In this study, a three-level Box-Behnken design of experiments combined with response surface methodology used to investigate the effects of the feed density, feed pressure and vortex finder diameter on the separation results (ash content and yield of the overflow) of a water-only cyclone. The coal used in the study was supplied from Soma, Turkey and crushed to below 1 mm. Experiments were conducted using a watter-only cyclone (WOC) which was operated in a closed-circuit test rig, overflow and underflow streams were collected and were sieved through 0.1 mm to simulate dewatering screens.The actual data collected from the tests were used to construct the empirical models representing clean coal ash and yield as process responses to the independent variables. The significance test of model fit for clean coal ash and yield were performed using analysis of variance (ANOVA). The results showed that ash content and yield of the clean coal models were significant.The results showed that with an increase in vortex finder diameter (VFD), feed density (FD) and inlet pressure (IP), ash content and yield of the clean coal increases. The results suggested that all main parameters affected the ash content and yield of the clean coal to some degree. The significance order of the effect of the variables on the ash content and yield was found as FD > VFD > IP and VFD > IP > FD respectively. The results of the numerical optimization in the range of the experimental data showed that it is possible to reduce the ash content of clean coal from 42.21 to 18.89.

Volatility is one of the most characteristic features in the all market types. In the raw material market, including the bituminous coal market, volatility is visible in the supply and demand variations, in consequence in the prices fluctuations. Market actors usually having opposite interests, for example buy low, sell high, are vitally interested in identifying the causes of these fluctuations.

Some of the factors causing the market fluctuations are quite common, others are more complicated because of circumstances complexity. This article attempts to examine the relationships between bituminous coal fines prices and the economic situation. Given the complexity of the issue, the research area has been narrowed down – territorially to Poland and temporarily – to the present decade.

The average prices of coal fines in Poland are presented by the Industrial Development Agency (Agencję Rozwoju Przemysłu SA) in the form of two indices: PSCMI 1 and PSCMI 2. Both indices are calculated based on the prices of pattern bituminous coal, produced by domestic manufacturers and sold on domestic markets, the energy and heat market respectively.

Statistical methods, because of their quantitative nature, are important in identifying the correlations between the coal fines prices and economic conditions. Therefore, the article presents examples of relatively strong linear correlations between the PSCMI 1 and/or PSCMI 2 and some indicators of the economic situation.