The presence of high clay in the ore results in difficulties in the operations process since the presence of clay in the ore causes changes in the rheological properties of the pulp, hinders particle settling, and affects fixed plant unit processes. This relationship between high clay ore and poor performance in some unit processes was observed at Chuquicamata.
The problem with high clay ores is that they can be wet, sticky, and hard to handle through the plant. The reported problems include the tendency to stick to conveyors, idlers, and screens, bridging and ratholing in storage bins, and hoppers, thereby interrupting feed into the crushers and mills. As a result, the plant might not even get started with a stable feed stream to the crushers and mills.
The presence of clays can affect the grindability and behavior of the milling operation, particularly in cases where the clays are combined with fine materials. In fact, according to the Chuquicamata thesis, the effect of clay minerals on milling performance is more pronounced if these minerals are poorly crystalline or are found along with fine silicates. The increase in grinding power requirement due to clay presence has been observed in a similar study involving cement production operations.
An increase in the amount of clay adversely affects the efficiency of flotation due to the formation of coatings on the mineral surface and increased pulp viscosity, reducing the opportunity for bubbles to attach particles. These phenomena associated with high clay flotation have been described by the listed high-clay flotation recommendations. It is noteworthy that a mill may experience reduced capacity and metal recovery simultaneously, making the reduction in production volume just one aspect of losses.
Clay minerals are particularly problematic in thickeners and filters since they retain water, inhibit settling, and increase yield strength. According to the Chuquicamata thesis, there is an instance where the system exhibited turbid overflow, non-flocculation of fines, and higher energy consumption by the rakes and pumps, resulting in occasional plant shutdowns. This thesis also underscores the fact that the clay mineral type, crystallinity, size, and combination of clay minerals can affect the flocculation process.
High clay content in ores also lowers the effectiveness of water recovery, an important consideration in a stationary plant that uses recycled process water. In cases where the solid particles fail to settle adequately, overflow clarity is reduced, and there is a decreased amount of water recycled back to the plant, resulting in increased make-up water usage and operating expenses. In dry mining areas, this problem leads to a restriction in production rather than simply a metallurgical one.
Plants with good performance handle the ore containing high clays as an operation specific issue, addressing it by ore analysis, ore blending, desliming, scrubbing, controlling water chemistry, and reagent adjustment. The references demonstrate that knowing clay mineralogy and adjusting flocculation technique to the ore will help achieve faster thickening rate, cleaner overflows, and increase thickener stability. Thus, one may conclude that the issue is not just that clay is “dirty” ore, but that it disrupts the whole fixed plant processing line.
