Beneficiation of Iron Ore and the treatment of magnetic iron taconites, stage grinding and wet magnetic separation is standard practice. This also applies to iron ores of the non-magnetic type which after a reducing roast are amenable to magnetic separation. All such plants are large tonnage operations treating up to 50,000 tons per day and ultimately requiring grinding as fine as minus 500-mesh for liberation of the iron minerals from the siliceous gangue.
Magnetic separation methods are very efficient in making high recovery of the iron minerals, but production of iron concentrates with less than 8 to 10% silica in the magnetic cleaning stages becomes inefficient. It is here that flotation has proven most efficient. Wet magnetic finishers producing 63 to 64% Fe concentrates at 50-55% solids can go directly to the flotation section for silica removal down to 4 to 6% or even less. Low water requirements and positive silica removal with low iron losses makes flotation particularly attractive. Multistage cleaning steps generally are not necessary. Often roughing off the silica froth without further cleaning is adequate.
The iron ore beneficiation flowsheet presented is typical of the large tonnage magnetic taconite operations. Multi-parallel circuits are necessary, but for purposes of illustration and description a single circuit is shown and described.
The primary rod mill discharge at about minus 10- mesh is treated over wet magnetic cobbers where, on average magnetic taconite ore, about 1/3of the total tonnage is rejected as a non-magnetic tailing requiring no further treatment. The magnetic product removed by the cobbers may go direct to the ball mill or alternately may be pumped through a cyclone classifier. Cyclone underflows usually all plus 100 or 150 mesh, goes to the ball mill for further grinding. The mill discharge passes through a wet magnetic separator for further upgrading and also rejection of additional non-magnetic tailing. The ball mill and magnetic cleaner and cyclone all in closed circuit produce an iron enriched magnetic product 85 to 90% minus 325 mesh which is usually the case on finely disseminated taconites.
The finely ground enriched product from the initial stages of grinding and magnetic separation passes to a hydroclassifier to eliminate the large volume of water in the overflow. Some finely divided silica slime is also eliminated in this circuit. The hydroclassifier underflow is generally subjected to at least 3 stages of magnetic separation for further upgrading and production of additional final non-magnetic tailing. Magnetic concentrate at this point will usually contain 63 to 64% iron with 8 to 10% silica. Further silica removal at this point by magnetic separation becomes rather inefficient due to low magnetic separator capacity and their inability to reject middling particles.
The iron concentrate as it comes off the magnetic finishers is well flocculated due to magnetic action and usually contains 50-55% solids. This is ideal dilution for conditioning ahead of flotation. For best results it is necessary to pass the pulp through a demagnetizing coil to disperse the magnetic floes and thus render the pulp more amenable to flotation.
Feed to flotation for silica removal is diluted with fresh clean water to 35 to 40% solids. Being able to effectively float the silica and iron silicates at this relatively high solid content makes flotation particularly attractive.
For this separation Sub-A Flotation Machines of the open or free-flow type for rougher flotation are particularly desirable. Intense aeration of the deflocculated and dispersed pulp is necessary for removal of the finely divided silica and iron silicates in the froth product. A 6-cell No. 24 Free-FlowFlotation Machine will effectively treat 35 to 40 LTPH of iron concentrates down to the desired limit, usually 4 to 6% SiO2. Loss of iron in the froth is low. The rough froth may be cleaned and reflotated or reground and reprocessed if necessary.
A cationic reagent is usually all that is necessary to effectively activate and float the silica from the iron. Since no prior reagents have come in contact with thethoroughly washed and relatively slime free magnetic iron concentrates, the cationic reagent is fast acting and in somecases no prior conditioning ahead of the flotation cells is necessary.
A frother such as Methyl Isobutyl Carbinol or Heptinol is usually necessary to give a good froth condition in the flotation circuit. In some cases a dispersant such as Corn Products gum (sometimes causticized) is also helpful in depressing the iron. Typical requirements may be as follows:
One operation is presently using Aerosurf MG-98 Amine at the rate of .06 lbs/ton and 0.05 lbs/ton of MIBC (methyl isobutyl carbinol). Total reagent cost in this case is approximately 5 cents per ton of flotation product.
The high grade iron product, low in silica, discharging from the flotation circuit is remagnetized, thickened and filtered in the conventional manner with a disc filter down to 8 to 10% moisture prior to treatment in the pelletizing plant. Both the thickener and filter must be heavy duty units. Generally, in the large tonnage concentrators the thickener underflow at 70 to 72% solids is stored in large Turbine Type Agitators. Tanks up to 50 ft. in diameter x 40 ft. deep with 12 ft. diameter propellers are used to keep the pulp uniform. Such large units require on the order of 100 to 125 HP for thorough mixing the high solids ahead of filtration.
