chromium ore grinder

laboratory grinding mill

laboratory grinding mill

Our Laboratory Grinding Mill comes standard with a 1 HP motor and optional variable speed drive ranging from 1 to 100 RPM. This Grinding Mill is fully enclosed with sound dampening material for a quiet workplace. The sheet metal steel fabrication provides full enclosure around the main frame and door. The yoke (grinding cylinder) is totally balance and vibration-free in its horizontal position of operation. Minimal effort is needed to swing it from horizontal to vertical position (loading/unloading). A unique feature this grinding mill has is the possibility to use different cylinders for research or pilot plants tests. Specifically, we offer different size of cylinders from 5 (127mm) inside diameter by 12 (305mm) long to 9 (229mm) diameter by 22 (560mm) long. All grinding mills cylinders are fully interchangeable and can be mounted/ removed rapidly. The cover is of a unique design that automatically seals the cylinder and is quickly mounted/removed. Another feature of this Grinding Mill are its heavy duty castors (wheels) for ease of laboratory movement/mobility.

Ore Grinding Mills are used for the fine grinding as the last step in the reduction of an ore prior to concentration (gravity or flotation) or cyanidation. Practice varies, depending upon the type of ore and the amount of reduction required. In addition, some of the older properties continue with methods that perhaps are not considered the best in light of recent improvements but that cannot be economically changed because of capital outlay. Present grinding practice is closely linked with classification, so that some overlapping of subject matter occurs. In this chapter some of the theory of grinding, different types of equipment, and flow sheets are discussed.

Most of the tonnage milled today is ground in one of the following types of equipment or a combination of two or more: ball mills, tube mills, rod mills, and stamps. Chilean mills and Huntington mills are used only in a few isolated cases today.

The term ball mill is generally used to refer to a cylindrical mill whose length is less than, equal to, but not much greater than its diameter. It was initially developed for relatively coarse grinding, but by using it in closed circuit with a classifier its use has been extended for fine grinding.

Ball mills have shells of cast iron or steel plates and are carried on hollow trunnions. Ore is fed through a scoop, drum-type, or combination feeder at one end and is discharged from the opposite trunnion.

Ball mills may be arbitrarily classified into two types, according to the method of pulp discharge. In high-level or overflow mills the pulp level builds up until it overflows and discharges through the trunnion. High- level discharge mills are made by a large number of manufacturers throughout the world. Low-level mills are typified by the Allis-Chalmers andMarcy (see Figs. 14 and 15) grate-discharge mills. The discharge end is fitted with grates; between the grates and the end of the mill are radial lifters which act as a pump to lift the discharge to the hollow trunnion. Drive is by spur or herringbone gear, direct connected or belt driven.

Ball mills are built in sizes ranging from small laboratory mills to a present maximum of 12 ft. diameter by 12 ft. long, the latter requiring close to 1000 hp.Liners are usually of manganese steel, of chrome steel, or white iron, 3 to 6 in. thick. Corrugated and shiplap construction is commonly used to increase the grinding action.

The Hardinge mill (see Fig. 16) differs from most ball mills in that conical ends are added to the cylindrical portion of the mill. The cone at the feed end has a larger open angle than that at theopposite end. Its makers state that the large balls concentrate near the feed end of the mill where the coarsest ore collects and the smaller balls act on the finer ore.

Rod mills (see Fig. 17) follow the general dimensions of tube mills with diameters from 3 to 6 ft. and lengths from two to three times their diameter. They differ from ball mills in that steel rods 3 or 4 in. shorter than the mill length inside the liners are used as grinding media. Rod mills are often run on tires and rollers instead of trunnions or on one trunnion and one tire and set of rollers.

Low-level discharge is obtained on Marcy rod mills by having a beveled annular ring at the discharge end. A stationary steel door fits close to this beveled ring and serves to hold the rods in the mill while pulp discharges between the mill and the door.

