Chlorine : Weldon Process

Chlorine was formerly made for producing bleaching liquor, from salt, manganese dioxide and sulphuric acid in stoneware jars heated in a water-bath. In 1836 Gossage began to condense the hydrochloric acid evolved in the decomposition of salt with sulphuric acid, in absorption towers. Hydrochloric acid became cheap and was used as a source of chlorine by heating it with manganese dioxide.

The operation is carried out, on a small scale, in chlorine stills, made of flagstones bound together, with a grid A on which lumps of pyrolusite rest.

Fig: Chlorine still

Hydrochloric acid is run in through the pipe B, with a liquid seal below, and the still is heated by admitting steam cautiously from a stoneware column, C. Chlorine is evolved through the pipe D, and deposits moisture in the pot shown. The residual liquid in the still contains manganous chloride, ferric chloride (from impurity in the pyrolusite) and undecomposed hydrochloric acid.

In 1837 Gossage attempted to recover the manganese from this liquor, by precipitating it with the theoretical amount of lime: MnCl₂ + Ca(OH)₂ = CaCl₂ + Mn(OH)₂. By blowing air through the manganous hydroxide, he hoped to convert it into manganese dioxide, which could be used again: 2Mn(OH)₂ + O₂ = 2MnO₂ + 2H₂O. The precipitation and oxidation were incomplete; Volhard showed that this was partly due to the acidic character of manganese dioxide, which combined with the basic manganous oxide to form MnO,MnO₂, or Mn2O₃, which is very stable. In 1866 Walter Weldon, working at Gamble's alkali works at St. Helens, found that if the precipitation of the manganese liquor is carried out in presence of 30-40 per cent. excess of lime, then on blowing air through the mixture the manganous oxide is completely precipitated and is largely oxidised to the dioxide, the latter combining with the lime, a stronger base than MnO.

The acid liquor from the stills is neutralised with limestone and the ferric hydroxide precipitated is allowed to settle out The liquor is then pumped into an oxidiser, consisting of a large cylindrical iron tank where it is treated with the requisite excess of milk of lime, is heated to 60° by steam, and a powerful blast of air is then forced through it. The compound CaO,MnO₂, or calcium manganite, is precipitated. More still-liquor is run in, and the blowing continued, when some of the compound CaO,2MnO₂ is formed. The suspension is then run into settling-tanks, where a thin black mud, called Weldon mud, settles out. The clear liquor containing calcium chloride is drawn off; the mud (calcium manganite) runs down into chlorine stills, where it is treated with hydrochloric acid and steam, producing chlorine and manganese liquor. The latter goes through the Weldon process repeatedly, but fresh manganese dioxide must be added to replace losses. The complete reaction in the Weldon process has been given as follows:

124MnCl₂ + 160CaO + 49O₂ = 36(CaO,2MnO₂) + 26(MnO,MnO₂) + 124CaCl₂.

Protein crystallography
Chemistry guide
Chlorine
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