In addition to effective removal of silica with low water requirements flotation is a low cost separation, power-wise and also reagent wise. Maintenance is low since the finely divided magnetic taconite concentrate has proven to be rather non-abrasive. Even after a years operation very little wear is noticed on propellers and impellers.
A further advantage offered by flotation is the possibility of initially grinding coarser and producing a middling in the flotation section for retreatment. In place of initially grinding 85 to 90% minus 325, the grind if coarsened to 80-85% minus 325-mesh will result in greater initial tonnage treated per mill section. Considerable advantage is to be gained by this approach.
Free-Flow Sub-A Flotation is a solution to the effective removal of silica from magnetic taconite concentrates. Present plants are using this method to advantage and future installations will resort more and more to production of low silica iron concentrate for conversion into pellets.
Silica sand low in iron is much in demand for glass, ceramic and pottery use, and for many of these applications clean, white sand is desired. Impurities such as clay slime, iron stain, and heavy minerals including iron oxides, garnet, chromite, zircon, and other accessory minerals must not be present. Chromium, for example, must not be present, even in extremely small amounts, in order for the sand to be acceptable to certain markets. Feldspars and mica are also objectionable. Generally, iron content must be reduced to 0.030% Fe2O3 or less.
Silica sand for making glass, pottery and ceramics must meet rigid specifications and generally standard washing schemes are inadequate for meeting these requirements. Sand for the glass industry must contain not more than 0.03% Fe2O3. Concentrating tables will remove free iron particles but iron stained and middling particles escape gravity methods. Flotation has been very successfully applied in the industry for making very low iron glass sand suitable even for optical requirements.Sub-A Flotation Machines are extensively used in this industry for they give the selectivity desired and are constructed to withstand the corrosive pulp conditions normally encountered (acid circuits) and also the abrasive action of the coarse, granular, slime free washed sand.
The flowsheet illustrates the more common methods of sand beneficiation. Silica may be obtained from sandstone, dry sand deposits and wet sand deposits. Special materials handling methods are applicable in each case.
The silica bearing sandstone must be mined or quarried much in the manner for handling hard rock. The mined ore is reduced by a Jaw Crusher to about 1 size for the average small tonnage operation. For larger scale operations two-stage crushing is advisable.
The crushed ore is reduced to natural sand grain size by Rod Milling. Generally, one pass treatment through the Rod Mill is sufficient. Grinding is done wet at dilutions in excess of normal grinding practice. A Spiral Screen fitted to the mill discharge removes the plus 20 mesh oversize which either goes to waste or is conveyed back to the mill feed for retreatment.
Sand from such deposits is generally loaded into trucks and transported dry to the mill receiving bin. It is then fed on to a vibrating screen with sufficient water to wash the sand through the 20 mesh stainless screen cloth. Water sprays further wash the oversize which goes to waste or for other use. The minus 20 mesh is the product going to further treatment.
The sand and water slurry for one of the three fore-mentioned methods is classified or dewatered. This may be conveniently done by cyclones or by mechanical dewatering classifiers such as the drag, screw, or rake classifiers.
From classification the sand, at 70 to 75% solids, is introduced into a Attrition Scrubber for removal of surface stain from the sand grains. This is done by actual rubbing of the wet sand grains, one against another, in an intensely agitated high density pulp. Most of the work is done among the sand grains not against the rotating propellers.
For this service rubber covered turbine type propellers of special design and pitch are used. Peripheral speed is relatively low, but it is necessary to introduce sufficient power to keep the entire mass in violent movement without any lost motion or splash. The degree of surface filming and iron oxide stain will determine the retention time required in the Scrubber.
The scrubbed sand from the Attrition Machine is diluted with water to 25-30% solids and pumped to a second set of cyclones for further desliming and removal of slimes released in the scrubber. In some cases the sand at this point is down to the required iron oxide specifications by scrubbing only. In this case, the cyclone or classifier sand product becomes final product.
Deslimed sand containing mica, feldspar, and iron bearing heavy minerals can be successfully cleaned to specifications by Sub-A Flotation. Generally this is done in an acid pulp circuit. Conditioning with H2SO4 and iron promoting reagents is most effective at high density, 70-75% solids. To minimize conditioning and assure proper reagentizing a two-stage Heavy Duty Open Conditioner with Rubber Covered Turbine Propellers is used. This unit has two tanks and mechanisms driven from one motor.
