Mineral Processing, Chemistry tutorial

Introduction:

Mineral processing is basically the art of treating crude ores and mineral products in order to separate the important minerals from the waste rock or gangue. This is the first method that most ores undergo after mining in order to give a more concentrated material for the methods of extractive metallurgy. The primary operations are communition and concentration, however there are other significant operations in a modern mineral processing plant, comprising sampling and analysis and dewatering. All such operations are illustrated in below.

What Is Ore?

An ore is a kind of rock which includes minerals with significant elements comprising metals which can be extracted from the rock at a profit. The ores are extracted via mining: these are then refined (often through smelting) to extract the valuable element(s). The grade or concentration of an ore mineral, or metal, and also its form of occurrence, will directly influence the costs related with mining the ore. The cost of extraction should therefore be weighed against the metal value contained in the rock to find out what ore can be processed and what ore is of too low a grade to be worth mining.

Basic Ore processing methods:

A) We feed ore to a sequence of crushers and grinding mills to decrease the size of the ore particles and expose the mineral. Water is as well added which turns the ore into slurry.

B) We drive this slurry to leaching tanks, where we add a weak cyanide solution to the slurry that leaches gold and silver to the solution. This method eliminates up to 93 % of the gold and 70 percent of the silver from the ore. Carbon granules are then added to the solution. The gold is pulled from the solution and links to the carbon.

C) We then 'strip' the gold from the carbon via washing it by a caustic cyanide solution. The carbon is recycled afterward.

D) Subsequently, we pump the bearing solution via electro-winning cells that extract metals from the solution by utilizing an electrical current.

E) Subsequent to gold has been processed; the leftover waste material is known as tailings. Tailings include small amounts of cyanide and other hazardous chemicals in such a way that they should be disposed of in an environmentally safe manner. The tailings are stored in tailings dams that are lined by impermeable layers. Whereas the cyanide levels in the dam are safe, steps are in use to keep wildlife away from the dams. Over time, the chemicals break-down and the solids settle to the bottom in such a way that the water can be returned to the plant to be utilized in processing.

F) We after that smelt, that melts it in a climate at around 2, 100°F.

G) We lastly send the bars to a refinery for further processing.

Analysis of Ore:

Sampling and Analysis:

The routine sampling and analysis of raw material being processed are assumed in order to get information essential for the economic evaluation of ores and concentrates. Moreover, modern plants have completely automatic control systems which conduct in-stream analysis of the material as it is being processed and make adjustments at any phase in order to generate the richest possible concentrate at the lowest possible operating cast.

Sampling:

Sampling is the elimination from a given lot of material a part which is representative of the whole yet of convenient size for the analysis. This is done either via hand or through machine. Hand sampling is generally expensive, slow and imprecise, so that it is usually applied only where the material is not appropriate for machine sampling (that is, slimy ore, for illustration) or where machinery is either not accessible or too costly to install.

Most of the different sampling devices are available, comprising shovels, pipe samplers and automatic machine samplers. For such sampling machines to give a precise representation of the whole lot, the quantity of a single sample, the total number of samples and the type of samples taken is of decisive significance. The number of mathematical sampling models has been work out in order to arrive at the suitable criteria for sampling.

Analysis:

Subsequent to one or more samples are taken from an amount of ore passing via a material stream like a conveyor belt, the samples are decreased to quantities appropriate for further analysis. Analytical methods comprise chemical, mineralogical and particle size.

=> Chemical analysis:

Even prior to the 16th century, comprehensive schemes of assaying (that is, measuring the value of) ores were recognized, by using methods which don't differ materially from such used in modem times. However, the conventional processes of chemical analysis are employed nowadays to detect and estimate the quantities of elements in ores and minerals; they are slow and not adequately accurate, specifically at low concentrations, to be completely appropriate for process control. As a consequence, to accomplish greater efficiency, sophisticated analytical instrumentation is being employed to the increasing level.

In case of emission spectroscopy, an electric discharge is established between the pair of electrodes, one of which is made up of the material being examined. The electric discharge vaporizes a part of the sample and excites the elements in the sample to emit feature spectra. Detection and measurement of the wave-lengths and intensities of the emission spectra disclose the identities and concentrations of the elements in the sample.

In X-ray fluorescence spectroscopy, a sample bombarded by X-rays gives off fluorescent X-radiation of wavelengths characteristic of its elements. The quantity of emitted X-radiation is associated to the concentration of individual elements in the sample. The sensitivity and accuracy of this technique are poor for the elements having low atomic number (that is, some protons in the nucleus, like boron and beryllium), however for slags, sinters, ores and pellets where the greater part of the elements are in the higher atomic number range, as in the case of lead and gold, the technique has been usually appropriate.

=> Mineralogical Analysis:

The successful separation of an important mineral from its ore can be found out via heavy-liquid testing, in which a single-sized fraction of a ground ore is suspended in the liquid of high specific gravity. The particles of less density as compare to the liquid remain afloat, whereas denser particles sink. Some of the different fractions of particles having the similar density (and, therefore, identical composition) can be generated, and the valuable mineral components can then be found out by the chemical analysis or via microscopic analysis of the polished parts.

