Electrostatic Separation

July 2nd, 2009 by zkmachine
ELECTROSTATIC SEPARATION is defined as “the selective sorting of solid species by means of utilizing forces acting on charged or polarized bodies in an electric field. Separation is effected by adjusting the electric and coacting forces, such as gravity or centrifugal force, and the different trajectories at some predetermined time. Separations made in air are called Electrostatic Separation. Separations made using a corona discharge device, are called High Tension Separations. Separations made in liquids are termed separation by Dielectrophesis, and if motion is due to polarization effects in nonuniform electric fields. Electrophoresis is when separations are made if motion is due to a free charge on the species in an electric field. There are no industrial applications of mineral concentrations by electrophoresis of dielectrophesis.” 1

Electrostatic separation is important in the production of minerals, also in the reclamation of other valuable materials, as well as the cleaning of some food products. When every effort is being made by Process Engineers to make use of all concentrating equipment available for the recovery of critical minerals and reclaimed materials, the subject of applied electrostatic separation is of interest. Refer to Fig.2, for a diagram of how standard electrostatic separators function.  

Fig. 2, Typical Electrostatic Separator Diagram
A very simple demonstration of electrostatic separation can be made by taking a handful of salted peanuts, rubbing the skins off, then taking a comb, rubbing it on fur or the coat sleeve until a static charge has been collected on the comb, and passing it over the peanuts and skins. The skins are easily removed from the peanuts. Hulls may be removed from ground coffee in the same manner. Under the influence of an electrostatic charge there is a difference in the susceptibility and behavior of most materials, minerals, salts, and food products. This can be controlled to a great extent by potential, polarity, temperature, and conditioning of the surface of the particles. Oftentimes, by a combination of these factors, the desired separation is closely controlled.

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July 2nd, 2009 by zkmachine

  One Mine’s Nickel Processing of Pyrrhoite,
Chalcopyrite and Pentalandite Ores

The ore is crushed to -5 inches in primary cone crushers, then reduced to -1/2″ in short head cone crushers. The ore is then ground to -100 mesh in ball mills. Using wet magnetic separators the magnetic ore is separated (pyrrhotite) and further reduced to -200 mesh in a ball mill. Classification is accomplished with screens and cyclones. The pyrrhotite is then sent to froth flotation cells, and produces a 3% nickel concentrate.

The non magnetic ore is sent to a series of rougher, cleaner flotation cells, and produces a 31% copper concentrate. The tailings from these cells is sent to another flotation cell, to recover the nickel, and the concentrate is combined with the 3% nickel concentrate to produce a 12% nickel concentrate.

Most nickel ores have several recoverable metals in them and the process if commonly a multiple stream of metals recovered. The copper concentrate is sent to the copper smelter and 99.99% copper cathode is produced in electrowinning cells. The nickel ore is sent to the nickel refinery, where a complex set of reactions is carried out. These include leaching in autoclaves using ammonia, heated to around 200 deg F at elevated pressures of 100-150 PSI. The application of heat and pressure dramatically speeds up the chemical reaction and produces the nickel much faster than leaching at room temperature and atmospheric pressure. There is a primary and a secondary leach circuit, where the solids remaining in the first circuit are sent to a second autoclave to recover any metals the first leach process missed. A slurry of the liquid and solids are then pumped through a thickening and filtration system, separating the non-valuable solids from the liquid containing the nickel. The nickel and ammonia solution contains 2:2 ratio of nickel to ammonia (molar), which is roughly 50 grams nickel per liter of solution.

The nickel-ammonia solution is then reduced with hydrogen in autoclaves, adding a a small amount of ferrous ammonium sulphate. This solution is heated to 250 deg F at 350 PSI, in a hydrogen atmosphere. The nickel is reduced by hydrogen and precipitates as fine metallic nickel (Ni). The nickel settles and the solution is pumped out to a holding tank. Since the solution still contains some nickel , it is re-processed again to recover the remaining nickel. The nickel can then be removed through a cone bottomed tank or by a thickener/filter operation to yield the nickel powder. It is then dried and sent to a briquette process where it is made into pellets or briquettes. It can also be melted and poured into ingots for sale to the market.

