Agglomeration means that the clays and fine particles contained in the ore adhere to the coarser particles and create a coating of fines around the coarse particles. When the ore has few fines and the percolation problems are not severe, the addition of a liquid may be all that is necessary to render the fines immobile in a heap. When the quantity of fines is substantial, e.g. 10% -200 mesh, a need may exist for a binding agent as well.

Use of Solution Only for Agglomeration: There are many forces that cause agglomeration and one of them is the surface tension of water. Water saturated particles collide with each other or with dry particles and the surface tension of the liquid film promotes coalescense and joins the two particles by a continuous film. Many such collisions gives an assemblage of particles. The forces of agglomeration are relatively weak when only solution (no binder) is used and the operator must be careful about the amount of solution that is sprayed on top of the heaps. If the spray rate is raised excessively, say more than 0.1 gpm/ft², or if a pipeline on top of the heap breaks, the flood of solution percolating through the heap could cause fines to dislodge from the coarse particles and to migrate downward in the heap. If the spray application rate is held within typical bounds of 0.003 to 0.006 gpm/ft², however, the slow moving fluid film on the particle surfaces has insufficient momentum to mobilize the fines.

Agglomeration works best if the coarse particles are wetted in preference to the fines. The coarse particles are then contacted with the dry fines and adhesion occurs. In practice, however, it is impractical to selectively wet the coarse particles.

The material should be wetted and mixed so that no minus 100 mesh fines remain unattached. The material is not wetted, however, to the extent that free moisture glistens on the ore. If too much water is added, the mixture becomes a mass of mud and becomes very impermeable. If the ore has a moderate amount of fines, properly agglomerated material will clump together when squeezed in the hand. In general, the moisture content varies depending upon the fineness and clay content of the ore. It is usually in the range of 10 to 15% by weight but it could be as low as 6% if the ore is very clean.

A simple method for estimating the proper moisture content is to make a slurry of ore and water and filter it on a vacuum filter. The dewatered filter cake will contain 1% to 3% more water than necessary for proper agglomeration.

There are no rules of thumb defining the proper amount of moisture to be added for agglomeration. Each ore will be different. In fact, the proper amount of moisture may vary during any one operating shift depending upon the moisture already in the mined ore and upon the amount of fines in the ore. So, even if the water and binder can be automatically added in proportion to the ore tonnage, observations of the agglomerated material at 30 to 60 minute intervals must still be made to be sure that the nature of the incoming ore has not changed significantly.

There is a tendency on the part of operators to add insufficient moisture to the agglomeration step. They do this because properly wetted ore becomes sticky and it tends to build up on agglomerating surfaces, at transfer points, and in storage bins.

Use of Binders in Agglomeration: When both solution and binder are added to the agglomeration step, the bond between fine and coarse particles is much stronger and more easily withstands excessive wetting, drying cycles, and rough handling.

The USBM and others (McClelland and Hill, 1981) have done considerable evaluation of binding agents and have concluded that Portland cement Type 2 is probably the best agent, usually added at the rate of 2 to 10 lbs/ton. Lime is a less effective binder. Both materials contribute to the protective alkalinity that is needed but the lime is better in this respect. Electron microscopy studies by the USBM showed calcium ions dispersed throughout the agglomerates but the nature of the bridging that holds the particles together was unidentifiable. They state, “we assume that particle agglomeration is similar to flocculation; that is, coagulation is caused by bridging of the colloidal clayey particles with the flocculating reagents and electrolytes.” The process yields very stable, porous agglomerates that do not break down under exaggerated leaching conditions, including flooded vat leaching.

The addition of a binder must be made to relatively dry ore so that the binder will mix thoroughly with the ore before the mixture is wetted. Binders should be added in the crushing circuit where they can mix with the ore as it is crushed. The binders also adsorb some excess moisture that may be in the ore, making it easier to move the ore through the crushing plant without sticking to screens and plugging transfer chutes. Adding the binder to the crushing circuit so it can be intimately mixed with the ore, especially if it is lime, puts the lime throughout the heap so that there is no chance of depleting the alkalinity when leaching first starts on a new heap.

The fresh mixture of ore, binder, and solution needs to cure for about 8 hours to develop strong bonds, according to the USBM (McClelland and Hill, 1981). No special curing facility is needed; the freshly mixed ore can be placed on the heaps and it will cure there in the 8 to 24 hours before a spray system can be set up and turned on if solution alone is used to achieve agglomeration, no curing period is needed.

Some companies have used flyash or kiln dust as a substitute for cement or lime but there is some evidence that these materials are not as effective as previously thought.

Milligan (1983) has suggested a method for testing the effectiveness of agglomeration by heating the fresh agglomerates at 90°C for six hours, cooling, and then dipping them ten times in water. The amount of fines generated is a measure of the strength of the agglomerates. Similarly, the agglomerates can be placed in a large burette with a plug of glass wool on top and then solution can be fed at varying flowrates for several hours with the solution being collected from the bottom of the burette and analyzed for solids. Either saturated (vat leaching) or unsaturated flow (heap leaching) can be simulated.