=> Size Analysis:

The coarsely ground minerals can be categorized according to the size by running them via special sieves or screens, for which different national and international standards have been acknowledged.

One old standard (that is, now obsolete) was the Tyler Series, in which wire screens were recognized via mesh size, as measured in wires or openings per inch. Modern standards now categorize sieves according to the size of the aperture, as evaluated in millimeters or micrometers (10-6 meters).

The mineral particles smaller than 50 micrometers can be categorized by various optical measurement techniques that make use of light or laser beams of different frequencies.

Types of method for the processing of ore:

The mineral processing can comprise four general kinds of unit operation:

  • Communition
  • Particle size reduction
  • Sizing
  • The separation of particle sizes via screening or classification
  • Concentration by taking benefit of physical and surface chemical properties
  • Dewatering
  • Solid or liquid separation

In all of such techniques, the most significant suppositions are the economics of processes and this is stated by the grade and recovery of the final product. To do this, the mineralogy of ore requires to be considered as this state the amount of liberation needed and the methods that can take place. The smaller the particle's methods, the greater the theoretical grade and recovery of the final product. However this is difficult to do with fine particles as they prevent some concentration methods from taking place.

Communition:

Communition is the particle size reduction of materials. Communition might be taken out on either dry materials or slurries. Crushing and grinding are the two main Communition methods. Crushing is generally taken out on 'run-of-mine' ore, whereas grinding (generally carried out after crushing) might be conducted on dry or slurried material. In case of Communition, the size reduction of particles is done by three kinds of forces-compression, impact and attrition. Compression and impact forces are broadly employed in crushing operations whereas attrition is the dominant force in grinding. The mainly used equipment's in crushing ate-jaw crushers, gyratory crushers and cone crushers while rod mills and ball mills, closed circuited by a classifier unit, are usually used for grinding purposes in a mineral processing plant. The crushing is a dry method while grinding is usually performed wet and therefore is more energy intensive.

The simplest sizing method is screening, or passing the particles to be sized via a screen or number of screens. Screening equipment can comprise grizzlies, bar screens, wedge wire screens, radial sieves, banna screens, multi-deck screens, fine screens, vibratory screen, flip-flop screens and wire mesh screens.

Sizing:

Sizing is basically a general term for the separation of particles according to their size.

The simplest sizing technique is screening or passing the particles to be sized via a screen or number of screens. Screening equipment can comprise grizzlies, bar screens, fine screens, wedge wire screens, radial sieves, multi-deck screens, banna screens, vibratory screen, flip-flop screens and wire mesh screens.

Screens can be static (that is, generally the case for extremely coarse material), or they can incorporate methods to shake or vibrate the screen. Some of the considerations in this method comprises the screen material, the aperture size, shape and orientation, the amount of close to sized particles, the addition of water, the frequency and amplitude of the vibrations, the angle of inclination, the presence of harmful materials such as wood and steel, and the size distribution of the particles.

Categorization refers to the sizing operations which exploit the differences in settling velocities exhibited via particles of different size. Classification equipment might comprise ore sorters, hydrocyclones, gascyclones, rotating trommels, rake classifiers or fluidized classifiers.

The significant factor in both Communition and sizing operations is the determination of the particle size distribution of the materials being processed, generally termed to as particle size analysis. Most of the methods for examining particle size are employed and the methods comprise both off-line analyses that require that a sample of the material be taken for analysis and on-line methods which allow for analysis of the material as it flows via the process.

Concentration:

There are a number of manners to increase the concentration of the wanted minerals: in any specific case the technique selected will based on the relative physical and surface chemical properties of the mineral and the gangue. Concentration is stated as the number of moles of a solute in a volume of the solution. In case of mineral processing concentration signifies the raise of the percentage of the important mineral in the concentrate.

a) Gravity concentration:

This is the separation of two or more minerals of different specific gravity via their relative movement in response to the force of gravity and one or more other forces (like centrifugal forces, magnetic forces), one of which is resistance to motion (that is, drag force) via a viscous medium like heavy media or water.

This separation method is one of the oldest methods in mineral processing however has seen a decline in its use since the introduction of methods such as flotation, categorization, magnetic separation and leaching. Gravity separation dates back to at least 3000 BC if Egyptians utilized the method for separation of gold.

It is essential to find out the suitability of a gravity concentration process before it is used for the concentration of an ore. Criteria termed as concentration criterion is generally utilized for this purpose. It is stated as-Concentration Criterion (CC) = (SG of heavy mineral-SG of fluids (SG of light mineral-SG of fluid), here SG is the specific gravity.

For CC > 2.5, appropriate for the separation of particles above 75 micron in size.

For 1.75 < CC < 2.5, appropriate for the separation of particles above 150 micron in size.

For 1.50 < CC < 1.75, appropriate for the separation of particles above 1.7 mm in size.

For 1.25 < CC < 1.50, appropriate for the separation of particles above 6.35 mm in size.

