ABSTRACT
In a laboratory float and sink test, the separation is clean and accurate as sufficient time is given to allow complete separation to take place. The data from such tests indicate what should be obtained under ideal conditions of operation. Such conditions do not exist in plant practice. In a continuously operating process, with constant discharge of float and sink, particles having specific gravity nearer to the medium may not have time to be separated and will be misplaced into the other product. Particles of high or low specific gravity are least effected. The difficulty, or ease, of separation depends on the amount of the material present having specific gravity nearer to the medium. Conversely, the efficiency of a particular separating process depends on its ability to separate material of specific gravity close to that of the medium. Some of the reasons for improper separation are:
1 Degradation of coal during separation process 2 Use of suspensions as the separating media 3 Disturbances created in separating medium due to the introduction of feed and
the removal of float and sink fractions 4 Agitation or upward currents used in the vessel to keep the separating medium in
suspension 5 Lack of sufficient retention time for the particles especially near gravity particles
to separate perfectly due to the use of high throughput
Different expressions for coal washing efficiency have been evolved to accurately reflect the quantitative and/or qualitative aspects of a washing operation. Evaluation of performance of a washer is based on two types of criteria namely dependent and independent.
