High Pressure Grinding Rolls

High Pressure Grinding Rolls

As we have noted often in these oages, High Pressure Grinding Rolls (HPGR) are becoming increasingly attractive as crushing technology in gold treatment. KHD Humboldt Wedag notes that “both their large capacity (up to 3,000 t/h per unit) and fine crushing performance [offer] advantages in gold treatment -demonstrated for practically all projects that are presently being realised (such as Boddington, Vasilkovka, SanShanDao) or under consideration.”

“Virtually all studies [by manufacturers and research bodies] indicate that, apart from the high unit throughput and low specific energy, significantly enhanced metal recoveries are generated. Two main phenomena are considered to be responsible for this. The first is improved accessibility of the gold in the minerals through so-called micro-cracks that are generated as result of lower ball mill grindability through weakening the crushed material by the high pressure applied, which significantly contributes to an improved downstream processing result.”

KHD Humboldt Wedag was contracted for the supply of a type RPS 7-140/110 for the 800 t/h tertiary crushing at the Sanshandao gold mine, China. The unit will be equipped with the patented KHD STUD-PLUS wear resistant roll surface. Another HPGR circuit for gold is currently commissioned by KHD at Vasilkovka gold deposit in Kazakhstan, with two RPS 16-170/180 units operating in parallel, in closed circuit with partial product recirculation. The units each process 1,440 t/h, generating a product of 80% < 5 mm from a 40 mm feed.

Grinding is the result of the pressure brought on a bed of material in the gap between the rolls. It is not dependent on the the high pressure applied to the ore in HPGR crushing. This results in an enhancement of gold recoveries in e.g. heap leaching, even from the coarser grain size fractions. Another advantage [of] HPGR is a finer product and a physical gap opening between the rolls, as is the case with conventional roll crushers. The high press force applied brings about a high reduction ratio and a significant micro-crack generation, liberating gold grains and enhancing amenability to gold recovery. The grains produced have been pre-stressed and weakened by micro-cracks.

This enhances leaching rates in (heap) leaching, and in virtually all cases improves grinding results by increasing the subsequent ball mill grindability (lowering the ball mill work index).

Much of the current emphasis of HPGR technology is to replace tertiary and quaternary crushing, and increase ball mill throughput by providing a finer mill feed at a reduced grindability and a high fines content. In many cases, a circuit of primary & secondary crushing, HPGR treatment, and ball mill grinding can be seen as a feasible alternative to SABC (SAG and ball mill circuits).

For heap leaching operations HPGR can replace tertiary and quaternary crushing stages to produce a leach feed with a moderate quantity of fines, and a permeable heap. Process studies and column leach tests do demonstrate a high permeability, increased recovery and faster leaching times than with conventional crushing technologies.

Bottle roll tests similarly indicate lower tailings Au assays and shorter leach times for more refractory ores.

Depending on the ores considered, it is generally observed in heap leaching (column leaching) that lower tails are obtained from HPGR even when conventionally crushed products are finer than the HPGR products in all size ranges and this is true for all rock types. For example it is effective working on rhyolite, quartz latite (a latite with a phenocryst modal composition containing 5-20% quartz; above 20% quartz, the rock would be classified as a rhyolite), feldspar, biotite or diorite, etc. The conclusion is that the micro-cracks developed in the ore particles by HPGR allow relatively more gold to be extracted than by conventional crushing. This has been reported in a large number of studies and the presence of these cracks is shown in microphotographs.

A detailed analysis of the leaching results for various HPGR tests shows that tails and thus recoveries are affected by specific press force. A higher specific press force gives a finer overall particle size distribution and leads to a greater density of micro-cracks and this will directly affect tails and thus recoveries. The lowest tails have consistently been obtained when the sample has been crushed in the HPGR. KHD says “the analysis suggests that HPGR is the best and possibly only alternative for preparing particles for heap leaching in a particular application and the specific press force is the key parameter.”

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