Coal Preparation » Fine Coal
In the Hunter Valley (HV), coal preparation plants often discard ultrafines < 125 micron because of the high clay and high ash content. Recovery of the coal that is currently lost in this fraction can potentially increase plant yield by 1 to 2 %. The use of small diameter hydrocyclones (100 mm) operating at a low cut size may significantly reduce the clay content in the feed to an ultrafine gravity process and have the potential to significantly reduce the capital and operating costs for that process.
The objective of ACARP project C19045 was to qualitatively and quantitatively assess the suitability and practicality of such cyclones as the first stage of a potential multi-stage beneficiation process and determine the criteria for optimal performance with the following specific objectives:
· Maximise the combustibles yield of the plus 30 to 40 micron material in hydrocyclone underflow (product) and maximise the desliming efficiency, by the removal of high-ash ultrafines (< 20 micron) from underflow;
· Minimise the loss of coal to tailings (hydrocyclone overflow) as the result of the density separation effect that is overlaying classification; and
· Minimise the volumetric split to underflow (product) resulting in a reduction of foot print requirements and consequently capital and operation costs for a potential subsequent beneficiation stage.
A test rig with a single 100 mm hydrocyclone was set up at a Hunter Valley coal mine. It was fed with primary classifying hydrocyclone overflow directly from the plant. As feed pressure, feed solids concentration and the ratio of feed nozzle area to vortex nozzle area significantly influence the process performance the controlled equipment parameters were feed pressure, feed nozzle size, overflow nozzle diameter and apex nozzle diameter. The uncontrollable parameter was the feed solids concentration in the test rig feed.
A large proportion of the feed to the coal processing plant contains bands of free swelling clays. Consequently the feed to the test rig was highly variable in feed solids concentration, feed ash and feed particle size distribution (PSD), where a high amount of <20 micron particles resulted in high feed ashes (<60 %). Test work was planned in three stages with the initial stage consisting of around 60 tests with individual parameter settings across a wide experimental space. Stage 2 was a repeat of the better tests of stage 1 and stage 3 finally represented the 'best case'.
Process performance could not be quantified by cut size d50c and imperfection alone because of the quantity of slimes remaining in the underflow due to the water split. Consequently the following parameters were adopted to quantify performance:
· Ash percentage in underflow vs feed (to be minimised);
· Ultrafines by-pass to underflow (to be minimised);
· Combustibles yield in underflow (to be maximised ) and loss of coal to overflow (to be minimised); and
· Volumetric split to underflow (to be minimised).
The most significant variables with the biggest impact on desliming efficiency were:
· Ratio of apex nozzle to vortex nozzle area; and
· Feed ash percentage.
The optimum performance of a desliming process that utilises small diameter hydrocyclones as a first rougher stage can be defined with the following achievable targets:
· Combustibles recovery in underflow: 75 % or above;
· Ash recovery in overflow: at least 35 %;
· Volumetric (slurry) split to underflow: less that 50 %;
· An ash reduction of 10 to 15% in the underflow with the high ash feeds;
· Proportion of ultrafines <20 micron in underflow: less than 50-60 %.
Optimum equipment and process parameter settings were found to be:
· Feed to vortex area ratio 0.75 - approx. 22 m3/h capacity per single cyclone;
· Apex nozzle size 35 mm;
· Feed pressure 190 kPa.
A single stage small diameter hydrocyclone set up is well suited as a first rougher stage in a multi stage process.
Subsequent process (beneficiation) stages can potentially be any of the following:
· Gravity separators that may avoid floatation units (e.g.hindered bed separators)
· Classification (eg hydrocyclones, and screens);
· Enhanced classification (eg water injected hydrocyclones, etc);
· Enhanced gravity separation (eg Kelsey jigs, Inline Pressure jigs, Knelson concentrators, etc);
· Classification in combination with dewatering (eg screen bowl centrifuges).
Additional test work in parallel with any of the above processes would be required to quantify the suitability of such process configurations.