In order to have a preliminary idea about the metallurgical performance of the samples, some scoping tests have to be performed. They can be carried out in the prospective mine site and/or special metallurgical laboratory. The final election is based on the equipment availability and gold mineralization. Test to be done include; concentration by gravity, flotation and leaching.
Rotary Sampler Splitter
It is a good practice to test several samples or composites. The sample preparation must be done under the following considerations:
- Combine the samples according to the geologist indications
- The sample has to be crushed to 100% passing 10 mesh
- Split the sample in one and/or two kilos charges
- Take samples for head analysis and perform assays for gold and silver.
- Other elements can be detected by ICP analysis
- Gold Panning:
Without any doubt, gold panning is the oldest prospecting tool and concentration device employed by prospectors. The method is based on the difference of specific gravity among gold and other minerals. Pans can be found in several presentations and the prospector must select the most appropriate according to gold mineralization. The typical gold pan is 16 inch diameter and 2 to 3 inches depth. Some pans have riffles a long one half and they are an extra help to enhance the separation of light and heavy particles.
Basically, gold and heavy metals remain at the bottom and the other minerals are discharged due to they are lighter. Before adding material into the pan, it must be classified with 6 mesh screen so that all the coarser particles can be separated. The finer particles will be processed by panning. It is important to mention that the appropriate skill can be only obtained with practice. In this way, the prospector will recognize minerals and minimize losses.
If the material does not have homogeneous particle distribution, some problems can be expected during the panning. For this operation the sample must be classified in 1000-500 g charges and each charge must be panned. Ideally, the sample must be mixed with water in order to prepare slurry at 30-35% solids. The water will be added slowly over the ore so that all the lumps can be broken. At the moment of adding water the supernatant layer will be dark and a lot of fine particles (slimes) will be floating. At this moment is important to shake the pan repeatedly several time in order to create a preliminary separation between slimes and the other particles, tilt the pan and discharge some dirty water and add more clean water. Repeat this procedure until the water is totally clear.
Now, the separation between heavy and lighter particles must be performed. For this, the pan must be inclined and shaken from side to side by making a circular movement. It is important to add water in each operation in order to observe the minerals and make easy the separation. Elimination of light particles is helped by moving the lip of the pan down and up. A gentler wash action is required as the amount of wash dirt remaining decreases. The previous operation must be repeated until only a small area (circle of 2.5-4.5 cm radio) of fine material can be observed.
If the material has free gold particles, they will be observed easily at the bottom of the pan. Most the time, by processing samples from placers, at the last part of the process will be observed black particles such as magnetite and zircon. Magnetite can be separated by using a magnet.
At the end of the process is important to be careful with the shaking motion due to there are flattened and tiny gold particles that tend to floats easily. For this reason, it is important to be careful during the panning. If the panning is perform with samples taken in a placer or any place without performing a classification or size reduction, the opportunity of watching a gold nugget is high and the coarse material must be rejected carefully, otherwise some free gold will be lost. For this reason is important to prepare the sample by crushing and screening operations. In other words, if the sample has homogeneous size distribution, losses are reduced.
It is important to mention that gold particles can be found in nature under different presentation such as large pieces coarser than 2.0 mm; fine particles, 2 – 100 µm, microcrystalline particles, 100 – 1 µm; sub-microcrystalline particles, associated with sulphides; and tellurium compounds.
A sluice is defined as a channel through which controlled amounts of diluted slurry. Sluice box and riffles are one of the oldest forms of gravity separation equipment. The size of this equipment is variable and range from small and portable metallic models to large units. Small models are employed for prospecting purposes. In general, sluices can be made of wood, aluminum, steel or plastic. A typical sluice section is 360 cm long and 30 cm wide. As a rule, a long narrow sluice is more efficient than a short wide one. The sluice must slope 10 to 30 cm per 360 cm (12 feet) depending on the water available, the coarse particle to be processed and the possible gold particle size.
To perform efficiently, a sluice needs large amounts of water. Enough water must be added to the feed to build up a sand bed in the bottom of the sluice. For maximum recovery, the flow should be turbulent, yet not forceful enough to wash away the sand bed. Russian studies have shown that recovery increases with the frequency of cleaning. For clean up, clear water is run through the sluice until the riffles are clear of gravel. A pan or barrel is placed at the discharge end to prevent loss of concentrate. Starting from the head of the sluice, riffles are carefully washed into the sluice box. According to the model, any bottom covering is removed and washed into a separate container. Clean up continues until all riffles are washed. Large particles of gold should be removed by hand, and then the concentrate is washed out of the sluice or dumped into an appropriate container. The concentrate can be smelted or cleaned more by panning. After clean up, the sluice is reassembled and more ore is processed.
