out of the total of 14 soda ash producing sites.
However, the size of the refined sodium bicarbonate plants in relation to the soda ash capacity is variable from the smallest representing just 3 to 4 % of the total plant output compared with the largest accounting for 17 %.
Approximately 0.7 tonne of raw soda ash and up to 1.1 tonnes of 40 % CO2 gas, depending on the CO2 gas concentration, CO2 conversion rate due to the equilibrium of the reaction and plant configuration [33, CEFIC-ESAPA, 2004], are used for the production of 1 tonne of refined sodium bicarbonate. It is assumed that, on average, some 550 kg 100 % CO2 is used for the production of one tonne of NaHCO3, of that 260 kg CO2 is captured by the product and 290 kg CO2 is released to the atmosphere.