AGITATORS IN MINING AND MINERAL PROCESSING
Agitators are the mechanical equipment employed during the chemical processing of excavated ores whereby the valuable minerals and metals are extracted. Extractive metallurgy focuses on the processes and methods of extracting metals from their mineral deposits. Ferrous and Non-Ferrous extractive metallurgy have common specialties, which are categorized as follows Mineral processing, Hydrometallurgy, Pyrometallurgy and Electrometallurgy; according to the processes adopted to extract the metal. Certain metals undergo several extraction processes, which are determined by the metals occurrence as well as chemical requirements.
The exploration and extraction of gold, platinum, nickel, copper and uranium alongside other base metals, provides for a large variation of different process applications where the agitators are carefully calculated and designed. Ensuring that the individual processes are understood, AFX engineers have spent time on site and continuously work closely with process engineers that we offer the best solutions.
AFX has an exceptional track record throughout mining and mineral processing. Our attention to detail as well as the years of accumulated knowledge has allowed AFX to become the preferred supplier of specialized
SX pumper mixers, Bioleaching, Cyanide Detoxification, Leach and CIP/ CIL, Iron Removal, High-pressure Autoclave agitators, Backfill agitators and many other applications requiring unique solutions.
Although the spans of mining and mineral processing are more extensive than what we have listed below, we have elected only a few specialized and specific processes below to highlight not only the extensive knowledge around the process application, but to offer peace of mind relating to our agitator and engineering solutions.
All metallurgical treatments have a common outcome namely the extraction of the mineral ore from the rock. The first step in the circuit would be the accurate grinding, comminution, into small particles.
GOLD EXTRACTION & PROCESSING
Principally, gold occurs as a native metal usually alloyed with silver as an electrum or with mercury as an amalgam. Gold may occur as sizeable nuggets, fine grains or flakes in alluvial deposits or microscopic particles embedded in rock minerals. Although comparatively rare, gold occurs in chemical compositions with other elements such as calaverite, sylvanite and petzite to name a few.
Historically, the extraction of the metal relied on gravity concentration, where simple tools such as pans and wash tables were used. Gravity concentration could be directly smelted too for gold bars, if the gold was present in the ore as coarse particles.
In todays’ era, which is continually demanding maximum yield and concentration from the mining process, there are a series of chemical procedures employed to extract the gold from the ore.
This is commonly used metallurgical extraction processing method. The process itself falls into the Hydrometallurgy category, which is the use of an aqueous solution to extract the gold from the ore. Most leaching applications sparge oxygen into the leach tanks. Cyanide is a lixiviant, which is used to leach gold from solid matrixes to form a gold-cyanide complex. The concentration and dose of cyanide used is very low in comparison to the live liquid volume in the tank. The pH of the resulting slurry would be carefully controlled by means of adding lime or another alkali ensuring that the cyanide ions do not change into toxic cyanide gas. AFX carefully calculate and design the required agitators for the process, knowing that the slurry requires the solids to be in full suspension throughout the tank for the chemical process to efficiently take place. The agitator will also ensure the thorough distribution of the alkali chemical(s) being dosed into the mixing environment. AFX are able to offer advice on the sizing and installation configuration of baffles within the mixing tanks, which are vital for the mixing process. An adsorption process is employed after cyanide leaching, where activated carbon or an organic carbon maybe used, depending on the ore.
The below processes refer either to a particular process in a metallurgic extraction or through various metallurgical applications.
CIL and CIP
Adsorption is most commonly recognized as Carbon in Leach (CIL) or Carbon in Pulp (CIP) the processes use activated carbon for the gold to leach onto.
CIL is a process of cyanidation as the carbon is added into the leach or reactor tanks. Leaching as well as adsorption will take place in the same leach tanks. CIL is the simultaneous process of leaching and adsorption. This process was specifically developed for the processing of gold ores containing “preg-robbing” materials i.e. organic carbon. These materials reduce the gold yield by attracting the gold meant for the activated carbon. The employment of leaching and adsorption assists in minimizing this problem.
CIP differs from CIL in that the leaching process and the adsorption process take place in separate reactors or tanks. During the CIP process, pulp flows through several agitated tanks. The addition of oxygen and sodium cyanide is done prior to the CIP circuit so in order to dissolve the gold into a solution, this is commonly known as gold dissolution. Activated carbon is present in the adsorption tanks, which the gold adsorbs onto. The activated carbon flows counter currently to the pulp. Separation screens sort the barren pulp from the gold loaded carbon.
Absorption onto activated carbon is commonly categorized as recovery. Preceding recovery one would expect to encounter solution concentration, stripping or electro-winning.
AFX have a countless number of successful installations in Leach, CIL and CIP applications worldwide, using our F3 hydrofoil impeller system, our agitators are robust in design and mechanically sound to withstand the harsh operating conditions some of these plants undergo.
