stanje i perspektive u rudarstvu i odrţivi razvoj mining present state ...

2.3. Content of coarse/hydroscopic moisture in samples:Moisture in gypsum V=24.67%Moisture in ash V=2.15%3.0. EXPERIMENTAL WORKIn order to have a successful pelletisation, the following conditions had to be met in terms of pelletcomposition and mechanical properties:The ratio between ash and gypsum in the agglomeration mixture has to be 1:4 because they areused in that ratio in cement production;The binding agent must not affect adversely the quality of cement;The size of final pellets should be between -20.0 and +2.0 mm in class of coarseness, whichcorresponds to the size of klinkers;Pellets must be capable of withstanding the transport and handling within the plant withoutresulting in class -2.0 over 10% mass.The tests regarding pelletisation were performed on the lab pelletisation plate of 40 cm in diameter,while the homogenisation was carried out in the lab planetary mixer for powdered materials. Portlandcement was used as an binding agent; it was added in quantities of 1, 3, 5 and 7 % mass in regard tothe cement mixture. The choice of binding agent seemed logical concerning the requirements that hadto be met (to ensure the satisfactory strength of the pellets, to prevent adverse effects on the cementquality).The resulting ―green‖ pellets were dried by letting them stay out, at room temperature, while themechanical properties were checked after 3, 7 and 28 days. After analysis of mechanical properties ofthe finished pellets, the sample showing the best ratio between mechanical properties and content ofbinding agent was chosen for milling with portland cement klinker and testing of physical andmechanical properties. The intention of these tests was to determine if the cement produced from theagglomerated gypsum and ash would yield the satisfactory properties.4.0. TESTING MECHANICAL PROPERTIES OF THE PRODUCED PELLETS4.1. Testing pressure resistance of pelletsThe testing of pressure resistance of the pellets was carried out in the standard laboratory press. Theinitial sample was sieved into four grain size classes; single pellets were placed in the press mould.The pressure was increased gradually until the destruction of pellets and then the values in kg/pelletwere recorded (Baker et al. 1982). Several tests for each grain size class were done and then theaverage pressure resistance value for the relevant class was calculated.Table 5 shows the test results for the pellet pressure resistance when using 1, 3, 5 and 7 % of cement,after 3 days of airing.Table 6 shows the test results of pellet pressure resistance when using 1, 3, 5 and 7 % of cement, after7 days of airing.Table 7 shows the test results of pellet pressure resistance when using 1, 3, 5 and 7 % of cement, after28 days of airing.