The distinction between tube mills and ball mills is not somarked as their names indicate. Mills from 4 to 6 ft. in diameter and from 16 to 22 ft. long are usually termed tube mills. This was the first type of rotary mill for metallurgical purposes. Because of the necessity of completing the grind during one passage (open circuit) of the ore through the mill, it was built with a large length-diameter ratio. The tube mill is still largely used in South Africa and to some extent in North America for fine grinding generally following some other primary mills.

Tube mills are usually supported on hollow trunnions, the feed entering through a feed scoop at one end and discharging through the other. Drive is by a large gear fitted over the mill shell. Various types of liners are used, as in ball mills.

All rotary mills must be fitted with some kind of replaceable liners. Chrome steel, manganese steel, and white iron are generally used. Shapes designed to give a corrugated or shiplap surface to the interior of the mill are often used to prevent slippage of the ball load. Pocket liners arealso common. These liners have pockets in which the balls become lodged to form the wearing surface.

Rubber liners have been tried experimentally but have not been adopted by the industry. According to Taggart, no fully satisfactory method of holding the liners in place was worked out, utility was limited to fine feeds and small balls, mill capacity was reduced, and while a slightly higher grinding efficiency was shown in comparative tests with steel liners, there was no indication that possible increased wear for rubber would offset its far greater cost.Silex liners with flint pebbles for grinding media are sometimes used where iron contamination must be avoided.

The grinding that takes place in mills of this type is usually ascribed to two actions, impact and attrition, although some authors do not believe that a sharp line of demarcation can be drawn between the two actions.

In rod mills there is line contact between the rods, there is less grinding by impact, and the action resembles that of crushing rolls. As a result, a rod-mill product usually contains a greater percentage near the limiting size with less extreme fines than ball or tube mills.

In selecting the correct grinding media it is important that the rods or balls supplied be large enough to break the largest particles of ore in the feed, and as already discussed, a seasoned load composed of balls of all sizes, which is the condition found in a mill that has been operating for some time, gives better grinding efficiency than a new charge.

The volume of the charge is limited to a maximum of about 50 per cent of the mill volume. If the charge is too large, its center of gravity shifts too near the axis of the mill and the power input falls.

The speed of the mill is limited by what is known as the critical speed. This is the speed at which (assuming no slippage) the charge starts to cling to the liners, or to centrifuge. It is given by the formula.

The percentage of solids in the pulp is usually maintained at 60 to 75 per cent, the principle being to keep the volume percentage of solids as high as possible without loss of mobility of the charge. The correct proportion of water present will depend on the kind of ore being handled, slimy ores in general requiring a higher dilution than ores that have a low slime content.

The size of mill required for a specific grinding problem will depend on the character and size of the feed and the product desired and whether open- or closed-circuit grinding is desired. An accurate estimate of capacity can be made only by an engineer familiar with the proper evaluation of the factors involved.

For rough estimating purposes Table 6 gives approximate capacities grinding to 48 and 100 mesh for several size mills. Connected horsepower is also shown. These figures are for what would normally be considered average siliceous ore and for nominal circulating loads of 2 or 3 to 1.

These capacities may be reduced by as much as 50 per cent in the case of a hard, tough ore which is highly resistant to grinding, and for this reason considerable thought has in recent years been given to methods for determining the relative grindability of different ores and to correlating laboratory figures with plant performance. F. C. Bond has published comprehensive grindability data based on work carried out by the Allis-Chalmers Manufacturing Co. and grindability tests are a regular part of the testing procedure of the Dorr Company at the Westport, Conn., laboratories.

When the tube mill was first introduced, grinding was done in open circuit; i.e., the ore was ground to pass the limiting screen size by one passage through the mill. It was found, however, that if sufficient time of contact between the ore and grinding media were provided to ensure that no unground particles (or oversize) discharged from the mill, an excessive amount of fines were produced. This meant that the ore was ground much finer than necessary and mill capacity was correspondingly reduced.

