The importance of gold gravity recovery to complement the performance of flotation and cyanidation has been a key aspect in the design of many gold mills. Essentially, gravity differs from other recovery methods in that most of the gold recovered by a gravity device would not be recovered by the circuit downstream, flotation and/or cyanidation. The technical justification of a gold gravity circuit is in some cases based on small margins. Considering economical aspects, flotation and/or cyanidation are considered relatively inefficient processes and labor costs low. Meanwhile, the gravity circuit involved a dynamic performance. During the eighties the gold mining industry started to experiment a new perspective about the gold recovery process by the implementation of new flotation cells such column or flash flotation cells, the formulation of selective collectors for floating gold bearing minerals and improved control systems, which enhance the performance of the gold recovery process. This trend decreased the interest for gold gravity circuits. The cyanidation of gold bearing minerals experienced some changes with the introduction of activated carbon and ion-exchange resin or an improved Merrill-Crowe process.
With the years, the design of gold gravity circuits has been an interesting alternative again, which with new equipment offer low capital and operating costs. Essentially, this has resulted in a relative change from the traditional criteria used to implement a gravity circuit. A good example is the introduction of centrifugal concentrators, and especially the Knelson concentrator, whose introduction has made possible to consider the design of gravity circuits in the primary grind circuit. Basically, it was possible to obtain better overall gold recoveries with a very simple gravity circuit. Other example is the Hemlo operation whose gold gravity circuit was equipped with a screen and a Knelson concentrator unit, whose concentrate feeds a Gemini table, to produce a 75% gold concentrate which accounted for approximately 25% of the gold production.
It is important to mention that low capital and operating costs are the key factors that favor the installation of gold gravity circuits on the basis of reduce the cost of other complementary processes such ion exchange elution. Some designers have considered the importance of gold gravity circuits based on an improved gold recovery. Obviously, this aspect is influenced by the mineralogy and the circuit design. In most cases the easy respond of the material to gravity concentration is the factor to be studied. In this way the metallurgist must be able to identify the performance by following a reliable testing procedure, which should produce important metallurgical statistical information, the evaluation of updated concentration units is important and without any doubt, the centrifuge units must be part of this testing program. Other important information is related to the global gold recovery and its recovery size-by-size; in order to identify the possible weak points of the unit to recover specific gold grain sizes.
A metallurgical testing program 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.