Impact of the Automatic Control of a Coal Separation Process in a Jig on its Economic Efficiency

Abstract

Abstract The beneficiation process of fine coal in jigs consists of two phases: stratification of coal grains in the bed according to their density and then splitting the stratified material into the product and the discharged refuse. At first, during subsequent water pulsations induced by opening and closing of air valves, the stratification of coal grains takes place due to varied velocity of their upward and downward movement. Grains of low density migrate to upper layers and grains of high density migrate to lower layers of the bed. The material travels horizontally on a screen along the jig compartment with the flow of water. The stratification of grains due to their density is not perfect, because the velocity of their upward and downward movement depends in part on their diameter, shape and the way in which the material loosens within a given pulsation cycle. The distribution of coal density fractions in the bed, characterized by the imperfection factor I, has been investigated by many researchers. The imperfection factor I is defined as the ratio of the probable error Ep and the separation density ρ50 (I = Ep/ρ50). The distribution of coal density fractions for an ideal and a real stratification process was compared. The maximum mass of the product of the desired quality (ash content) can be achieved for the ideal process when the imperfection I = 0. The stratified bed is then, in the end part of the jig, split into the product which overflows the end wall of the compartment and the refuse (or middlings) discharged through the bottom gate. The separation density (cut point) is established by the tonnage of the discharged bottom product (opening of the discharge gate). The separation density depends also on the tonnage of raw coal feeding the jig, and its washability characteristics. The impact of variations in the separation density on product parameters has been analysed. The mass of the product is always greater when the separation density is constant over a given period of time – even if in spite of its variations the process renders the same average ash content. Hence, the conclusion is to stabilise the separation density at the desired value as accurately as possible. The analysis was performed for raw coal washed in a three-product jig at the separation densities of 1.5 and 1.8 g/cm3. Percent contents (in brackets) of density fractions in raw coal were: <1.35 g/cm3 (40%), 1.35–1.50 g/cm3 (12%), 1.50–1.65 g/cm3 (4%), 1.65–1.80 g/cm3 (4%), 1.80–1.95 g/cm3 (12%, >1.95 g/cm3 (30%) (average ash in raw coal was 35.5%). In the analysis, an increase in the imperfection by 0.02 resulted in the decrease of the product tonnage by ΔQc = 1.0%. In this case, separation densities were set to ensure the same ash content in products (for I = 0 the change in tonnage was accepted at ΔQc = 0). The simulation analysis presented in the paper focused on the impact that fluctuations in separation density have on the economic effects of a jig operation. The influence of the separation density fluctuations on the product tonnage turned out to be nonlinear; for ±0.04 g/cm3 (control system with the radiometric density meter) the decrease in the product tonnage was ca. 0.5 % and for ±0.12 g/cm3 it was ca. 5.0% (control system with a float). The above results indicate that the operation of a refuse discharge system in a jig plays an important role in the final results of coal separation process defined in terms of tonnage and quality of the product.