Degradation of coal in a port and shiploading environment

Degradation characteristics of South Blackwater and Ensham coal were tested by using a drop test apparatus, a twin pendulum device and a tumbling machine. Together these three tests cover the basic coal degradation mechanisms for the handling processes in a ship loading facility. The data was used to develop a degradation model. The model can be used to recommend practices that will reduce the fines generated during coal handling at Gladstone Port Authority, GPA.

The research was commissioned by GPA because of the strict particle size requirements for the COREX steel making process. GPA loads ships with coal for a COREX plant in India. The COREX process cannot tolerate coal particle sizes below sixteen millimetres. In this research the fines component is defined as the coal fraction that is less than sixteen millimetre particle sizes.

The series of drop tests were performed to determine the critical drop height below which the fines generated is a minimum. The results showed as expected that the rate of fines generated increased as the drop height increased but more importantly that the fines generated below a 3 m drop height was small for both types of coal. Hence, the critical drop height for both South Blackwater and Ensham coal was 3 m. The coal samples were dropped onto a steel surface from heights ranging 1 m to 10 m. Multiple drops were used to simulate drop heights up to 30 m.

As bulk coal drops onto a surface, the larger particles or lumps are cushioned from impact by the smaller sized particles, particularly by the fines component. Five types of cushioning tests were performed. The different tests were:

Non-cushioned coal with all fines removed for each drop.

Cushioned coal with all fines included for each drop.

Cushioned coal with an additional initial 10% of fines include in the first drop.

Cushioned coal with an additional initial 30% of fines included in the first drop.

Cushioned coal with an additional initial 50% of fines include in the first drop.

All cushioned coal tests showed significant differences to the non-cushioned coal tests in the amount of fines generated. There was little difference between the coal cushioned with an initial 10% of fines and the standard cushioned coal test.

The drop tests were also carried out onto a steel surface, a simulated conveyor surface and a simulated coal surface. The coal samples dropped on the steel surface produced the most fines compared to other two impact surfaces.

The drop test results for weathered coal samples shows that the rate of fines generated was increased with the period of weathering.

The breakage of coal lumps was found to decrease with an increase in the mass of the sample.

A computer monitored twin pendulum device was used to measure the comminution energy utilization pattern in the single particle breakage tests. In this novel apparatus individual coal particles are crushed between the two pendulums. The breakage products that resulted showed that the fineness of the products increased with the energy used to break the particle. The breakage energy per unit mass of the particle is known as the specific comminution energy. The results showed that the product size distributions from the breakage of a single particle could be expressed by a single parameter, the "t₅₀" parameter, which was related to the specific comminution energy.

Tumbling tests are best used for determining the degradation caused by abrasion of the particle as the drops inside the drum are limited to heights less than 2 m. Two types of tumbling mill tests were performed. These were partially-cushioned coal tumbling tests where the products of degradation were removed from the tumbling drum at regular intervals and cushioned coal tumbling tests where no degradation products were removed. For both tests the results showed that breakage occurred in two distinct and identifiable stages. The first stage was identified by a high initial breakage rate and the second stage by a lower and almost constant rate of breakage. The relationship for the increase in fines generated as tumbling time continued was determined from these tests.

A degradation model was developed with parameters that account for:

The height of the drop called height parameter.

The number of handling operations during a drop test called the handling parameter.

The effect of cushioning by smaller particle sizes called the cushioning parameter.

The effect of previous load history for a sample called the stabilization parameter. The effect of exposure of the coal to the environment called the weathering parameter.