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Crude Sodium
sulphate from
evaporation plant
Dissolver
Fine maternal
returned to process
C austic soda
Sulphur dioxide
Sulphuric acid
Classifier
Treatment
tanks
Pure sodium
Sulphate to silos
Cyclone
Filter
Precipitated solids
Exhaust gases
Air
Crystalliser
Dryer
Air
Fuel
C entrifuge
Liquor recycled
to crystalliser
Figure 1.12: Flow scheme of <strong>the</strong> production of sodium sulphate from <strong>the</strong> chromium process
[64, CEFIC-SSPA, 2004]
1.6.2.4 Mannheim furnace process (hydrochloric acid)
As mentioned in Section 1.6.1, direct production processes based on <strong>the</strong> reaction of solid
sodium chloride with sulphuric acid or sulphur dioxide/oxygen, have become less important, as
– apart from <strong>the</strong> natural product – a by-product sodium sulphate from o<strong>the</strong>r processes is readily
available. However, in some locations syn<strong>the</strong>tic sodium sulphate may be by-produced by <strong>the</strong>
Mannheim furnace process jointly with hydrochloric acid [64, CEFIC-SSPA, 2004].
Approximately two tonnes of 100 % Na2SO4 are produced per one tonne of 100 % HCl,
equivalent to approximately three tonnes of HCl solution which is in fact obtained in <strong>the</strong>
process. Hydrochloric acid is widely applicable in various industries, including <strong>the</strong> chemical,
tannery, textile, ceramic and pharmaceutical industries.
In <strong>the</strong> Mannheim furnace process, sulphuric acid reacts with sodium chloride producing sodium
sulphate and a 32 % hydrochloric acid solution obtained through absorption. Combustion gases,
obtained as intermediate products, are used to heat up <strong>the</strong> combustion air.
Producing sodium sulphate, that is free of sulphuric acid and hydrochloric acid, requires
equivalent amounts of acid (approx. 0.73 t of 100 % H2SO4 per tonne of Na2SO4) and salt
(approx. 0.83 t of 100 % NaCl per tonne of Na2SO4). The salt is measured out by means of a
screw, <strong>the</strong> quantity of acid is measured by flow. Based on raw material analyses, <strong>the</strong> quantity
ratio of <strong>the</strong> materials is <strong>the</strong>n controlled by an operator.
The reaction between sulphuric acid and sodium chloride requires a long reaction time and a
high temperature. This is achieved in a muffle furnace with a hearth which is heated with
burners to approx. 600 °C. The air volume/oil ratio is kept constant. To prevent any problem of
exhaust gas after <strong>the</strong> HCl absorption, <strong>the</strong> air is not blown into <strong>the</strong> process.