Unit operations
Contenido
El procesamiento de minerales puede involucrar cuatro tipos generales de operación unitaria:
- Conminución") – reducción del tamaño de las partículas;
- Dimensionamiento – separación de tamaños de partículas mediante cribado&action=edit&redlink=1 "Cribado (ingeniería) (aún no redactado)") o clasificación;
- Concentración aprovechando las propiedades físicas y químicas de la superficie; y
- Eliminación de agua – separación sólido/líquido.
En todos estos procesos, las consideraciones más importantes son los aspectos económicos de los procesos, que están dictados por la ley y la recuperación del producto final. Para hacer esto, es necesario considerar la mineralogía de la mena, ya que esto dicta la cantidad de liberación requerida y los procesos que pueden ocurrir. Cuanto más pequeñas sean las partículas procesadas, mayor será la ley y la recuperación teóricas del producto final. Sin embargo, esto se vuelve difícil de hacer con partículas finas, ya que evitan que se realicen ciertos procesos de concentración.
Comminution
Comminution") is the reduction in the particle size of materials. Comminution can be carried out on dry or suspended materials. The primary comminution processes are crushing and grinding. Crushing is normally carried out on the ore as it arrives from the mine,[15] while grinding (which is normally carried out after crushing) can be carried out on dry or suspended material. In comminution, the reduction in particle size is It is carried out by three types of forces: compression, impact and friction. Compression and impact forces are widely used in crushing operations, while friction is the dominant force in grinding. The equipment mainly used in crushing includes jaw crushers, gyratory crushers and cone crushers, while rod mills and ball mills, usually in closed circuit with a classifying unit, are commonly used for grinding purposes in a mineral processing plant. that grinding is generally done wet, so it consumes more energy.
Sizing
Sizing is the general term for separating particles based on their size. The simplest sizing process is screening, which consists of passing particles through one or more screens. Screening equipment may include grates,[16] bar screens, wedge wire screens, radial screens, banana screens, multi-deck screens, vibrating screens, fine screens, flip flop screens, and wire mesh screens. Screens may be static (typically for very coarse material) or may incorporate mechanisms to shake or vibrate the screen. Some considerations in this process include the screen material, opening size, shape and orientation, number of close-sized particles, addition of water, amplitude and frequency of vibrations, angle of inclination, presence of harmful materials such as steel and wood, and particle size distribution.
Sorting refers to sizing operations that exploit differences in sedimentation velocities exhibited by particles of different sizes. Sorting equipment may include ore classifiers, gas cyclones, hydrocyclones, rotary trommels, rake classifiers, or fluidized classifiers.
An important factor in comminution and sizing operations is the determination of the particle size distribution of the processed materials, commonly known as granulometric analysis. Many techniques are used to analyze particle size, and the techniques include both off-line analysis that requires taking a sample of the material for analysis, and on-line techniques that allow analysis of the material as it flows through the process.
Concentration
There are several ways to increase the concentration of the desired minerals: in any particular case, the method chosen will depend on the physical and chemical properties of the mineral and gangue surface. Concentration is defined as the number of moles of a solute in a volume of solution. In the case of mineral processing, concentration means increasing the percentage of the valuable mineral in the concentrate.
Concentration by gravity
Gravity separation is the separation of two or more minerals of different specific gravity by their relative motion in response to the force of gravity and one or more additional forces (such as centrifugal forces, magnetic forces, buoyancy forces), one of which is the resistance to movement (drag force) by a viscous medium, such as heavy media, water, or, less commonly, air.
Gravity separation is one of the oldest techniques in mineral processing, but has seen a decline in use since the introduction of methods such as flotation, sorting, magnetic separation and leaching. Separation by gravity dates back to at least 3000 BC. C. when the Egyptians used the technique to separate gold.
It is necessary to determine the suitability of a gravity concentration process before using it for the concentration of an ore. The concentration criterion is commonly used for this purpose, designated as in the following equation (where represents the specific gravity):.
Although the concentration criterion is a useful rule of thumb in predicting amenability to gravity concentration, factors such as particle shape and the relative concentration of heavy and light particles can dramatically affect separation efficiency in practice.
There are several methods that use differences in weight or density of particles:[17].
