Critical size material is defined as ore that is too small to be effective grinding media and too large to be ground. This data showed that both specific energy consumption and critical size production increased during the winter months. The increased energy consumption was in part a result of an increase in the viscosity of the mill pulp at colder pulp temperatures. However, viscosity could not fully explain the changes in critical size production. These changes are most likely due to changes in the breakage characteristics of the ore.
Theory
Brittle materials are those requiring little energy to fracture. They fracture easily because of defects in their microstructure, and high yield strengths (high yield strengths prevent the material from deforming plastically). Defects, such as preexisting cracks, increase stress levels in their vicinity under external tensile forces. The amount of plastic deformation that can occur to relieve the stress is limited by the high yield strength.
Decreasing the temperature increases the brittleness of a material by increasing its yield strength. As yield strength increases, the material has less plasticity. This prevents the crack tip from being blunted, and causes the cracks to propagate under lower applied forces.
Thermal stresses also contribute to fracture in brittle materials. Rocks contain several different minerals. The grains of each vary in size, shape and have different coefficients of thermal expansion. However, grains at the center warm more slowly and thus a strain differential between the surface and center of the rocks is developed making the rock more susceptible to fracture.
Freezing temperatures were hypothesized to affect critical size production in two ways. The first would occur during winter blasting. If the ore is colder and becomes more brittle, compared to summer, cracks would propagate easier.
Experimental
Sandstone, basalt, and summer and winter blasted iron ore were studied. Both summer and winter blasted iron ore were obtained from the same open pit mine. The orebody is a banded iron formation composed of chemically deposited iron rich rocks with interbedded deposits of sediments, all of which have been metamorphosed the mineralogy of the crude ore consists predominantly of recrystallized martite and chert with subordinate hematite and locally earthy hematite, goethite, and magnetite.
Drop tests were used in the experimental work for four reasons. First, it is a quick test which ensures frozen samples will remain frozen throughout the test. Second, large, more representative samples (45 kg), can be used. Third, large rocks 178 mm to 152 mm in diameter, which are common in autogenous mill feed, can be studied. Last, the test measures a rock’s resistance to breakage in an impact collision after free fall under its own weight, which is similar to what happens to rocks in an autogenous mill.
Results and Discussion
Results for the summer and winter blasted iron ore drop tests were performed. The objective of these tests was to determine if brittleness increased at lower temperatures, and if winter blasted ore was fractured more than summer blasted ore. An increase in brittleness would make a rock fracture easier on impact. This would be represented by a decrease in size stability in dry samples at lower temperatures.
A noticeable trend in the data is that nearly all the wet frozen samples had a higher size stability than dry frozen samples, while wet and dry samples at room temperature showed no systematic differences. This shows that frozen pore water increases a rock’s resistance to breakage, also, the amount of scatter in the wet frozen samples is less than wet samples at 25°C and dry frozen samples. This further confirms the effect of frozen water on a rock’s size stability.
Since sandstone has high permeability and porosity, and can absorb plenty of water, it was expected to show large changes in its resistance to breakage due to frozen pore water. The data for the wet samples definitely confirm this. Wet samples have a much higher size stability than dry samples at -25°C. At 25°C the size stability of the wet sandstone is much lower than that for dry samples.
In addition to the laboratory drop test, industrial research data concerning the effects of temperature on rock breakage were also examined. This data consisted of studies on borehole temperature profiles of the orebody and bond impact tests.
From the preceding results, it is concluded that the results from the bulk iron ore drop tests indicate that the brittleness of the ore is a function of size as well as a function of temperature. Further work involving single rock drops is necessary to clearly define the brittleness effect