The difficulty was overcome by placing a classifier in the circuit to separate out oversize from the mill discharge and return it to the mill feed. In closed-circuit grinding no attempt is made to finish the grind in one passage through the mill, but every effort is made to remove finished material as soon as it is released, thus reducing over-grinding and preventing the fines from hindering the grinding action on yet unreduced particles. In this way the tonnage that a given mill will grind is much greater than it is possible to grind in open circuit.

By using wide classifiers with high raking capacity, circulating-load ratios are now being carried to 4:1 or higher. The direct result of the increased capacity is reduced power, liner, and grinding media consumption per ton of finished ore.

There is, of course, a limit as to how large a circulating load can be carried in practice. While capacity continues apparently to improve, though at a decreased rate, it becomes increasingly difficult to move the growing volume of material through the system.

There is some controversy in the literature as to the definition of ratio ofcirculating load. The term used by most millmen is the ratio of sand tonnage returned to the mill to the tons of original feed.

If the mill-classifier circuit is fed into the classifier instead of into the mill, the sand contains oversize from the original feed as well as oversize from mill discharge, and thus the definition is not entirely accurate. The ratio of circulating load can be calculated from screen analyses by using the following formulas:

Circulating-load ratio = d o/s d where d = cumulative percentage 0n any mesh in the mill discharge o = cumulative percentage on same mesh in the classifier overflow s = cumulative percentage on same mesh in the classifier sand

There are many types of flow sheets in use today. The tendency in new mills is to crush relatively fine ( to in.). Single-stage ball mills in closed circuit with classifiers are used for grinds coarser than 48 mesh, but when a finer product is desired, two stages of ball mills in closed circuit with classifiers is usual. Efficiency must necessarily be sacrificed to some extent in small mills by capital requirements, and even greater reduction ratios are justified in a single-stage grinding unit.

With the large classifiers used for high circulating loads it is quite often necessary to use some kind of auxiliary device to complete the closed circuit. A large motor-driven scoop lifting the mill discharge to the classifier has been successful.

Stamp mills were built to parallel the operation of a mortar and pestle, working continuously and on a large scale. Ore is fed into a mortar and is crushed by the dropping of the stamp on a die at the bottom of the mortar. The crushed ore discharges through a screen in the side of the mortar.

The shoe that forms the wearing surface on the dropping stamp is attached to a steel stem and is replaceable. The stem is lifted by a cam operating against a tappet which is bolted to the stem. A common camshaft activates usually five stamps in a battery.

Milling was done in unique, crude wooden stamp mills developed by the ingenious Antioquenan miner. Made entirely of hand-hewn hardwrnod (except for cast-iron shoes, several bolts, and a few nails) these molinos Antioquenos have a stamp duty of approximately 0.4 tons per 24 hr. They are powered by overshot water wheels, 18 to 24 ft. in diameter, mounted directly on the 18- to 24-in. wooden camshaft of the mill. Up to 56 drops per minute can be obtained with a water-wheel speed of 14 r.p.m. The stamps, 6 by 7 in. by 14 ft. in dimensions, weigh 450 to 500 lb. including the cast-iron shoe. The mills are usually built with three stamps to the mortar box and as many as three sets (nine stamps) per mill. Battery-box screens are usually made of tin from 5-gal. gasoline cans perforated with a small nail. Stamp guides, cams, and the hardwood camshaft bearings are lubricated with beef tallow.

The stamp mill was originally devised as a combination grinding and amalgamating device before the days of cyanidation. Its use continued with theintroduction of the cyanide process, where it was well suited to the comparatively coarse crushing used, the distribution of the ground pulp over amalgamation plates, and the steps of separate cyanidation of sand and slimes that followed. As the all-sliming method became more generally adopted, however, with the need for fine grinding in ball mills and preferably in cyanide solution, the stamp mill tended either to be used as a secondary crusher or to be replaced altogether by dry-crushing equipment.