These processes can be classified as separation by density or separation by gravity (weight). In separation in dense media, a medium is created with a density intermediate between the density of the ore and gangue particles. When subjected to this medium, the particles float or sink depending on their density relative to the medium. In this way, the separation is carried out solely by differences in density and is not based, in principle, on other factors such as the weight or shape of the particles. In practice, particle size and shape can affect separation efficiency. Separation in dense media can be performed using a variety of media. These include organic liquids, aqueous solutions, or suspensions of very fine particles in water or air. Organic liquids are not commonly used due to their toxicity, difficulties in handling, and relative costs. Industrially, the most common dense medium is a suspension of fine particles of magnetite and/or ferrosilicon. An aqueous solution is used as a dense medium in coal processing in the form of a Belknap scrubber, and air suspensions are used in water-scarce areas, such as some areas of China, where sand is used to separate coal from gangue minerals.
Gravity separation is also called relative gravity separation, as it separates particles due to their relative response to a driving force. This is controlled by factors such as the weight, size and shape of the particles. These processes can also be classified into multi-G and single-G processes. The difference lies in the magnitude of the driving force for separation. Multi-G processes allow separation of very fine particles (in the 5 to 50 micron range) to occur by increasing the separation driving force to increase the rate at which particles separate. In general, single-G processes are only capable of processing particles larger than about 80 microns in diameter.
foam flotation
Flotation is an important concentration process. This process can be used to separate two different particles and works through the surface chemistry of the particles. In flotation, bubbles are introduced into a pulp and the bubbles rise through the pulp.[19] In the process, hydrophobic particles adhere to the surface of the bubbles. The driving force for this adhesion is the change in surface free energy when adhesion occurs. These bubbles rise through the suspension and are collected from the surface. To allow these particles to adhere, the chemistry of the pulp needs to be carefully considered. These considerations include pH, Eh, and the presence of flotation reagents. pH is important because it changes the charge on the surface of the particles and pH affects the chemisorption of collectors on the surface of the particles.
The addition of flotation reagents also affects the operation of these processes. The most important chemical added is the collector. This chemical adheres to the surface of the particles since it is a surfactant. The main considerations in this chemical are the nature of the head group and the size of the hydrocarbon chain. The hydrocarbon tail must be short to maximize selectivity to the desired mineral and the head group dictates which minerals it attaches to.
Foaming agents are another important chemical addition to the pulp or slurry, as they allow the formation of stable bubbles. This is important because if the bubbles coalesce, the minerals will break away from their surface. However, the bubbles should not be too stable, as this makes transport and dehydration of the concentrate formed difficult. The mechanism of these foamers is not completely known and more research is being done on their mechanisms.
Depressants and activators are used to selectively separate one mineral from another. Depressants inhibit the flotation of one or more minerals, while activators allow the flotation of others. Examples of these include CN, used to depress all sulfides except galena, and this depressant is believed to act by changing the solubility of chemisorbed and physisorbed collectors in sulfides. This theory originated in Russia. An example of an activator is the Cu ion, used for the flotation of sphalerite.
There are several cells that can be used for mineral flotation, including flotation columns and mechanical flotation cells. Flotation columns are used for finer minerals and typically have a higher grade and lower mineral recovery than mechanical flotation cells. Cells currently in use may exceed 300 m. This is done because they are cheaper per unit volume than smaller cells, but they cannot be controlled as easily as smaller cells.
This process was invented in the century in Australia. It was used to recover a sphalerite concentrate from tailings produced by gravity concentration. Further improvements have emerged in Australia in the form of the Jameson Cell", developed at the University of Newcastle, Australia. This operated by using an immersion jet that generated fine bubbles. These fine bubbles have a higher kinetic energy and, as such, can be used for the flotation of fine-grained minerals, such as those produced by the IsaMill.
electrostatic separation
There are two main types of electrostatic separators. These work in a similar way, but the forces applied to the particles are different and these forces are gravity and electrostatic attraction. The two types are electrodynamic separators (or high voltage rollers) or electrostatic separators. In high voltage rollers, the particles are charged by a corona discharge. This charges the particles which subsequently travel over a drum. The conductive particles lose their charge in the drum and are removed from the drum with acceleration. centripetal. Electrostatic plate separators work by passing a stream of particles") across a charged plate. The conductors lose electrons to the plate and are pulled away from the other particles due to the induced attraction towards the plate. These separators are used for particles between 75 and 250 microns and for efficient separation to occur, the particles must be dry, have a close size distribution, and be uniform in shape. Of these considerations, one of the most important is the water content of the particles. This is important as a layer of moisture on the particles will cause the non-conductors to act as conductors since the layer of water is conductive.
