The variation in the hardness of ores is one of the factors that limit the production capacity of a fixed plant since the processing unit must run based on the hardest material rather than the average. Harder ores are broken slowly in the grinding mill; hence an excess load may lead to overloading and subsequent reduction in tonnage.
When the ore is softer than the assumed design conditions, the throughput capacity may increase, but this is bound by limitations at the downstream end and by the control of the particle size. Conversely, when the throughput is fixed, the ore being softer will lead to over grinding, which wastes power while producing too many fine particles that may affect the recovery processes at the downstream end.
The variations in hardness have a similar effect on availability by increasing the likelihood of upset conditions, clogging, and premature wear. Harder ore causes high circulating loads, mill overload conditions, and coarse agglomeration. Variations from one extreme to another, particularly from soft to hard ore feed, cause more wear to liners and processing equipment, leading to an increased number of unplanned stops for maintenance activities. This means that even though the plant might be physically operating, it is not operationally available at full capacity.
According to verified literature in the design of plants, hardness of the ore and the degree of variance are key factors when considering design tonnage for the comminution circuit. An industry publication points out that maintaining the nominal tonnage often demands that the hardness value be designed above the average hardness of the deposit by considering the 75th or 80th percentile, which is intended to mitigate the effects of harder ores on capacity. That is why variance is not only an operational problem but a fundamental design risk.
In effect, variations in hardness will affect the location of the bottleneck along the process flow. Hard ores may locate the bottleneck in the primary SAG mill itself, whereas soft ores will push the bottleneck farther down into the ball mills, classification, and transportation facilities, which means that the “real” throughput capacity will continue to move with the ore blend.
Economic impact is normally a double cost: fewer tonnes milled during hard ore stages and more energy, wear, and maintenance expenses during varying feed situations. It should also be noted from industrial advice that inadequate domain control may result in reduced liberation, increased tailings loss, and blocking or wear at certain parts of the processing circuit, resulting in less recovered value per unit of time that the plant operates.
The primary effect of variability in ore hardness results in decreased predictability for stationary plants and thus low productivity. The best method to address this issue is to assess hardness per domain, incorporate such information in mine planning and scheduling, and process the feed material to remain within the stable operating range of the plant.
