Simply put, there is only one direction for energy efficiency in the mining and mineral-processing industry, and that is up. We need to do things a whole lot better than we are doing right now if we are to meet growing demand efficiently in terms of energy and costs.
Making the right investment
The comminution process, which includes both crushing and grinding, is one of the world's most energy-intensive industrial processes. Comminution uses at least 3% of total global electricity production, and according to the Coalition for Energy Efficient Comminution (CEEC), it accounts, on average, for more than 50% of a mine's energy consumption and 10% of total production costs.
Mines rarely have control over the cost of energy, so it is vital that comminution processes meet process objectives while consuming as little energy as possible.
Energy efficiency is one of the pillars of comminution circuit design. Ignoring this can be the difference between a profitable operation and a money loser.
There is no magic design that is the best for all circuits. Generally, when a circuit is designed, a number of alternative circuit configurations are considered in the early stages. Comprehensive ore characterisation work on both composite and variability samples is key in determining the right comminution circuit design.
In hard-rock applications the designer may favour a design that incorporates more crushing upstream of an HPGR (high-pressure grinding roll) circuit for energy efficiency. In cases where there are a lot of clays present or there is high moisture, an AG (autogenous grinding) or SAG (semi-autogenous grinding) mill may be appropriate. For smaller throughput scenarios, a three-stage crushing plus rod mill plus Vertimill may make the most sense. It all depends on the ore characteristics and the overall plant requirements. It is very important to think about the pros and cons of each option, especially considering the ore types to be processed, the location, the environmental considerations and the size of the operation.
Is going bigger always better?
Some designers prefer having two smaller mills instead of one larger mill. The thinking is that when one mill goes down, the other is still operating and you still have 50% or more plant throughput. But having one large piece of equipment means a lot of cost saving at every stage of the operation - less civil and structural costs (concrete and steel) and less conveyors, bins, chutes and materials-handling equipment. Essentially, less overall moving parts and maintenance. This is the initial reason why SAG mills started becoming so popular.
The trend over the past 30-40 years has been towards bigger and bigger equipment, but this may not always be the most efficient choice. As a general rule, the larger the reduction ratio that a comminution device is required to achieve, the less efficient it is.
Comminution machines can be much more energy efficient if they target a smaller size range, i.e.. F80 to P80. Therefore, a really energy-efficient circuit may mean three stages of grinding instead of only two, and each stage can be slightly smaller and more efficient than one big one, e.g. a SAG mill (or HRC) followed by a ball mill and with a tertiary Vertimill. These options should at least be considered in the prefeasibility stage to see if the energy savings overcome the cost of an extra stage of grinding.
Reducing fuel consumption with IPCCs
As conveyors are generally far more energy efficient than trucks, IPCC (in-pit crushing and conveying) and reductions in fuel consumption are an integral part in comminution efficiency. Combining IPCC and coarse ore sorting, for example, by rejecting waste rock after secondary crushing in the pit, can be a game changer. Not only is there the benefit of reduced energy consumption in the mills, but the cost to convey material out of the pit goes down proportionally to the amount of material rejected.
Traditionally conveyor systems have been known to be less flexible than haul trucks, and, in many cases, there is a risk that if a conveyor stops, the whole operation stops. But these concerns are continuously being addressed with more flexible systems, remote monitoring and easier change-outs of parts while operating.
This blog post is part of Metso's four-piece series on how to ensure energy efficiency in mining. Read the original here.