Electrostatic plate separators are generally used for currents that have small conductors and not thick conductors. High voltage rollers are generally used for currents that have thick conductors and not fine conductors.
These separators are commonly used to separate mineral sands"), an example of one of these mineral processing plants is the CRL processing plant at Pinkenba in Brisbane, Queensland. In this plant, zircon, rutile and ilmenite are separated from silica gangue. In this plant, separation is carried out in several stages with primary separators, cleaners, recoverers and recleaners.
Magnetic separation
Magnetic separation is a process in which magnetically susceptible material is extracted from a mixture using magnetic force. This separation technique can be useful in mining iron, as it is attracted by a magnet. In mines where wolframite was mixed with cassiterite, as at the South Crofty and East Pool mine in Cornwall, or with bismuth, as at the Shepherd and Murphy mine in Moina, Tasmania, magnetic separation was used to separate the ores. A device called the Wetherill magnetic separator (invented by John Price Wetherill, 1844–1906) was used in these mines. In this machine, the raw ore, after calcination, was fed to a conveyor belt that passed under two pairs of electromagnets, under which other belts ran at right angles to the feeder belt. The first pair of electromagnets was weakly magnetized and served to remove any iron ore present. The second pair was strongly magnetized and attracted wolframite, which is weakly magnetic. These machines were capable of treating 10 tons of ore per day. This process of separating magnetic from non-magnetic substances in a mixture with the help of a magnet is called magnetic separation.
This process operates by moving particles in a magnetic field. The force experienced in the magnetic field is given by the equation f=m/k.H.dh/dx, where k=magnetic susceptibility, H=magnetic field intensity, and dh/dx is the gradient of the magnetic field. As seen in this equation, separation can be driven in two ways, either through a gradient in a magnetic field or the intensity of a magnetic field. Different driving forces are used in different concentrators. These can be with or without water. As in the spirals, the washing water helps in the separation of the particles, while increasing the drag of the gangue in the concentrate.
Automated ore classification
Modern automated sorting applies optical sensors (visible spectrum, near-infrared, X-ray, ultraviolet), which can be coupled with electrical conductivity and magnetic susceptibility sensors, to control the mechanical separation of ore into two or more categories on a rock-by-rock basis. New sensors have also been developed that exploit material properties such as electrical conductivity, magnetization, molecular structure and thermal conductivity. Sensor-based sorting has found application in the processing of nickel, gold, copper, coal and diamonds.
Dehydration
Dehydration is an important process in mineral processing. The purpose of dehydration is to remove water absorbed by the particles, which increases the density of the pulp. This is done for several reasons, specifically, to facilitate ore handling and allow concentrates to be easily transported, allow additional processing to be performed, and remove gangue. Water extracted from ore through dehydration is recirculated for plant operations after being sent to a water treatment plant. The main processes used in dewatering include dewatering screens, sedimentation, filtration and thermal drying. These processes become more difficult and expensive as particle size decreases.
Dewatering screens operate by passing particles over a screen. Particles pass over the sieve while water passes through the sieve openings. This process is only viable for coarse ores that have a close size distribution, as the openings can allow small particles to pass through.
Sedimentation operates by passing water into a large thickener or clarifier. In these devices, particles settle out of suspension under the effects of gravity or centripetal forces. These are limited by the surface chemistry of the particles and the size of the particles. To assist in the sedimentation process, flocculants and coagulants are added to reduce repulsive forces between particles. This repulsive force is due to the double layer formed on the surface of the particles. Flocculants work by binding multiple particles together, while coagulants work by reducing the thickness of the charged layer on the outside of the particle. After thickening, the slurry is often stored in ponds or reservoirs. Alternatively, it can be pumped to a belt press or membrane filter press to recycle process water and create a dry, stackable filter cake, or "tails".[20].
Thermal drying is generally used for fine particles and to remove low water content in the particles. Some common processes include rotary dryers, fluidized beds, spray dryers, hearth dryers, and rotary tray dryers. This process is generally expensive to operate due to the fuel requirements of the dryers.