The conditioned pulp is diluted with water to 25-30% solids and fed to a Sub-A Flotation Machine especially designed for handling the abrasive, slime free sand. Acid proof construction in most cases is necessary as the pulps may be corrosive from the presence of sulfuric acid. A pH of 2.5-3.0 is common. Wood construction with molded rubber and 304 or 316 stainless steel are the usual materials of construction. In the flotation step the impurity minerals are floated off in a froth product which is diverted to waste. The clean, contaminent-free silica sand discharges from the end of the machine.
The flotation tailing product at 25 to 30% solids contains the clean silica sand. A SRL Pump delivers it to a Dewatering Classifier for final dewatering. A mechanical classifier is generally preferable for this step as the sand can be dewatered down to 15 to 20% moisture content for belt conveying to stock pile or drainage bins. In some cases the sand is pumped directly to drainage bins but in such cases it would be preferable to place a cyclone in the circuit to eliminate the bulk of the water. Sand filters of top feed or horizontal pan design may also be used for more complete water removal on a continuous basis.
Dry grinding to minus 100 or minus 200 mesh is done in Mills with silica or ceramic lining and using flint pebbles or high density ceramic or porcelain balls. This avoids any iron contamination from the grinding media.
In some cases it may be necessary to place high intensity magnetic separators in the circuit ahead of the grinding mill to remove last traces of iron which may escape removal in the wet treatment scrubbing and flotation steps. Iron scale and foreign iron particles are also removed by the magnetic separator.
In general most silica sands can be beneficiated to acceptable specifications by the flowsheet illustrated. Reagent cost for flotation is low, being in the order of 5 to 10 cents per ton of sand treated. If feldspars and mica must also be removed, reagent costs may approach a maximum of 50 cents per ton.
Laboratory test work is advisable to determine the exact treatment steps necessary. Often, attrition scrubbing and desliming will produce very low iron silica sand suitable for the glass trade. Complete batch and pilot plant test facilities are available to test your sand and determine the exact size of equipment required and the most economical reagent combinations.
Silica sand for making glass, pottery and ceramics must meet rigid specifications and generally standard washing schemes are inadequate for meeting these requirements. Sand for the glass industry must contain not more than 0.03% Fe2O3. Concentrating tables will remove free iron particles but iron stained and middling particles escape gravity methods. Flotation has been very successfully applied in the industry for making very low iron glass sand suitable even for optical requirements.
Sub-A Flotation Machines are extensively used in this industry for they give the selectivity desired and are constructed to withstand the corrosive pulp conditions normally encountered (acid circuits) and also the abrasive action of the coarse, granular, slime free washed sand.
The flowsheet illustrated is typical for production of glasssand by flotation. Generally large tonnages are treated, forexample, 30 to 60 tons per hour. Most sand deposits can be handled by means of a dredge and the sand pumped to the treatment plant. Sandstone deposits are also being treated and may require elaborate mining methods, aerial tramways, crushers, and wet grinding. Rod Mills with grate discharges serve for wet grinding to reduce the crushed sandstone to the particle size before the sand grains were cementedtogether in the deposit. Rod milling is replacing the older conventional grinding systems such as edge runner wet mills or Chilean type mills.
Silica sand pumped from the pit is passed over a screen, either stationary, revolving or vibrating type, to remove tramp oversize. The screen undersize is washed and dewatered generally in a spiral type classifier. Sometimes cone, centrifugal and rake type classifiers may also be used for this service. To clean the sand grains it may be necessary to thoroughly scrub the sand in a heavy-duty sand scrubber similar to the Heavy-duty Agitator used for foundry sand scrubbing. This unit is placed ahead of the washing and dewatering step when required. The overflow from the classifier containing the excess water and slimes is considered a waste product. Thickening of the wastes for water reclamation and tailings disposal in some areas may be necessary.
The washed and dewatered sand from the spiral-type classifier is conveyed to a storage bin ahead of the flotation section. It is very important to provide a steady feed to flotation as dilution, reagents and time control determines the efficiency of the process.
Feeding wet sand out of a storage bin at a uniform rate presents a materials handling problem. In some cases the sand can be uniformly fed by means of a belt or vibrating-type feeder. Vibrators on the storage bin may also be necessary to insure uniform movement of the sand to the feeder. In some cases the wet sand is removed from the bin by hydraulic means and pumped to a spiral-type classifier for further dewatering before being conveyed to the next step in the flowsheet.
Conditioning of the sand with reagents is the most critical step in the process. Generally, for greater efficiency, it is necessary to condition at maximum density. It is for this reason the sand must be delivered to the agitators or conditioners with a minimum amount of moisture. High density conditioning at 70 to 75% solids is usually necessary for efficient reagentizing of the impurity minerals so they will float readily when introduced into the flotation machine.
