The recovery of gold by gravity was practiced before amalgamation, flotation or cyanidation, by thousands of years. Also in the design of modern gold recovery plants, it is often a misunderstood subject, even with people familiar with other applications of gravity recovery or with other gold recovery methods. It is important to indicate that this misunderstanding stems from the high circulating load of gold, which makes possible the unique approach used for its recovery, slowly bleeding if from the circulating load of the grinding circuit.
Many times, as little as 2-3% of the gold in the circulating load is recovered at each pass and overall gold recoveries of 20 to 50%, or one two thirds of the gravity recoverable gold, are usually obtained. Contrast this with most mineral circuits in which feed is exposed to rougher-scavenger recovery only once, and recoveries near 100% of the recoverable species of value at acceptable concentrate grade are the objective of the recovery process. Other important differences are the very high upgrading ratios, low weight of concentrate, and much smaller size and capacities of the cleaning circuits when compared to the roughing stage. The basics steps required to get a first design of the gravity circuits comprise the evaluation of the need for gravity, the gravimetric characteristics of the material, estimation of the gold recovery and selection of the optimum recovery effort to obtain an acceptable operation.
It is important to understand the economic impact of gold gravity recovery. Basically, the amount of gold recoverable depends on the quantity and size distribution of the gravity recoverable gold. Also, gold gravity recovery depends on how often gold is recycled in the grinding circuit, which is largely related to the product fineness of primary grinding circuit and the effectiveness and capacity of the recovery units. Also, the design must considers some aspects such as the potential recovery of gold carriers in cyanidation circuit tailing streams, pre-concentration of gravity recoverable gold and gold in gold carriers to reduce the size of the downstream circuit.
The preliminary investigation programs can proceed as soon as the drilling program has obtained the samples. Sometimes, it can be considered to carry out metallurgical test with chip samples if the metal content is interesting and it is necessary to know the behavior of the gold ore. Equipped with the results of the mineralogical examination, the metallurgist will generally begin the program by conducting crushing grinding and gravity concentration test. Most the time is required carry out tests to determine the time needed to grind the minus ten mesh ore to various sizes in a laboratory rod or ball mill. Attempts are made to establish grinding times which result in particle size distributions coarser and finer than the liberation size indicated by the mineralogist.
Once completed the preliminary tests, the metallurgists may select a flowsheet for treating the gold ore. The selection of these parameters is based on the experience of the metallurgist coupled with the current metallurgical processes. In other words, the test engineer will select the simplest conventional technology which has been used to treat similar ores successfully. He conducts preliminary tests to acquaint himself with the gold ore and determine the amenability of the deposit to the gravity recovery.