Schematic view of sluice
This equipment is known as Cradle. After the gold pan, rockers have been employed to evaluated placers in order to obtained concentrates with free gold. The equipment is good to work in small scale and can be operated easily in the same place or any lab. A rocker needs water and is necessary to be sure about the water source. One to three cubic yards, can be processed in a rocker per man-shift, depending on the water source, the character of the sample, and the size of the equipment. Basically, rockers are considered a small sluice and can vary in shape and size. They are 60 to 150 cm in length and 30 to 60 cm in width. The height is variable ranging from 15 to 60 cm. At the top, a screen is installed in order to reject coarse particles.
The rocker has three parts, screen, apron and sluice. The main body contains riffles that retain heavy particles. Screens aperture is 12 mm. Fine particles pass the screen and go onto the apron that feeds the material to the inclined surface with riffles (sluice). The rocker must be operated carefully in order to avoid loss of fine gold. With this machine, the washing task is not very tedious. However, to remove material trapped by riffles can be a little annoying.
The material is fed into the screen; the operator sits by the side the equipment and shakes it, at the same time water is added in order to wash and disintegrate any lumps formed on the screen. All the material minus 12 mm is transported on the apron that is equipped with a blanket so that fine gold can be trapped, and the other particles are transported on the riffled section. Technically, coarse, free gold particles and heavy material are trapped. By the shaking motion, lumps are disintegrated and screen apertures are not blocked, otherwise gold and heavy particles would be lost.
Ideally, two operators are required to operate the equipment and the water addition must be continuous and the flow has to be appropriated in order to don’t have excessive turbulence. Approximately the amount of water to be required is three to five time the weight of material to be processed. In general, the water addition depends on the mineralogical properties of the material to be tested.
Schematic view of rocker
- Long Tom
This equipment is basically a rocker with two sluices distributed in two levels. The first sluice is approximately 2.5-3.5 cm long and 40-50 cm wide. The second sluice is similar and the distance between both sluices is 15-20 cm. The design considers the lower end of the first sluice at an angle of 40-45° and connected with a screen which rejects coarse material and the fine material pass to the second sluice. Some designs consider the first sluice without riffle and others include riffles in both sluices.
During the operation the ore is fed in slurry form. If the material is too big, it must be removed manually and lumps have to be disintegrated. Basically, the equipment is able to concentrate coarse and free gold particles. The material can be fed by using a shovel and adding water, but the operation must be done carefully so that the equipment can’t be overcharged and the gold losses can be minimized. For this reason, to feed homogenous slurry is a good option. Similarly to the rocker, the water flow is a key element in the operation and it is extremely important to maintain a constant flow through the sluices. The second sluice must trap most the gold and sometimes during the process is necessary to clean the riffles in order to remove the heavy material.
The heavy material can be cleaned by using a gold pan in order to improve the concentrate quality or sent to the lab for gold and silver assays. It is important to perform a mass balance so that we can know the metallurgical performance.
There are other equipments similar to the rocket and were used to reduce the water consumption or perform concentration an amalgamation at the same time. Some of these old equipments are the Pudding-Tub and the Siberian Trough. They work under the same principle as the rocker and the operation performance is related to the operator’s skill.
Schematic view of Long Tom
- Flat Spiral
Sometimes, when there is not enough weight of material to perform many concentration tests, the metallurgist has to look for small equipments. The Flat Spiral is a good option and was developed based on the Humphreys spiral that was created a long time ago. The equipment is a rotary circular plate with a spiral on its surface. The plated is tilted and supported on a structure and the water is added from one side to the center. During the operation the material is fed on one side and the tails are discharged at the bottom and the concentrate is going to the center and discharge by a hole located in the center of the circular plate.
The equipment operates based on the action of gravity and hydraulic forces created by the rotation of the spiral and the water addition. Essentially, during the rotation, the heavy material is forced to pass into the canals. When the water is added the concentrate is cleaned and the lighter particles are directed to the tails.
The material to be tested by the equipment must be fine, approximately 100% passing 600 µm. Try to pass coarser material is more difficult and the separation efficiency and operatively of this equipment are reduced. Some manufacturers of this kind of equipment modify the number of spirals and the number. By doing this change the route to be followed by the heavy material is modified by the time exposed to water flow.
