Process for washing a solid substance impregnated with a liquid substance

A continuous process of washing nitrocellulose prepared by reaction of cellulose with a mixture of sulfuric acid and nitric acid and which is impregnated with said acids, which consists of subjecting said nitrocellulose to sequences of successive washing operations with a washing liquid comprising at least a first and last washing operation in a washing zone without any cocurrent movement of the washing liquid with respect to the nitrocellulose to be washed, and wherein:

• (1) in the course of the same sequence the successive washing operations are carried out with a washing liquid the content of the acids to be eliminated therein decreasing from the first washing operation towards the last washing operation;
• (2) during each sequence a quantity of liquid rich in the acids is removed from the washing liquid used during the first washing operation and an equivalent amount of the liquid free of the acids is introduced into the washing liquid used during the last washing operation;
• (3) during each washing operation of the same sequence a quantity of washing liquid is used, the quantity of liquid rich in the acids to be eliminated in (2) above being small with respect to the quantity of washing liquid in (3), a substantial quantity of the washing liquid being recovered to carry out the corresponding washing operation during the following sequence and with the exception of the first washing operation, a small quantity of this washing liquid is transferred into the washing liquid corresponding to a preceeding washing operation of the same sequence whereby the content of the acids is maintained constant in said washing liquid in each washing operation for the following sequence and wherein
• (4) each washing operation is carried out in a particular washing zone with the nitrocellulose to be washed being successively moved into each particular washing zone;
• (5) in each zone the washing is carried out with a quantity of the washing liquid which is almost totally recycled in the same zone thus forming a washing circuit, the washing circuits in all the zones being arranged in series;
• (6) the quantity of the washing liquid which circulates counter-currently from the last zone towards the first zone establishes a communication between the successive washing circuits;
• (7) in each washing zone the small amount of liquid which circulates counter-currently is introduced into the washing circuit at a point which is downstream, with respect to the direction of flow of the nitrocellulose to be washed, of the point at which the washing liquid is placed in contact with said nitrocellulose and downstream of the point at which the small amount of liquid which flows counter-currently and which is directed towards the preceeding washing zone is extracted, said liquid flowing counter-currently with respect to the direction of movement of the nitrocellulose.