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This invention relates to a process for reducing the residual sodium chloride content of granular rock by contact of the granules with a fluid which has been saturated with an acid and passed through a membrane.
The residual sodium chloride in granular rock (hereinafter referred to as “sand”) is usually as a result of the water contained in the sand being drawn from it as a result of weather processes; for instance by rain-dumping and wind-drifting during storms, or it can result due to contamination with other chemicals that are added to water, for instance fertilisers in a garden area, industrial products, etc. This phenomenon of an increase in the sodium chloride content becomes increasingly apparent particularly through the process of ground water absorption, whereby the sodium chloride content of natural earth is lowered, and its place taken by potash and the other minerals, and that water absorbs the sodium chloride which then leads to an increased water hardness. The residual sodium chloride in granular rock can also result from sand which has been contaminated from other sources at the mine, before the granular rock is produced for market, or during the production of some goods from the rock.
The presence of residual sodium chloride in granular rock results in a less pure sand product in the finished form. When the final form of the good is that of a brick or cement product, the presence of residual sodium chloride leads to a less marketable product. The presence of sodium chloride in the sand, moreover, also leads to difficulties in connection with cement production.
The quality of the rock can in some cases be improved by grinding and screening the sand, and can be of an adequate quality for a variety of uses. Granularity is typically given as the product quality, and this is normally determined by the relative density of the granular product.
Several methods are known for improving the quality of the granular rock products by washing off the residual sodium chloride, or by other means. Pouring the granular rock through or over large rotating discs in water is one of these methods, but this is not a fully satisfactory solution since it only operates in a batchwise and discontinuous manner, is expensive and has low effectiveness.
The problem of reducing the residual sodium chloride content of rock has also been alleviated by methods employing chemical solutions. Sodium hydroxide solutions are frequently used in Sweden, Canada and Australia and also in Spain and