The soda and salt production factory in 1921
(P A Wagner)
  |
|
|
Salt And Soda Mining |
|
| For thousands of years the Tswaing crater has been a source of salt for human and animal consumption. It was widely known among the black communities north of the Vaal River, who collected salt here until the early 20th century. During the dry winter months and in years of drought, when most of the crater floor was covered with crystallised salt deposits, the salt could be obtained by simply collecting these deposits. In seasons when the crater floor was covered with a shallow lake, a different technique was used. Lake water was first filtered to remove algae and other impurities by pouring it into a suspended cowskin basin filled with fine sand and collecting it in purified form below. The purified water was then boiled in a large clay pot until it evaporated, leaving only crystallised salt deposits. A drawback of this technique was that the porous clay walls of the pots rapidly absorbed salt crystals, which eventually caused the pots to crack and disintegrate. Hundreds of such pot fragments have been found on a level spot west of the crater floor. Up to the end of the 19th century, Boer settlers also used Tswaing as a source of salt. Salt was obtained from shallow evaporation ponds and then purified by mixing it with water and boiling the liquid in iron pans until the salt recrystallised. To facilitate access to the crater floor, a road suitable for ox-wagons was constructed, which is still in use today. The right to collect salt was leased by the government of the day in succession to various individuals and small companies who, in turn, controlled the exploitation of salt by farmers and other people. The fact that the crater also contained valuable deposits of natural soda, and was actually a soda and not a salt pan, was discovered accidentally when the first deep borehole was sunk in 1896 under the supervision of Dr F H Hatch. The systematic exploitation of these deposits began in 1912 by a company known as South African Alkali Limited. It took ten years of expensive trial and error operations to perfect a technique for extracting the soda from the crater deposits. Production began in earnest in 1922 and ran until 1956. The best years were the 1930s and 1940s, when the annual production of soda ash was between 2 000 and 3 000 metric tonnes. More or less the same amount was sold annually, worth £30 000-£40 000. Production took place at a factory south of the crater, of which only the ruins remain. Soda-salt liquid (brine) was pumped from a number of boreholes sunk into the crater floor to a collecting reservoir at a pumping station on the crater floor. From here the brine was pumped through a pipe uphill to a storage reservoir, holding 180 000 litres, situated on the southwestern crater rim. From here the brine flowed downhill through a pipe to open concrete storage tanks at the factory. Next, the brine from these tanks was pumped into a three-stage evaporating machine to remove some of the moisture. The brine from the evaporator was then cooled to air temperature in a cooling tower, and after that pre-cooled to 15 degrees C. The pre-cooled brine was then brought to large rectangular tanks at the factory, made of concrete and insulated with cork, each with a capacity of 27 000 litres. Here the brine was cooled by means of a number of horizontal pipes filled with ammonia. By cooling the brine to a temperature of -10 degrees C, decahydrated sodium carbonate, or washing soda, crystallised from the brine and adhered to the cooling pipes, leaving behind sodium chloride liquid. The soda deposits were periodically removed from the pipes by mechanical scrapers. Ever half-hour a sludge, a mixture of the scraped-off decahydrated sodium carbonate and soda-salt liquid, was drained from each refrigeration tank and pumped to a series of centrifugal separators. These machines separated the washing soda from the rest of the liquid, which went to another part of the factory to be turned into salt. The washing soda crystals were bleached, melted and vacuum-boiled, so that the decahydrate lost 90% of its water, thereby producing a new mixture containing monohydrated sodium carbonate. After further separation, this mixture was dried in a gas-fired furnace into a white powder, containing 97%-98% soda ash and 1,2% salt. The soda ash was packed into bags and transported to the nearest railway station at Hammanskraal, about 22 kilometres away. The waste liquid was pumped from the salt factory through a cutting in the crater rim (Mauss' Cutting) back into the crater. Salt was only produced as a by-product. After the separation of washing soda, the remaining liquid mixture, which was still ice-cold after leaving the refrigeration tanks, flowed to a large number of shallow cement ponds next to the factory. Here the liquid warmed up to air temperature. It was then pumped to a salt factory, where it was filtered to remove impurities. Next, salt was obtained through a process of evaporation and drying. The salt from Tswaing was pure enough to be used for the salting of butter and curing of meat, but not sufficiently pure for use as table salt. South African Alkali Ltd employed between 90 and 130 workers annually during the 1930s and 1940s. Most of them came from Zimbabwe, Zambia and Malawi. They lived in company-built brick houses east of the factory, of which the foundations are still visible. White staff lived in large brick houses along the southern crater rim. All these were demolished in the early 1970s. A school was provided for the education of the children of workers. The closest post-office was known as Silberman's Post at the present Zoutpan Trading Store and functioned during 1923-1963. Both the soda and salt extraction processes were complicated and expensive, involving many machines, a lot of energy (steam, electricity, gas) and a large number of workers. Increasing production costs, competition from cheaper imported soda ash after 1945 and the gradual decrease in the soda and salt content of the brine pumped from the crater floor eventually forced the company out of business. In 1950 operations were temporarily suspended, mainly to find out whether a period of rest, during which no waste liquid would be pumped back into the crater, would help to increase the soda and salt content of the brine. Production resumed in 1954 but only continued for another two years until 1956, when mining at Tswaing finally came to an end. The content of soda and salt in the brine had been reduced through years of mining to such a level that the production of soda ash and salt was no longer profitable. The machinery was removed from the factory and over the years its buildings have become a picturesque ruin. Another company, Barta & Partners (Pty) Ltd, worked the crater between 1957 and 1970, but the government terminated their mining lease. The Pratley Mineral Exploration Co (Pty) Ltd then acquired a prospecting lease over the property in 1971, but appeared to concentrate on the clay content of the deposit and sunk a borehole for this purpose in 1972. After a number of years of exploration and laboratory work they allowed the prospecting lease to lapse. Akasia Mynbou (Edms) Bpk then acquired a prospecting lease for base minerals over the crater with the intention of prospecting for salt and soda. Akasia carried out very little prospecting and when the prospecting lease expired in 1992, the government declined a further application by Akasia. A chemical engineering company, Palframan & Horner, which attempted to produce salt, used part of the old factory during 1958-1961. This operation was unsuccessful. Although it is known that saline mud continue to a depth of 170 metres below the surface of the crater floor, it would appear that further exploitation of this deposit is no longer economically viable. |
|
| (Source: REIMOLD, W U, BRANDT, D, DE JONG, R C, HANCOX, J, 1999, Tswaing Meteorite Crater. An introduction to the natural and cultural history of the Tswaing region including a description of the hiking trail. Popular Geoscience Series 1. Pretoria: Council for Geoscience. Available at the Council for Geoscience, Pretoria, South Africa) | |
| All intellectual property rights, including but not limited to copyright and trademarks, vested in the material contained on the NFI website is held by the NFI and may not be copied, reproduced, adapted, published or distributed in any form whatsoever without the prior written consent of the responsible person at the NFI. | |