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Flotation & Gravity Concentration

July 2nd, 2009 by zkmachine
A bank of 4 Denver Sub A Flotation Cells, 750 Cubic Foot per cell, overflowing the froth product.  Flotation is used in gold, mineral processing, copper, coal, to recover fine particles, typically finer than 0.5 mm.  Chemical and physical attachment of the solid particle to the bubble, induced by the chemical reagents and conditioners would be a over simplified description of froth flotation.   
Deister Table, being used in a small gravity gold concentrating plant located in Ecuador.  This particular mine processes both placer (loose rock and gravel) ore as well as lode or vein ore.  They crush the ore in a KueKen Primay Jaw crusher, further reduce the ore in Denver dual roll crushers and finally use rod and ball mills to reduce the particle size of the ore to between 60 and 100 mesh.  The crushed ore is then processed in a slurry form, fed to deister tables, such as the one in this photo, with gold and certain heavy minerals being separated from the abundant quartz.   The final concentrate is then “cleaned” on a secondary series of Deister Tables, to separate the gold from the other heavy minerals, such as zircon, garnet, ilmenite and magnetite    
A 6′ x 10′ Ball Mill ,with a spiral classifier, classifying the product size and he oversize and recirculating the oversize back to the mill for re-grinding.  This is also located at the same site in Ecuador.   
A Eimco 12 B Mucker, is featured in this photo, previously identified as a “Ore Car”. The Eimco mucker was a ingenious invention, operated by compressed air, that simulated mechanically the shoveling motion of a human. It would lift the broken ore and throw it backward, into the car, just like the miners did with their shovels. It had a air motor, and ran on light rails, so it moved forward as the ore was removed. This increased production from underground hard rock mines (especially gold) in the early part of the 20th century.

For narrow vein stoper mining, these old Eimco 12B Muckers (the smallest made) are still popular, and some local companies rebuild them for sale. Stoper mining is still the most economical method to mine a narrow vein of gold, even though much research has been made on equipment to do this type of mining. Using a Eimco 12B mucker, in a 5 foot wide by 7 foot high tunnel, approximately 5 tons per 8 hour shift per man can be achieved, stoper mining. This compares to about one ton per man per shift without a mechanical mucker. They will operate in a mine tunnel that is only 5 feet wide and 7 feet high.

Eimco made a full size range of muckers, which went out of fashion with the newer loading and hauling equipment, such as the load haul dump (LHD’s) machines, bucket loaders (muckers), and articulated underground tractors for hauling ore underground. As for the ore car photo that the old caption was supposed to have been matched with, it apparently never made it to the web site.

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Cement, How It Is Produced

July 2nd, 2009 by zkmachine

Cement - The Short Version

Cement, or Portland cement, is defined as “a hydraulic cement, obtained by burning a mixture of lime and clay to form a clinker, then pulverizing the clinker into powder. The greenish gray powder is composed primarily of calcium silicates, calcium aluminates, and calcium ferrites. When mixed with water (Hydrated), it solidifies to an artificial rock, similar to Portland stone.” A Portland Stone is a yellow limestone from the Isle of Portland, in Great Britain.

Historically, cement can be traced back to the early Roman Empire, and contributed to the building of the great structures of the Roman Empire. By varying the amounts and types of the same basic ingredients, cement with various properties may be obtained. By further varying the ingredients, even more differing cements are manufactured.

*”Cement manufacturing is the basic processing of selected and prepared mineral raw materials to produce the synthetic mineral mixture (clinker) that can be ground to a powder having the specific chemical composition and physical properties of cement.” Cement manufacture, like many other manufacturing processes, begins at the mine, where the raw materials like limestone, silica, aluminates, feric minerals and others are obtained. Some typical materials used for calcium carbonate in cement manufacturing are limestone, chalks, marbles, marls, and oyster shell. Some typical materials used for alumina in the cement manufacturing are shale, clay, slags, fly ash, bauxite, alumina process waste, and granite. Some typical materials used for silica in cement manufacturing are sand, clay, claystone, shale, slag, and fly ash. Some typical materials used for iron in cement manufacturing are iron ores, blast furnace flue dusts, pyrite clinker, mill scale, and fly ash.


