Coal Preparation » Process Control
Large-scale coal preparation plants require the subdivision of solid/liquid slurries at a number of stages. The unit operations are characterised by preferred feedrates of slurry, solids concentration and particle size distributions for optimal efficiencies. The ideal feed subdivision device would provide a consistent slurry at the same feedrate to each downstream unit.
There are numerous examples of the current technology producing biased slurry substreams. This project has focussed on the subdivision of slurries under pressure. Two types of slurry subdividers have been studied in this project; the T-type and a multi-port type.
Experiments were conducted using a T-splitter, in which the incoming slurry encounters a side branch of the same diameter at right angles to the straight through flow. Slurries of glass beads and pulverised coal were tested, with the splitter mounted both vertically and horizontally.
The slurry does not split evenly into two substreams in terms of particle size distribution and solids content, as the inertia of the larger particles makes it difficult for them to change direction. The use of streamline partition walls and stratifiers were found to reduce bias for both vertical and horizontal cases, with the stratifier providing the greater improvement. This result holds for a wide range of slurry flowrates and solids concentrations.
A pilot-scale experimental system was constructed, which can support up to five 100mm hydrocyclones operating from a mulitiport distributor with an overall feedrate of 110m3/hr. A purpose-built sampler was used to simultaneously collect samples from all substreams without introducing sampling errors or disturbing the flow in the system.
The degree of bias between the substreams, in terms of solids concentration and particle size distribution, varies with feed flowrate, solids concentration, and the density of the solids in the slurry. It has been demonstrated in the experiments that the primary cause of poor subdivision is non-uniform solids segregation due to low velocity within the distributor. Unbiased slurry subdivision was achieved by reducing the diameter of the distributor. A practical limitation of this finding is the number of outlets of the required hydrocyclone inlet diameter that can be fitted around the distributor.
A simple model has been developed to predict the extent of particle settling in a distributor for given conditions. The model related the degree of particle settling with particle size distribution, particle density, solids concentration, feed flowrate and the geometric parameters of the distributor. A linear relationship was derived between the model prediction and the amount of bias detected in the pilot scale investigation.
Sampling campaigns were conducted at two operating coal preparation plants, to establish a baseline of current performance. The predictive model was validated against this plant data. This model was used to develop a new approach to designing multi-port distributors. The new approach needs to be further validated against data from other distributor types, preferably from operating plant data.