Bioleaching is a method of extracting metals from their ores with the use of living organisms. Bioleaching is a cleaner and more environmentally accepted means of extraction when compared to traditional cyanide leaching. Bioleaching is categorized under biohydrometallurgy and can be used in the recovery of copper, zinc, lead, gold silver, cobalt and nickel to name a few. Bioleaching also known as Bio-oxidation, uses bacteria for the further oxidization of the ore. The bacteria oxidize the ore, whilst regenerating the chemical oxidant. Bioleaching or Bio-oxidation applications require the induction of oxygen into the process tanks, as the bacteria use the oxygen in order to carry out their required oxidizing tasks. As exploration into bioleaching’s possibilities has expanded, AFX working hand in hand with specialist, have been at the forefront of innovative solutions. In order to deliver the most calculated solution for optimal output results. AFX designed and commissioned pilot plants, which were used for testing. Once the testing was completed and the client satisfied with the results of the agitators and spargingequipment, AFX was awarded the contract for the full-scale plant.
Pressure oxidation, POX, can be employed for the leaching of ores and concentrates in the mining and mineral processing industry. Autoclaves operate under pressure as well as high temperature conditions in the presence of pure oxygen gas being required in order for the chemical reactions to take place. Efficient oxygen uptake as well as dispersion throughout the autoclaves is of utmost importance. AFX have spent time focusing on a mechanical solution of the autoclave systems. A dual agitator impeller system, with a P4 and P3 impeller are installed with a mechanical seal, housed in a pedestal. The mechanical seal is rated to withstand the high temperatures as well as pressure of the autoclave systems. AFX provide the solution, which has a proven industry track record throughout the mining and mineral processing groups, internationally.
Solvent extraction is the process used to recover base metals and uranium from solutions generated in atmospheric leaching reactors, as well as in pressure oxidation autoclaves. This solution is always referred to as the pregnant leach solution (PLS) and contains copper, nickel, uranium and other minerals in the solution. The metals are removed from the solution using the solvent extraction (SX) process. The SX plant consists of three or four mixer or settler units which is a mixing tank, attached to a large circular or rectangular settler.
The organic solvent used in the application, extracts the metal from the solutions and at the same time concentrates it into a much smaller volume, similar to the gold CIP process, where the gold absorbs onto the carbon. As an example, copper PLS is contacted with an organic solvent in the extraction settlers of the SX plant. The extraction is normally carried out in three to four stages, with the two-phase (PLS aqueous and organic solvent) moving counter currently. The two phases need to be mixed thoroughly with the organic solvent to extract from the metal. The aqueous phase gravitates, while the organic is pumped. Extraction mixers operate in the organic phase. The two phases then separate in the settlers, with the lighter organic phase being the top layer. The next stage is the scrubber mixer-settlers, where the solvent is washed with a low pH solution to remove impurities like iron and silica. Similarly, in the strip mixer-settler units, the Copper is stripped from the organic again, using high concentration sulphuric acid solution. This solution (electrolyte) is then pumped to the electrowinning plant to produce pure Copper cathodes.
PERISTALTIC PUMPS IN MINING
Mining operations challenge most conventional pumps. Handling abrasive and corrosive fluids typically used in mining operations presents a challenge to any pump manufacturer. High solids content and strong acidity create problems for diaphragm, centrifugal or other types of pumps where the product comes in contact with the working parts of the pump.
To overcome these problems, mine operators have had to purchase specialised pumps, often constructed from acid-resistant and wear resistant materials or put up with frequent, costly pump maintenance or replacement. Often rotors or impellers on slurry pumps last only weeks and diaphragm pumps clog, leak or fail after only a few months.
Replacing centrifugal slurry pumps with peristaltic hose pumps makes economic sense. High density thickener underflow slurries are too high to allow centrifugal pumps to deliver the correct flow rate and abrasive wear causes regular costly repairs. Because abrasives in the slurry do not affect peristaltic hose pumps, mine operators are now able to reduce downtime and achieve dependable pump operation at the required flow rate. Often peristaltic hose pumps were overlooked as being “low flow”. The AFX150 pump however has one of the highest flowrates in the world at 150m3/hr.
Because peristaltic hose pumps contain the fluid entirely within the hose, the hose is the only wearing part. The rubber hose is highly abrasion resistant and pumps can easily pump abrasive fluids like tailings, metal slurries and thickener underflow. Due to the high solids content of these slurries other types of pumps often fail because the product comes into contact with the rotors, stators, impellers and seals of the pump. In a peristaltic hose pump however, the hose never fails due to abrasion.
Unlike most rotary pumps, peristaltic pumps have no shaft seals to flush with water, coupled with the ability to handle slurries with high solids concentrations plants use much less water with peristaltic hose pumps, saving the plant considerable amounts in both maintenance and water usage. In the mining industry, water is money and the less you use the better. Flushing pump seals and diluting thickened slurries is incredibly costly to a mine because the water added has to be removed or treated.
AFX peristaltic pumps are designed to operate at slower speeds and have lower energy consumption than most traditional peristaltic hose pumps. Actual tests have seen up to 50% savings on power consumption.