The Heavy-duty Duplex Open-type Conditioner previously developed for phosphate, feldspar, ilmenite, and other non-metallic mineral flotation is ideal for this application. A duplex unit is necessary to provide the proper contact time. Circular wood tanks are used to withstand the acid pulp conditions and the conditioner shafts and propellers are rubber covered for both the abrasive and corrosive action of the sand and reagents.
Reagents are added to the conditioners, part to the first and the balance to the second tank of the duplex unit, generally for flotation of impurities from silica sand. These reagents are fuel oil, sulphuric acid, pine oil, and a petroleum sulfonate. This is on the basis that the impurities are primarily oxides. If iron is present in sulphide form, then a xanthate reagent is necessary to properly activate and float it. The pulp is usually regulated with sulfuric acid to give a pH of 2.5-3.0 for best results through flotation.
A low reagent cost is necessary because of the low value of the clean sand product. It is also necessary to select a combination of reagents which will float a minimum amount of sand in the impurity product. It is desirable to keep the weight recovery in the clean sand product over 95%. Fatty acid reagents and some of the amines have a tendency to float too much of the sand along with the impurities and are therefore usually avoided.
After proper reagentizing at 70 to 75% solids the pulp is diluted to 25 to 30% solids and introduced into the flotation machine for removal of impurities in the froth product. Thepulp is acid, pH 2 .5 to 3.0 and the sand, being granular and slime free, is rapid settling so a definite handling problem is encountered through flotation.
The Sub-A Flotation Machine has been very successful for silica sand flotation because it will efficiently handle the fast settling sand and move it along from cell to cell positively. Aeration, agitation and selectivity due to the quiet upper zone can be carefully regulated to produce the desired separation. The machine is constructed with a wood tank and molded rubber wearing parts to withstand the corrosive action of the acid pulp. Molded rubber conical-type impellers are preferred for this service when handling a coarse, granular, abrasive sand.
Flotation contact time for removal of impurities is usually short. A 4, and preferably a 6 cell, machine is advisable. Cell to cell pulp level control is also desirable. A 6 cell No. 24 (43 x 43) Sub-A Flotation Machine in most cases is adequate for handling 25 to 30 tons of sand per hour. If the impurities are in sulphide form a standard machine with steel tank and molded rubber parts is adequate provided the pulp is not acid. Otherwise acid proof construction is essential.
The flotation tailing product is the clean sand discharging from the end of the flotation machine at 25 to 30% solids and must be dewatered before further processing. Dewatering can be accomplished in a dewatering classifier and then sent to storage or drying. Top feed or horizontal vacuum filters are often used to remove moisture ahead of the dryer. Dry grinding of the sand to meet market requirements for ceramic and pottery use is also a part of the flowsheet in certain cases.
This particular sand was all minus 20 mesh with only a trace minus 200 mesh and 70% plus 65 mesh. Iron impurity was present as oxide and stained silica grains. The plant which was installed as a result of this test work is consistently making over a 95% weight recovery and a product with not over 0.02% Fe2O3 which at times goes as low as 0.01% Fe2O3.
Si02, minimum..99.8 per cent Al2O3, maximum..0.1 percent Fe2O3, maximum..0.02 per cent CaO + MgO, maximum.0.1 percent For certain markets, a maximum of 0.030 per cent Fe2O3 is acceptable.
Natural silica-sand deposits generally contain impurityminerals such as clay, mica, and iron oxide and heavy iron minerals which are not sufficiently removed by washing and gravity concentration. Flotation is often used to remove these impurity minerals to meet market specifications.
Anionic-type reagents, such as fatty acids, are used to float some impurities in alkaline pulp. Cationic-type reagents such as amines or amine acetates are also used with inhibitors such as sulphuric or hydrofluoric acids to float certain impurity minerals and depress the silica.
Also called silica sand or quartz sand, silica is silicon dioxide (SiO2). Silicon compounds are the most significant component of the Earths crust. Since sand is plentiful, easy to mine and relatively easy to process, it is the primary ore source of silicon. The metamorphic rock, quartzite, is another source.
Silicon (Si) is a semi-metallic or metalloid, because it has several of the metallic characteristics. Silicon is never found in its natural state, but rather in combination with oxygen as the silicate ion SiO44-in silica-rich rocks such as obsidian, granite, diorite, and sandstone. Feldspar and quartz are the most significant silicate minerals. Silicon alloys include a variety of metals, including iron, aluminum, copper, nickel, manganese and ferrochromium.
In almost all cases, silica mining uses open pit or dredging mining methods with standard mining equipment. Except for temporarily disturbing the immediate area while mining operations are active, sand and gravel mining usually has limited environmental impact.
