The Process Mineralogy Today blog has now been running for over 12 years and in that time, we have built a wealth of information on mineral processing, process mineralogy and geometallurgy. While technology has advanced and we are rapidly moving into the Digital Age of mining many of the core concepts we have explored remain deeply relevant in development and operation of mineral projects. We are taking the opportunity to dig down into the archives and present some of the more relevant articles from over the years. We you find these interesting and that they help drive a deeper understanding of minerals for a new generation.
Every mineral processing operation, from a 50,000oz pa gold plant to a 400,000 tpa Cu operation has areas that are inefficient, costing unnecessary money every day. Understanding where these areas are and how value losses occur is a fundamental aspect of continual process improvement that needs to be constantly monitored. Metallurgists have a growing suite of tools to identify chances for improvement but often the basic causes come back to key areas that we see repeatedly.
This article was originally published almost 8 years ago but in that time, we have continually seen these same issues in both existing operations looking to improve and during project development, when looking at flowsheet optimisation. Thinking about whether any of these issues affect an operation is a great way to begin a forensic approach to continuous improvement and systematically identify where gains, either in reduced costs or increased recovery can be made.
Constantly changing material composition in the feed will lead to an ongoing state of change in the mill operating parameters. Any change in feed conditions will affect the grinding conditions and will require changes by the operators in reagent additions for flotation or leaching. However, by the time a variation in material feed has usually been identified and the operators are able to respond time has passed where sub-optimal recovery conditions were present in the process. Even with an experienced team of operators there is an additional period in which changes are made and the process is made stable again. Through this period product losses occur and generally overcompensation occurs in reagent addition, leading to excess reagent consumption.
Feed variability occurs in every operation, it is in the very nature of mineral deposits. However, when the feed ore type or blend changes multiple times in one day, or even within one shift, significant losses can occur with a direct result on the efficiency of the process. MinAssist has performed audits on several operations where poor blending processes and mine planning led to variations in the feed on almost an hourly basis, leaving the process in a constant state of flux. In these situations, recovery improvements of up to 10% could be achieved simply by evening out the feed composition, adding significantly to the value of the operation and saving costs through reduced reagent consumption.
In MinAssist’s experience the variability of ore feed to an operation is the primary driver in losing money and is often the catalyst for many of the other areas for losses detailed below.
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 represents one of the most significant areas that operations lose money. These inefficiencies can be caused by variability in the hardness of the feed material, which may lead to softer material progressing faster through the circuit while harder material spends more time in the circulating load.
Another key area where inefficient grinding occurs is when throughput is pushed too high to meet production demands. The grinding circuit is often a bottleneck in process throughput and the temptation exists to increase throughput and benefit from the economies of scale. If not properly planned this can result in under-grinding of the material and poor value mineral liberation. This has a flow-on effect resulting in poor recovery or concentrate grades in downstream process circuits. Problems also occur when insufficient throughput is present (I.e. At times of restricted supply from the mine). This can result in insufficient charge in the mills and over-grinding of the material, leading to higher proportions of slimes through the process.
Inefficient or poorly maintained cyclones or screens can result in poor classification of material. This in turn can lead to material that has already been sufficiently ground being directed back to the grinding circuit or poorly ground material progressing through the flowsheet when it should be sent back to the grinding circuit. In the first instance overgrinding may occur, which not only expends energy that could otherwise be saved but runs the risk of reducing the particle size distribution of value elements to below the optimum range for recovery. Both have a direct cost impact on the operation and could be avoided if sufficient understanding of the classification circuit was maintained.
A direct result of under-grinding is that mineral grains targeted for recovery remain locked within host particles. This means that when those particles enter the flotation or leaching circuit they pass directly through to the tailings stream. In some cases, the mineral grain may be partially liberated but attached to a larger gangue particle, which in the case of flotation would require greater reagent addition to recover to the concentrate and when recovery was achieved, dilution of the final concentrate grade.
The use of excess reagents in flotation or leaching is generally a symptom of other issues in the process, however, it is a primary area in which operations lose money. If the feed material is poorly understood and recoveries are not being maintained the natural response of operators is to increase reagent dosage to compensate for variations. This leads to reagent addition schemes that are naturally tailored for the worst-case conditions, which may only occur rarely, and at all other times excess reagents are present. This is a direct cost to the operation that can be avoided with a better understanding of the material being presented to the circuit, whether it is flotation or leaching.
Every operation needs to target recovery of saleable commodities, however, often the presence of deleterious minerals or elements in a concentrate can result in penalties to the price or in extreme circumstances a concentrate that is un-saleable. Understanding the distribution of these deleterious components is vital for any operation looking to sell concentrate and strategies to minimise their recovery should be put in place.
Consideration of the water quality in an operation and effect that it may have on each of the process circuits can have a direct impact on costs. The impact of water quality can range from increased maintenance expenses if saline water is used in a flowsheet not designed for it to detrimental effects on flotation or leaching recovery caused by interaction of ions with reagents. For processes operating a metal extraction circuit the quality of water in Ion Exchange or Solvent Extraction is of significant importance in how efficient the circuit may run and which reagents are most appropriate.
Although make-up water quality is a primary consideration, understanding the water balance and how impurities build up within the circuit is of equal importance. Poorly controlled build-up of impurities can have direct impacts on reagent consumptions or recovery. However, the inadvertent concentration of environmentally detrimental impurities prior to final discharge can have significant environmental impacts, leading to at best increased waste processing costs and at worst an impact on the sites social licence to operate.
While excess reagent addition is a big cost, efficient recovery of those excess reagents can mitigate the impact. In systems such as cyanide leaching for gold, the recovery of cyanide both minimises the required cost of the reagent but also reduces costs associated with waste management where free cyanide is present in tailings dams. In many cases the recovery of reagents is not possible but when it is every effort should be made to ensure that it is undertaken.
The definition of residence time within specific unit operations should be determined by design criteria, however, changes in processing conditions or ore types may mean that optimisation might be possible. Constant monitoring of the optimum residence time can allow increased throughput if residence time can be reduced or increased recovery if it is identified that increased residence time is required.
A simple area in which operations can run less than efficiently is through productivity of their operators. This may be due to inexperience or poor workplace morale but can have significant impacts on process efficiency through lack of monitoring or slow reaction to process changes. Operator training is a critical part of any continuous improvement program. Engaging with operators and providing them with the skills to identify inconsistencies gives them greater investment in the process and helps to naturally improve productivity.
The issues identified in this article are just a snapshot of the myriad of areas where operations can lose money. What they show is that a good continuous improvement program, supported by data and properly trained metallurgists and operators can be used to systematically improve any operation. Most operations have some form of continuous improvement but it is always worthwhile re-assessing the process and thinking about whether the approach is systematic and rigorous. Executed well this can mean sustained incremental gains for any operation.