The crushing and grinding circuit in any process flowsheet is a major contributor to cost and should be a major focus of any continuous process improvement program. While the direct costs (i.e. power and maintenance) within the crushing and grinding circuits are generally the primary consideration for optimisation, the indirect costs associated with insufficient liberation or over grinding can have as profound an impact on downstream processes. Care should be taken when evaluating comminution circuit optimisation that efforts to increase throughput or reduce energy requirements don’t have a negative impact on the liberation characteristics of the material and result in reduced downstream recovery.
Comminution processes account for 3-4% of the world’s energy usage and are responsible for at least 40% of energy usage in mining and mineral processing. Inefficient crushing and grinding therefore represents one of the most significant areas that operations lose money. Various groups do great work in optimisation of comminution circuits and if you would like to find out more about some of these the best place to visit is the Coalition for Eco-Efficient Comminution (CEEC). What we will focus on in this post is how characterisation can be used to evaluate where value is being lost and where cost savings can be made.
A key area that impacts on the operation of a process flowsheet is an inability to maintain consistent throughput. The texture of an ore has a direct impact on its hardness and incomplete characterisation of ore domains for both mineralogy and standard hardness and grind ability measures can lead to unexpected variation in throughput. Take the simple example of a copper operation with a mix of massive sulphide ore and quartz rich ore. The massive sulphide ore may present excellent grinding characteristics, allowing for high throughput in the comminution circuit; whereas, the quartz rich material may exhibit poor grinding characteristics requiring a higher grinding residence time and consequently a lower throughput. In this example the massive sulphide ore is likely to be higher grade and a sudden change to this ore type could flood the downstream flotation circuit, leading to dilution of the concentrate through entrainment of deleterious sulphide minerals and increased losses to tailings. In this example, characterisation of the changes in ore types to the comminution circuit may be readily accomplished through geological logging but many subtle changes in gangue mineralogy or ore texture can occur that have a pronounced impact on grind ability without obvious indicators. To maintain consistent load on the comminution circuit with a steady throughput it is therefore necessary to understand the variations in ore texture coming from the mine and use this information to implement an appropriate blending or batch crushing plan.
Another key area that leads to inefficiency in the comminution circuit is inappropriate or poorly maintained classification units. Hydrocyclones or screens are typically used to classify material that has achieved the desired particle size distribution (PSD) and is ready to progress to downstream recovery circuits. If these units are not set correctly for the ore material being processed or are not adequately maintained it can result in under-ground material progressing through the circuit or adequately ground material being sent back into the comminution circuits. In the case where classification causes material to remain in the grinding circuit too long over-grinding will occur resulting in a direct cost associated with the power used for unnecessary size reduction and potentially further indirect costs downstream where over-ground material may result in issues such as sliming or entrainment of gangue minerals to a flotation concentrate. Routine evaluation of ore texture in the final mill product, along with characterisation of recycle streams, such as the cyclone underflow, can greatly assist in evaluating whether the classification is operating as desired or whether optimisation is warranted. If the classification circuit is operating poorly then this is an area where significant cost savings can be made through a simple characterisation program to identify and correct any issues.
One of the greatest causes of value losses is inefficient size reduction, leading to poor liberation of the target minerals. The comminution circuit is one of the first stages of processing and if adequate liberation of minerals is not achieved at this stage it will have a direct impact on the capability of downstream circuits to achieve target recoveries or concentrate grades. In this regard the health of an operation’s comminution circuit is often a direct reflection on the overall efficiency of the flowsheet. Although it is important to ensure optimum mineral liberation is achieved, this should be done economically and consequently achieved with the minimum energy in size reduction. This balance is difficult to achieve and requires a deep understanding of the ore texture to ensure that both conditions are met. In some instances, where multiple populations of target minerals with different size distributions are present, this may mean that less grinding is performed in the main comminution circuit and a regrind circuit as part of the downstream recovery circuits is the optimum solution.
Overall, it is important to maintain an open mind when approaching optimisation of your comminution circuit. As it represents one of the largest single operating costs for most operations the efficient running of this circuit should be a major priority, but care should be taken that all decisions are made on an economic basis and efficiency in energy consumption isn’t achieved at the detriment of recovery or concentrate grade. Of course, if an economic case exists to make cost savings in grinding at the expense of total recovery then these should be considered. The take home message should be that one of the most potent tools in optimisation of comminution circuits is characterisation of the mineralogy and ore texture as this provides a fundamental understanding of the optimum target grind size for liberation and can be correlated with empirical grind ability and hardness measures.
For those interested in evaluation of their operations grinding circuit check out MinAssist’s Grinding Health Check as a simple method for mineralogical and ore texture characterisation.