For CC < 1.25, not appropriate for any size.

b) Electrostatic Separation:

There are two major kinds of electrostatic separators. These work in an identical ways, however the forces applied to the particles are different and such forces are gravity and electrostatic attraction. The two kinds are electro-dynamic separators (or high tension rollers) or the electrostatic separators.

In high tension rollers, particles are charged via a corona discharge. This charges the particles which afterward travel on a drum. The conducting particles lose their charge to the drum and are eliminated from the drum by centripetal acceleration. Electrostatic plate separators work via passing a stream of particles past a charged anode. The conductors lose electrons to the plate and are dragged away from the other particles because of the induced attraction to the anode. Such separators are employed for particles between 75 and 250 micron and for proficient separation to take place, the particles requires to be dry, encompass a close size distribution and uniform in shape. Of such considerations, one of the most significant is the water content of the particles. This is significant as a layer of moisture on the particles will turn into the non-conductors as conductors as the layer of the water is conductive.

Electrostatic plate separators are generally employed for streams that encompass small conductors and coarse non-conductors. The high tension rollers are generally utilized for streams which have coarse conductors and fine non-conductors. Such separators are generally employed for separating the mineral sands; an illustration of one of such mineral processing plants is the CRL processing plant at Pinkenba in Brisbane Queensland. In this plant, rutile, zircon and ilmenite are separated from the silica gangue. In this plant, the separation is carried out in a number of phases by roughers, cleaners, scavengers and recleaners.

c) Magnetic Separation:

The Magnetic separation is a technique in which magnetically susceptible material is extracted from the mixture by using a magnetic force.

This separation method can be helpful in mining iron as it is attracted to a magnet. In mines where wolframite was mixed by cassiterite, like South Crofty and East Pool mine in Cornwall or with bismuth like at the Shepherd and Murphy mine in Moina, Tasmania, magnetic separation was employed to separate the ores. At such mines a device termed as a Wetherill's Magnetic Separator (discovered by John Price Wetherill, 1844-1906) was utilized. In this machine, the raw ore, after calcination was fed to a moving belt that passed beneath the two pairs of electromagnets under which further belts ran at right angles to the feed belt. The first pair of electromagnets was weakly magnetized and served up to draw off any iron ore present. The second pair were strongly magnetized and attracted the wolframite that is weakly magnetic. Such machines were capable of treating 10 tons of ore a day. This method of separating the magnetic substances from the non-magnetic substances in a mixture by the help of a magnet is known as magnetic separation.

This method operates via moving particles in a magnetic field. The force experienced in the magnetic field is provided by the equation f = m/k. H. dh/dx having k = magnetic susceptibility, H-magnetic field strength and dh/dx being the magnetic field gradient. As observe in this equation, the separation can be driven in two manners either via a gradient in a magnetic field or strength of the magnetic field.

The various driving forces are utilized in the different concentrators. These can be either with water or without. Similar to the spirals, wash-water helps in the separation of the particles whereas raises the entrainment of the gangue in the concentrate.

Dewatering:

Dewatering is the significant technique in mineral processing. The main aim of dewatering is to eliminate water absorbed via the particles that increases the pulp density.

This is done for a number of reasons, particularly, to allow ore handling and concentrates to be transported simply, allow further processing to take place and to dispose of the gangue. The water extracted from the ore via dewatering is re-circulated for plant operations after being sent to the water treatment plant. The main techniques which are employed in dewatering comprise dewatering screens, sedimentation, filtering and thermal drying. Such techniques increase in difficulty and cost as the particle size reduces.

Dewatering screens operate via passing particles over the screen. The particles pass over the screen as the water passes via the apertures in the screen. This method is only viable for coarse ores which encompass a close size distribution as the apertures can let small particles to pass through.

Sedimentation operates via passing water to a large thickener or clarifier. In such devices, the particles settle out of the slurry under the influence of gravity or centripetal forces. These are limited via the surface chemistry of the particles and the size of the particles. To assist in the sedimentation method, flocculants and coagulants are added to decrease the repulsive forces between the particles. This repulsive force is because of the double layer made on the surface of the particles. The flocculants work via binding multiple particles altogether as the coagulants work via reducing the thickness of the charged layer on the outside of the particle.

Thermal drying is generally employed for fine particles and to eliminate low water content in the particles. Some of the common methods comprise rotary dryers, fluidized beds, spray driers, hearth dryers and rotary tray dryers. This method is generally expensive to operate because of the fuel necessity of the dryers.

=> Froth Flotation:

Froth flotation is the significant concentration method. This method can be employed to separate any two different particles and operated via the surface chemistry of the particles. In flotation, bubbles are introduced to a pulp and the bubbles increase via the pulp. In the process. Hydrophobic particles become bound to the surface of bubbles. The driving force for this attachment is the change in the surface free energy whenever the attachment takes place. Such bubbles rise via the slurry and are collected from the surface. To allow such particles to attach, careful consideration of the chemistry of the pulp requires to be made. Such considerations comprise the pH, Eh and the presence of flotation reagents. The pH is significant as it changes the charge of the particles surface and the Eh influences the chemisorption of collectors on the surface of the particles.

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