Schematic view of Long Tom
- Dry Washer
If the concentration tests have to be performed in the same field and the water is not abundant, the Dry Washer is an interesting alternative in order to prospect and evaluate preliminary a deposit. Basically the equipment is a rocker or sluice without water addition and the particles are fluidized by adding air under certain pressure and flow. The equipment has a screen that classify the material to be concentrated, coarse particles are separated and the fine particles pass to the riffled section, which received a pneumatic action to separate light and heavy particles. The riffled section is equipped with bellows. Basically the equipment receives a shaking and classification action that separates the particles according to their specific gravity and shape. Dry washer separates gold from sand by pulsations of air through a porous medium.
One condition very important at the moment of employing this equipment is to be sure that the material is totally dry and there are not lumps. If the material does not present both conditions, it is necessary to prepare the sample. The bellows are a very important part and the operator must assure the appropriate addition of air. Approximately, the bellows operate at 200-250 strokes per minute and the final value is obtained during the test because all the materials have different minerals and the separation parameters are not the same.
When all the material was processed, the heavy particles deposited on the riffles must be removed and cleaned by using a panning dish. It is a good idea to pan each riffle separately in order to compare the concentration action on the riffled area.
Schematic view of Dry Washer
- Centrifugal concentration
There are two centrifugal concentrators employed in metallurgical tests and industrial plants, the Knelson and Falcon. The first one is a centrifugal concentrator can recover fine particles of free gold. The pulp is screened in order to avoid coarse particles such as 4 mm or more. The material in slurry form is fed centrally to the bottom of a ribbed inner cone rotating at a high speed. Centrifugal force causes the first feed to fill the bottom rib with solids and the next higher slightly larger diameter rib with solids and the next higher slightly larger diameter rib and subsequent ones are sequentially filled in few seconds. After that all the feed moves upward as thin film over the conical surface of the sand filled ribs and out. Compaction of solids trapped in the ribs of the cone is prevented by adding water through a series of holes in the cone wall. In this way, the trapped solids are fluidized.
Laboratory Knelson Concentrator Laboratory Falcon Concentrator
The Knelson concentrator employs the principles of hindered settling and centrifugal force. A central perforated cone containing horizontal ribs is rotated in order to get a centrifugal force of 60g (g is gravity acceleration). The cone is surrounded by a pressurized water jacket that forces water through holes to keep the bed of heavy particles fluidized. In this way, the force of the water acts against the centrifugal force of the rotating cone.
If there are free gold particles or particles transporting gold in the film of slurry that is passing over the fluidized solids trapped in the ribs of the cone, they will penetrate the fluidized bed under enhanced gravity situation and displace lighter particles of similar volume trapped in the rib. This process is repeated several times until saturate the ribs with heavy particles. In other words, this is a trading process that exchange light particles for heavy particles.
The Falcon concentrator consists of a fluidized bed spinning bowl gravity concentrator. It is has been designed primarily for gold recovery from grinding circuit hydrocyclone underflows or alluvial operations. The concentrator is capable of fine and coarser feed applications. Enhanced gravity concentrators accelerate feed slurries in a rotating cone shaped bowl to take advantage of the difference in density of valuable minerals and gangue in order to perform a separation.
The material is stratified according to specific gravity and then passed over a concentrate bed fluidized from behind by pressure water. The fluidized bed is required to retain coarse particles. Free gold or gold bearing minerals are retaining in the concentrate bed in preference to light minerals. Initially the design considered non-continuous operation, but with the years the design was improved and this centrifugal concentrator can work in continuous or not. There are several types of equipments to each need. The equipment functions and rising cycle are automated with variable frequency drive device.
Generally, the higher the field or the stronger separation gravity forces between different particles of different density, the more fast and efficient is the separation. Within limits, an enhanced gravity concentrator can treat more material and recover finer particles if it is spun faster. Falcon concentrators are able of centrifugal fields of 300g. The equipment can change this value due to the unit is equipped with a variable speed motor. Concentrating surface area is the single most important determinant for enhanced gravity concentrator. Similarly to jigs or shaking tables, the larger the surface area, the higher the overall capacity since performance is a function of residence time.
Many metallurgical laboratories have both concentrators and ideally the testing program can evaluate the performance obtained by using a Knelson or Falcon Concentrator according to the ore to be tested. It is important to mention that minerals present in different samples modify the results to be obtained. Results give a preliminary idea and the final test must be carried out with 20 – 30 kilos whose feed was done at steady state. Laboratory tests are done with 3-5 kilos and the reproducibility of the results is sometimes difficult. For this reason the final test is done with more material.
Centrifugal Concentrate cleaned in vanning dish
- Flotation tests
During the testing program is important to evaluate several concentration processes in order to get the highest gold recovery. Gravity concentration tests give a partial recovery and the sample must be tested again by other method in order to study the global performance. Considering the fact that gravity equipment is able to recover free gold particles, fine particles need to be recovered by other techniques. Froth flotation is an old process that allows recovering fine particle of free gold or associated with sulphides.