Ball Mill In A Cement Plant


 		 The general mining methods are surface mining, while some silicates, such as sand, are commonly mined using dredges, from lakes, rivers and waterways. There are a few underground limestone mines, but most are pits on the surface. Cement plants are typically located central to the minerals required to make the cement, which saves the transportation costs and reduces the price of the cement. Once the ore material to be used for cement has been mined, it is transported to the crushing/screening plant, where it is crushed and screened, to produce the desired particle size. Ore from the mines are typically reduced to about ¾ inch and stored in a coarse stockpile. From there, the ¾ inch ore is typically reduced to a powder in a large ball mill.  



 		 Sometimes, the raw materials are wet ground in the ball mill by adding water, and form a slurry. In either event, wet or dry, the ground powder is then blended, using the “Chemist’s Secret Mixture”, which can be a closely guarded “recipe”, and are then transported to the rotary kiln for heat processing. In the rotary kiln, first the carbon dioxide is driven off of the calcium carbonates, then the raw material is fused at a temperature somewhere around 2,700 degrees F. The discharge from the kiln is called clinker, as it resembles small rocks or residue from a blast furnace. The clinker is the cement in “lump” form. The particle size range for clinker is from about 2 inches to about 10 mesh. The clinker is then ground in a ball mill and shipped to users as Portland Cement.

 

There are five general types of cement. First, Type 1 cement is general cement for general use, typically the type used in construction. Type 2 cement is still a general cement, but it has resistance to sulfates and heat of hydration. Type 3 cement is for high strength properties in the early stages of the cement’s life. I.e., immediately after curing. Type 4 cement is used where very low heats of hydration are desired. And Type 5 cement is used where a very high sulfate resistance is required.

In the United States, the American Society of Testing Materials, the American Association of State Highway Officials, the American Concrete Institute, the US Corps of Engineers are generally the primary driving force of the cement quality and technology standards and specifications.

Concrete is a mixture of gravel, sand and cement. Concrete is NOT cement, but it is made with cement.
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Magnetic Separation

July 2nd, 2009 by zkmachine
When minerals are placed in a magnetic field, there are three reactions which may occur. First, they are attracted to the magnetic field. Second, they are repulsed by the magnetic field. And third, no noticeable reaction to the magnetic field occurs.

Particles that are attracted to the magnetic field are called magnetic. But, there are two classifications of magnetic particles, strongly magnetic particles, such as iron and magnetite, and weakly magnetic particles, such as rutile, ilmenite, and chromite. Strong magnetic particles may be easily separated with a separator having a low intensity magnetic field of 400 - 600 gauss. Paramagnetic particles (weakly magnetic) require a higher intensity magnetic field to separate them, generally ranging from 6,000 to 20,000 gauss.

Particles that are repulsed by a magnetic field are called diamagnetic. Other than levitation of carbon and an occasional frog, little practical use has been made of diamagnetism. However, using a similar principle, passing a eddy current through material, can cause the conductive material to be separated from the non conductive material. A line of separators called Eddy Current Separators, takes advantage of eddy current and conductive particles, separating them from other non conductive material. One of the largest uses currently is in the recycling industry, where wire and metals made from copper and aluminum are separated from plastics. When product, such as aluminum, passes over the eddy current separators , the spinning magnets inside the shell generate an eddy current in the aluminum thus creating a magnetic field around the piece of aluminum. The polarity of the magnetic field of the aluminum is the same as the rotating magnets, causing the aluminum to be repelled away from the separator. Product such as plastic, glass, or other process materials simply fall off the end of the separator. An eddy current is defined as the currents caused by voltages induced by changing flux, and tend to oppose the change of the flux.

