Grindin' at the Mill - investigating bottleneck in grinding circuit

Lead Mentor: Brent Sullivan, Senior Metallurgist

Challenge Description

Mt Keith was originally designed as a 6 Mt/a concentrator in 1994. Subsequent expansions and upgrades have taken the nominal throughput to 10.5 Mt/a, however the circuit can rarely maintain a feed rate that matches that. Mill throughput is a key driver for maintaining profitability at Mt Keith because of the low head grade (~0.5% Ni) and inherently low nickel recoveries due to the mineralogical makeup of the ore.

The Nickel West Mt Keith Concentrator (NMK) has 2 parallel grinding modules consisting of one SAG Mill and one BALL Mill per module. A third ball mill takes a split of feed from each module to assist maintaining a target product size. Both modules are fed from the same primary crushing circuit via a Coarse Ore Stockpile (COS) which provides surge capacity. The grinding circuit is designed to produce a product size suitable for the downstream froth flotation process.

User Story

There are multiple factors that can affect mill throughput at Mt Keith. Firstly, there is a large variation of ores being fed through which leads to certain ore types being harder than normal leading to lower throughput under normal grinding conditions. These harder ores can sometimes be inferred through mineralogical data, but not always. It is forecast that these harder ore types will represent a greater proportion of Mt Keith's feed over the coming years. The main problem with these harder ore types is that they hold up in the SAG mills, restricting throughput. The SAG mill feed is controlled by the mill weight, measured by 4 load cells on the mill bearing system. Therefore if the ore does not grind quickly enough in the SAG mills, the mill weight increases and the control system reduces feed to the mill to maintain the desired mill weight. 

While the two SAG mills at Mt Keith are identical in design, their operation is not due tosegregation of the coarse and fine material in the stockpile that provides feed to the grinding circuit. Therefore, the two circuits operate under different conditions and this has always presented a challenge for maximising throughput and optimising grind size for the downstream flotation circuit.

Other variables which can be manipulated to have an impact on mill throughput include:

• Mill feed and discharge water flow

• Use of different cyclone configurations, pressures or densities to manipulate the particle size leaving the SAG mill

• Varying the speed of the feeders to preferentially feed finer or coarser material from the stockpile, however this is not sustainable over the long term

• Changing the closed side setting (CSS) on the primary crusher to produce smaller rocks – this however limits throughput at the crusher, which may not be sustainable under certain conditions.

Additional Questions

• Are there any variables within the milling circuit that have a disproportionate impact on mill throughput?

• Are there any new technologies out there that may help us diagnose problems within the circuit?

• Is there anything that can be done to mitigate the effects of harder ore in the Mt Keith milling circuit without resorting to large amounts of capital expenditure?

• Are there any novel ways of countering the observed segregation in the coarse ore stockpile?