In addition to tool steels, an example of alloy steels, ferrosilicon is used in the manufacture of stainless steels, carbon steels, and other alloy steels. An alloy steel refers to all finished steels other than stainless and carbon steels. Stainless steels are used when superior corrosion resistance, hygiene, aesthetic, and wear-resistance qualities are needed.
Silicon is used in the aluminum industry to improve castability and weldability. Silicon-aluminum alloys tend to have relatively low strength and ductility, so other metals, especially magnesium and copper, are often added to improve strength.
In the chemicals industry, silicon metal is the starting point for the production of silianes, silicones, fumed silica, and semiconductor-grade silicon. Silanes are the used to make silicone resins, lubricants, anti-foaming agents, and water-repellent compounds. Silicones are used as lubricants, hydraulic fluids, electrical insulators, and moisture-proof treatments.
Semiconductor-grade silicon is used in the manufacture of silicon chips and solar cells. Fumed silica is used as a filler in the cement and refractory materials industries, as well as in heat insulation and filling material for synthetic rubbers, polymers and grouts.
Silicon is considered a semiconductor. This means that it conducts electricity, but not as well as a metal such as copper or silver. This physical property makes silicon an important commodity in the computer manufacturing business.
Foundry silica sand is the quartz as the main mineral composition, particle size of 0.020 mm to 3.350 mm refractory particles, according to the mining and processing methods of different can be divided into artificial silica sand and sand washing, sand washing, selection of natural silica sand such as sand (flotation).
1, crushing process directly.Its technological process is: the run of mine ore grizzly jaw crusher crushing and screening to cone crushing and screening, more paragraphs to roll the crushing and screening products.
3, since the mill grinding process.Its technological process is: the run of mine ore grizzly since the mill to air classification system, qualified products and super fine powder (coarse grained grinding cage hexagon screen the final product).
4, wet rod mill magnetic separation process.This is China during the period of > study of success and is promoting the new technology, its technological process is: in ore to coarse crushing, crushing and screening to rod mill, the high frequency fine screen to hydraulic classification, magnetic separation and iron products.
Material: silica sand Capacity: 100TPH Country: Malaysia Feeding size: 6mm Raw mineral description: 1. Sand particle distribution: <1mm 40%; 1-3mm 30%; 3-6mm 30% 2. Mineral composition: silica, SiO2 94.7%; Iron as Fe2O3 0.35%; Alumina, Al2O3 0.78%Customers requirements: Silica concentrate with >98.5% of SiO2.
Project: extraction of scrubbed silica sand (use for glass, cement industry) Material: silica sand Capacity: 50TPH Country: Indonesia Mineral condition description: raw silica sand contains slime and barren rock, which is up to 25-30 CM in diameter. Customers requirements: wash the mud off and sieve the barren rock out.
Material: silica sand Capacity: 65TPH Country: Malaysia Feeding size: 0-1mm Raw mineral description: 1. Mineral composition: silicon, SiO2 94%; Iron as Fe2O3 0.35%; others 2. Customer roughly washed the sand and sieved 0-1mm fine sand out.
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Flotation response of iron ore depends on its impurities.Silica rich ores float better compared to alumina rich ores.Clay and gibbsite coat on hematite and hinder flotation.SEM studies support less coating of quartz due to coarser size.Silica in the form of jasper present in BHJ ore is less floatable.
Indian low grade iron ores mostly contain quartz, gibbsite, and clay as the major impurities. Depending upon the ore characteristics, in many instances froth flotation has to be used to recover the hematite and other iron oxides from the ground ores. In this context, the difference in silica and alumina content in low grade iron ores is brought to bear on the prospects of iron ore flotation. For this purpose, pure minerals like hematite, quartz, gibbsite, and clay have been used to prepare synthetic mixtures and analyzed to determine the difference in floatability. The results are compared against natural iron ores with variations in silica and alumina content. The flotation results with oleic acid and dodecylamine show better recovery of iron values in the hematitequartz mixture as well as in the naturally occurring high silica ore compared to the hematiteclay mixture and the high alumina ore respectively. Similarly results on a variety of iron ores show that high silica content as quartz in the ore causes less hindrance in the flotation of iron ore, whereas the presence of silica as clay inhibits the flotation response of iron ore. The scanning electron microscopic (SEM) studies indicate that clay particles cover the surface of hematite, making it less selective for interaction with the collector. The surface potential studies of clay and quartz suggest that charge reversal takes place for quartz treated with dodecylamine (DDA), but for the DDA adsorption on clay, negative potential values are noted beyond a pH of 4.6.