Non magnetic particles, such as gold, quartz, and pyrite, are not amenable to magnetic separation, but some magnetic material may be removed from the feed. For instance, in a few situations, plants using gravity concentration for recovering gold, used magnetic separators to remove the high concentration of magnetite that was recovered with the gold, prior to further processing.

Magnetic separation is generally a low cost method of recovery, unless high intensity separators are required. The electro-magnetic high intensity separators that produce 20,000 gauss, tend to be expensive. However, the rare earth magnetic separators are relatively inexpensive and can produce magnetic fields around 6,000 gauss. So, when looking for a process to recover valuable minerals, magnetic separation should not be overlooked, if some of the material is magnetic or para-magnetic.

And as a comment on choosing magnetic separators, when I was working on a project involving separating some magnetite, rutile from a spiral concentrator concentrate, a new engineer just out of school, came up to me and said that he had a way to change the process and both eliminate a stage of separation, while saving money. I asked him to explain this idea and he began telling me that he would simply eliminate the first stage of low intensity separators and send everything directly to the high intensity magnetic separators. This would eliminate the need for one complete circuit, and would only “slightly” overload the high intensity separators. He further explained that by slightly increasing the size of the high intensity magnetic separators, this problem would be solved. Then he stood back, waiting for my approval.

“Well” I began, “you have definitely been giving this some thought. But you do not have two critical pieces of information, that would make your plan unacceptable. First, by sending highly magnetic material to a high intensity magnetic separator, it would rapidly fill the magnetic surface and blind off the separator working area for any weakly magnetic material, so you would reduce the efficiency of the separator by 50% to 70% or more. The magnetic separator salesman would like you, though because he could retire after you purchased enough of the very expensive high intensity magnetic separators to make your idea work. Which brings me to the second point, low intensity drum separators cost a fraction of what a high intensity magnetic separator would cost. These 20,000 gauss separators cost several hundred thousand dollars each, while this drum separator costs around $20,000, each. Almost a factor of 10. So, while it is a interesting concept, it would cost more to recover the titanium from the rutile than one could ever sell it for, which makes it not feasible. But don’t let that discourage you from looking at alternate methods, because that is how discoveries are made.”

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Spiral Concentrators

July 2nd, 2009 by zkmachine

  Spiral Concentrators

Single Start Spiral

Splitter and discharge port of spiral 

 Spiral concentrators are a gravity based concentrating device, that separates light density granular and sandy (10 mesh to 200 mesh (2 mm to 0.075 mm)) consistency material from heavier density material. In order to have a good separation, there should be a difference in SG’s of at least 1.0. One main benefit of spiral concentrators is they have no moving parts. The feed range, in percent solids, to a spiral ranges from 20% solids up to 40% solids. Depending upon the material characteristics, a maximum efficiency will usually be reached somewhere in this range. All that is required are some slurry pumps, the slurry to be separated and the banks of spirals with a feed distributor.

Slurry is pumped to the top of the spiral (typically 13′ to 15′ from the floor), and it enters a feed distributor that evenly distributes the feed to each spiral concentrator. The design and shape of the spiral make it work, when combined with gravitational acceleration. As the slurry travels the spiraling path down the spiral, mineral grains settle and start sorting according to size, density and to a lesser extend shape. Low density particles are carried with the bulk of the water towards the outside of the spiral (perimeter), while particles with the greatest density migrate towards the inside of the spiral

A cross section of a spiral concentrator can be divided into various regions, with each region describing the effect it has on the slurry traveling through it. On the outer most region (1) (perimeter), will have mostly water, with fine particles, trapped by the high velocity of the moving water. Moving inward towards the center of the spiral, the next region(2) would consist of a very small area where the maximum water velocity exists, and prevents any separation to occur. This region is defined since it separates the next region (3) from the first region.

