If we talk to talk about agglomeration, it is important to mention the work developed by T.C. Scrutton in 1905. He developed a unique technique for obtaining rapid vat leaching of a clayey gold ore in which gold was finely disseminated. He considered important to roll the material down a chute at 60 degrees to form agglomerates or balls readily permeable by the cyanide solution. He had to face a problem, the lack of rigidity and the solution at that time was to pile the material in layers of one meter. If this height was exceeded, difficulties were experienced in obtaining uniform leaching and washing. Some years later, in 1937, Shepard et al, investigated the addition of lime and carbon dioxide or calcium carbonate to gold –bearing tailings to form agglomerates suitable for vat leaching. Good results were obtained in 90 g scale tests, but the reagent requirements were excessive.
Probably, one of the most difficult gold ores to treat efficiently by heap leaching are those characterized by the presence of clayey ores and /or fine produced by crushing. Basically, the presence of excessive quantities of slimes, usually classified as 100% passing 50µm, in the heap leach feed will slow the percolation rate of the cyanide solution, cause channeling or produce dormant or unleached zones within the heap. This situation creates unreasonably long leaching times and low gold recoveries. In some cases, slimes and clays can seal the ore heap, causing the leach solution to run off the sides of the heap rather to penetrate the ore bed.
The basic problem of heap leaching ores with fines can be aggravated during stacking and preparation of the heaps due to segregation of coarse and fine material can occur. This situation creates an excessive concentration of fines in the center and larger rock fragments on the lower slopes and base of the heap. In this way, when the heaps are leveled, additional segregation is produced due to fine sift through the coarser particles. Considering that segregation results in localized zones having different permeability, the leaching solutions percolate through the coarse ore zones and bypass or barely wet areas that contain large amounts of fines particles and slimes. In this way, to employ a heap leaching process, needs methods that achieve uniform size distribution during preparation of the heaps and better slime control before leaching.
Agglomeration and balling of crushed ore to produce a porous and uniform material for heap leaching is a viable method for treating clayey gold ores. The U.S. Bureau of Mines performed a research program in the mid-1970 to study the agglomeration of clayey ores and the results were improved and was demonstrated the viability of this option.
Three important agglomeration parameters are important to agglomerate clayey gold ores, quantity of binder (Cement) added to the dry feed, moisture added to the binder-ore mixture and the curing period to form calcium silicate bridging. It has been noted that most crushed ores less than 25mm can be agglomerated by mixing 5.0 kg/t cement, wetting with water or cyanide solution to 10-12%, and mechanically tumbling the wetted mixture and curing for more than 8-9 hours before applying conventional heap leaching.