Laboratory Flotation Machine
Gold concentration by flotation is opened up due to there are several factors that can modify the surface mineral with the assistance of reagents which make the valuable particles floatable. Collectors and frothers are necessary to make possible the concentration. The adsorption of collectors may be influenced by adding other reagents which can be either supportive or inhibitive in effect. In these cases is common to make reference to activators and depressants. Air is added to transport the particles to the surface. Other reagents can be added when the non-valuable minerals interfere with the process.
The first part of the program includes rougher kinetics tests conducted on the gravity separation tailings or the whole sample. By these tests is possible to evaluate the effect of primary grind product, pH, collectors, modifiers and frothers on gold recovery. Sometimes is critical to select the collector, but as base line is common to select Potassium Amyl Xanthate (PAX) as the main collector and a Dithiophosphate (e.g. A-208, A-404) as a secondary collector. The frother can be MIBC and the first tests can be done at natural pH. PAX is selected because is a strong collecting agent whose action is sometimes enhanced by a secondary collector. If there aren’t frothing problems, MIBC is a good option, otherwise a strong and less selective frother such Pine Oil or Dowfroth D-250 can be tested. Since lime tends to affect gold flotation, to perform flotation tests at natural pH is the first option. About the particle size to be evaluated, it is common to evaluate flotation at 150, 125, 100, 90, 80 and 75 µm K80. Results indicate the most appropriate particle size.
Flotation tests by using a mechanical skimmer
Based on rougher flotation results, the next stage is to evaluate the fineness of regrind in the cleaning circuit. Since the rougher concentrate contains non-liberated particles is necessary to evaluate the effect of regrind on gold concentrate quality. Also, sometimes, the concentrate is reprocessed by cyanidation and this process is performed at 85-95% passing 75µm. Then, this evaluation has two important effects for the process. In this way, is common to evaluate the cleaning circuit at 70, 50, 45, 40, 35, 30, and 25 µm. Then, the program must try to find the optimum particle size. If the design consider gravimetry and flotation, the particle is not necessary to be very fine, but is must be enough to assure the separation among valuable minerals and gangue.
Some programs are based on experimental design techniques that try to evaluate several conditions and specific targets. In this way, the program can be focused in getting recoveries as high as 90% and concentrates with high content of precious metals. Tests are selected under specific conditions and results are evaluated mathematically. Obviously, the metallurgist must be able to judge these results against the reality due to the numbers are an indication and need an interpretation.
- Cyanidation tests
Auriferous deposits present different mineralogical and metallurgical characteristics that affect the concentration process to be selected. Some basic parameters to be considered are to identify gold mineralogy, gangue, gold content, gold particle size, and mineralogical associations. Based on these factors, free and coarse gold particles are recovered by gravity, and fine particles are recovered by flotation and/or cyanidation. The latter is perhaps the last option and the evaluation must be done methodically.
In this way, to obtain a high gold extraction by cyanidation, the testwork program must evaluate the particle size, leaching time, the presence of substances or compounds that retard the cyanidation such as oxidized copper minerals or organic matter.
There are several cyanidation processes and all the testing programs start with agitated leaching tests by using a mechanical stirrers or roller drivers to perform bottle tests. Typically these tests are performed at 80-85% passing 75µm and the ore is leached in slurry form (30-33% solids). The variables to be evaluated are sodium cyanide dosage, pH, leaching time, and oxygen level. For example the program can evaluate 0.5, 1.0, 1.5, 3.0, and 6 gpl of NaCN, the pH can vary from 10.0 to 11.5. Tests can last 48 or 72 hours, and the effect of chemical reagents such as hydrogen peroxide or lead nitrate on the leaching rate.
Leaching test by agitation Checking leaching pH
During the tests is common to take leaching solution samples in order to know the initial gold dissolution. At this moment, the researcher must determine the free cyanide and make the adjustments in order to have the initial dosage. The same idea is valid for lime or other chemical reagent.
At the end of the tests, the pregnant solution and residue are separated by filtration, and the residue is weighed and dried. Solution and residue are assayed for gold and silver. The solution is analyzed by atomic absorption and the residue by fire assay. With these results the balance is calculated. If during the tests was noted something strange such as the presence of copper minerals, assays must include copper so that we can know its effect on cyanidation. Low gold recoveries are due to the presence of oxidized copper minerals or gold liberation was not obtained. If the gold is refractory to cyanidation, recovery is low and the ore is a problem because needs special treatment previous to cyanidation.