Region 3 is a very active region where the velocity begins to slow down and most of the separation occurs, as more dense particles settle to the bottom and the water velocity keeps the light density particles in the stream near the surface, where they eventually wind up in the outer regions (2 and 1). The next region is actually where two regions overlap (region 3 and 5), and is a very narrow region (like region 2). Next to the last region (region 5) is where the heavy density concentrates collect. The remaining low density particles in this region find their way to the top of the slurry surface and are carried off by the fast flowing water to the perimeter of the spiral, with the bulk of the water and the low density solids. Some spirals have a wash water section, where additional water is added to free any trapped light density material in the concentrates, and on a wash water spiral, the innermost portion is where this water is added, and it is called region 6. From the innermost region of the spiral, the concentrates then flow to the bottom section of the spiral, where splitter “bars” actually make a cut of the material, channeling the inner most material to the heavy concentrate port, a ‘middlings’ splitter can be used to channel the intermediate to a separate discharge port, and the majority of the water and the light density material is cut to a low density port. Some spiral concentrators, especially those used in coal cleaning, have the capability of removing the high density material at multiple positions in the vertical spiral. Coal can consist of as much as 50% high density material, compared to a typical heavy mineral with only 5% to 10% high density content. These cutter bars are adjustable, and are usually set up during the start up and rarely moved, unless differing material is processed. They can be changed easily, to accommodate differing feed material.

Spiral concentrators can be made with multiple starts (multiple spirals interwound) to save floor space. I have seen up to 3 start spirals, which would essentially give 3 spirals in about the same space that one would take up. Spiral concentrators are normally used in banks of multiple spirals. Typical capacities for spirals run from 1-3 tons per hour of feed for minerals and 3-5 tons per hour for coal. Typical construction of a spiral concentrator is fiberglass and urethane to reduce wear from abrasion.
 

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Processing of Bauxite Ores

July 2nd, 2009 by zkmachine

  Processing of Bauxite Ores
To Obtain Aluminum


Bauxite, the primary aluminum ore, uses the Bayer Process to extract alumina from the ore. The processing begins with crushing to 1-2 inch particles and wet screening, to remove some silica fines, which are generally present with bauxite.

From here, the ore is digested in a heated, pressurized vessel at temperatures up to 450 defrees F and pressures of 500 psi. Generally, there are differing types of alumina ores in the bauxite deposit, and these require different temperatures to digest. The low temperature digestor typically operates at 275 to 290 degrees F. Sodium Hydroxide (NaOH) is used to dissolve the alumina in the digestor. The Pregnant liquor is separated from the red mud tailings using a series of thickeners set up in counter current decantation arrangement, to continuously wash the alumina bearing liquor from the tailings, until only barren tailings remain. Various contaminates (iron, organic matter) can play havoc with the process and may require sub routines to remove them.

Filtration, such as vacuum drum or pressure filters, remove the silica and low solids from the clarified alumina bearing liquor. The liquid containing the dissolved alumina is pumped to tanks called crystalization or precipitation tanks. The liquid is cooled with water from the counter current decantation thickeners, and as it cools, alumina hydrate slowly precipitates from the tank, according to the formula:

The precipitated liquor containing the white alumina is then filtered, to remove the solid alumina from the liquid, using vacuum drum filters or rotary pan filters, where it can be washed as it is filtered. The alumina hydrate (AlOH3) is then dried. It can be further calcined to Al2O3, alumina, in a rotary kiln at 800 defrees F.

From the kiln, the alumina is allowed to cool and it is stored for shipment to the aluminum plant. To produce aluminum, the alumina is reduced in a electrical reduction cell which produces pure Al plus carbon dioxide (CO2). Now, aluminum ingots or billets are stored for manufacturers of aluminum products, such as automobiles, pots and pans, and the ubiquitous aluminum siding that covers many houses. The electrical reduction plants are power intensive, using up to 16,000 KW of electricity per ton of aluminum produced.

Information provided by Charles Kubach, Mining and Mineral Processing Engineer
Reference: SME Mineral Processing Handbook 

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Hello world!

July 2nd, 2009 by zkmachine

Welcome to ttnet Blogs. This is your first post. Edit or delete it, then start blogging!

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