Planning

Geometallurgical planning must be practiced during the evaluation of new gold projects and all the life of mine. When the prospect is evaluated, the planning and strategies are focused on the viability and profitability of the project. If the project was accepted, the forecast and considerations taken at the early stages of the project must be improved. Also, when the ore is being mined and treated, the planning is based on metallurgist results obtained on samples to be processed within three or six months. This period of time is variable and depends on the type and size of project. In this way the sample must be characterized for several metallurgical factors such as hardness, work index, recovery by gravimetric concentration, flotation recovery, leachibility, liberation and reagent consumption. Obviously, the tests must be done according to the project selected.

To perform metallurgical tests on samples to be processed in the future is an advantage that metallurgists and plant operators need to study in order to confront the current results and the possible new results. If there are bad recoveries, the metallurgist will start to evaluate new alternatives oriented to solve the problem. In this way, the economy of the mine will not be affected. The economist or person responsible for marketing has to evaluate the results based on the new market trends, specifically the gold international price. For example, if the plant treats an ore by cyanidation and recoveries are close to 90%, and the new mine zone to be processed is partially leachable, the presence of a possible refractory ore is a warning voice that affects the process and profitability of the operation. If the plant was not designed for refractory ores, there are two possible options, redesign part of the plant or mix the problematic material with the non-problematic ore in order to get a balance on the operation. If the problem is very serious, the ore will not be processed and cash flow estimated during the early stages of the project is affected. This problem could have been detected during the scoping tests, but probably the lack of appropriate sampling procedures overlooked this problematic material.

The planning must be supported by mineralogical studies. When these studies are done in limited number and some problems appear during the operation, these studies are demanded in order to understand the problems of low gold recoveries. The process must be supported on mineralogical information no matter if gold recoveries were extremely high. It is important to cover all the mineralogical characteristics of the different ore types.

In general, geometallurgical planning needs the interaction of geologists, miners and metallurgist in order to coordinate and program the different ore types to be processed and any special requirement or treatment to be considered in the operation. Many times, the personnel responsible for the operation have to get a solution without having enough information.

The planning must define a production program based on mining and processing requirements during all the life of the mine so that the project can be supported economically. For many years, the valorization of gold deposits did not consider detailed information on the metallurgical characteristic of the resource. In this way, try to include many metallurgical parameters into the valorization modifies the definition or categorization of economical resource. An aspect very important in the evaluation is to optimize the Net Present Value (NPV), which is a projection of the profits in the time and consequently, the net cash flow must be optimized during each period.

It is well know that many gold operations valorize their resources considering the cut-off grade which has influence on the value of the ore block. However, there are other factors to be included such as gold recovery, processing arte or smelter costs. These factors are referred per tonne milled and not consider the value of the money in the time. The next table shows information on two ore zones.

If we consider gold grade and recovery, the main ore source is block 2. Now, the gold ore will be processed by cyanidation and the process includes comminution and leaching operations. An aspect very important of any comminution circuit is the power and each ore has different productivity due to rock breakage properties are different, especially hardness.

Under this new analysis, block 1 is more important than block 2 and will generate more money for the company due to during the same period of time will produce 2% more of gold. Basically, the example tries to show the benefits of employing a cut-off grade, recovery and the processing rate together. A strategy based on the cut-off and production may have meaningful influence on the ore block to be mined and processed initially. For this reason is important to quantify the value to be generated when the processing rate is included in the economical evaluation of the project.

It is important to study the impact or influence of the blocks or mineralized zones on the economy of the project without modifying the mining plan. Don’t forget that the processing plant will have to handle any variation in the ore sent to the operation. Losses can be avoided if previous studies analyzed the different options of treatment. The inventory of resources must be used to prepare a detailed mining plan year by year, which may have restrictions such as the mineable ore, the treatment capacity or the ratio between waste material and total ore.

The objective is to define production plans according to the ore and objectives of the operation. The procedures or considerations used must include operative costs, gold price, capital investment and financial aspects. Ideally, planning must be developed thinking in the future, considering exploration programs, cut-off grades and the extraction program of mined and waste material. The main steps to follow are to study the development of the mine, design the phases of extraction, select the process and evaluate the profitability of the project.

The project and specifically the processing plant can be affected by the geographical location of the plant, climate, physical characteristics of the ore body and precipitations. Whereas the location of the ore body cannot be modified, the plant must be located in the right place. The location has an important impact on the operative cost. For example, if the mine is located in geographical places of low temperatures such minus 5°C, the design must consider special buildings, otherwise the efficiency of the process and some equipment may be affected. If the design did not consider a building and some problems appear in the first months of operations, the company will have to redesign the plant and expend more money than the initial consideration. Consequently, the plant is not able to operate with high efficiency and the economy of the project is affected. The metallurgist involved in the design must be able to analyze the effects of low temperatures on gold recovery.

The previous operations prior to send the ore to the plant could affect the nature of the ore. For example, a copper-gold ore must be treated by flotation and the final product is a copper concentrate with gold values. According to the feasibility study copper and gold are associated and recoveries must not be lower than 87%. It is well known that some copper minerals are affected by the weather. If the ore is exposed more time than necessary to the environment, copper minerals will suffer alterations due to the action of oxygen and the recovery is affected. Any geometallurgical planning or production projection is affected when the analysis is not complete.

Many considerations for large operations are not necessarily true for small operations or projects and the main restriction depends on economical aspects. In general, the geometallurgy of small projects has a serious impact on the economy and profitability of the project.