These two types of mill are practically obsolete. In these mills rollers driven from a central gear-driven spindle revolve around a pan. In the former the rolls crush against a ring in the bottom of the pan, and in the latter centrifugal force holds the rollers against the ring at the side of the pan. Chilean mills were used at the Golden Cycle up to a few years ago for grinding roasted ore.

chromium ore - an overview | sciencedirect topics

chromium ore - an overview | sciencedirect topics

Chromium ores are often classified based on their elemental composition and industrial use into: high-chromium (4655% Cr2O3, Cr:Fe >2.1) used for metallurgical processes, high-iron (4046% Cr2O3, Cr:Fe 1.52.1) used for chemical and metallurgical purposes, and high-aluminum (3338% Cr2O3, 2234% Al2O3) used widely as refractory material [1].

Knowledge of the chromium ore minerals proper and their associated rocks-forming minerals is needed to explore new chromite fields and to predict the efficiency of individual methods for the beneficiation of chromite ores. It is important also to know the physicochemical properties of individual mineral fractions of chromite ores and preparation of ore (or concentrates) for electric smelting, the fabrication of refractory materials, and the production of chemical compounds of chromium. The leading minerals of common chrome ores are chromium spinelides, which belong to the spinel group (natural spinel mineral MgAl2O4). All of these minerals crystallize in space group Fd3m. The spinel group includes not only oxides, but also certain sulfides Cr3S4, FeCr2S4 (daubreelite), and others. A unit cell of a spinel lattice contains 8 AB2O4 units (i.e., A8B16O32). These 32 oxygen atoms constitute the closest cubic packing of 64 tetrahedral and 32 octahedral positions (Fig. 8.13). Out of 96 cation sites per unit cell, only 24 are included in this face-centered cubic cell.

A distinction is drawn between normal, inverse, and mixed structures of spinel. In a normal spinel, bivalent A2+ cations occupy tetrahedral sites and trivalent B3+ cations are located in octahedral sites. In inverse spinels, the octahedral A2+ cations occupy tetrahedral positions, and half of the B3+ cations (in the ideal case) are in octahedral sites. The normal and inverse arrangements of cations in natural spinels represent the ultimate limits of spinel structures when the spinel structural formula is presented in the form (BxA1-x)tetr.(AxB2-x)octO4. Normal spinel has x=0 and inverse spinel has x=1. Natural spinels are usually classified into four mineral species as series with Al3+, Fe3+, Cr3+, and Ti4+. All spinels with normal and inverse structures form solid solutions that are stable at certain temperatures and pressures (Lyakishev and Gasik, 1998).

For example, chromites from certain ore deposits in Kazakhstan include chromium spinelides with a predominant content of (Mg, Fe)Cr2O4, which contains some Al3+ in octahedral sites. The structural formula of chromium spinelides might be expressed as (Mg5.44Fe2.568)(Cr12.64Al2.78Fe0.58)O32. The composition ratios of chromium spinelides from different countries are shown in Table 8.2.

The following minerals might accompany chromium spinelides: chrome dioxide, chrome actinolite, chrome garnet (uvarovite Ca3Cr2Si3O12), kammererite (Mg,Fe2+)5Al(Si,Al)4O10(OH)8, clinochlore (Mg,Fe2+)5Al(Si,Al)4O10(OH)8, rhodochrome, serpentine (chrysolite 3MgO2SiO22H2O), chrysolite asbestos, actinolite Ca2(Mg,Fe2+)5Si8O22(OH,F)2, magnetitecalcite MgCO3CaCO3, antigorite (Mg,Fe)3Si2O5(OH)4 olivine (A,B)2SiO4, magnetite Fe3O4, quartz, opal, pyrite FeS2, chalcopyrite CuFeS2, goethite FeOOH, limonite (FeO(OH) nH2O), and hydrous ferruginous aluminosilicates. Chromite ores of certain deposits contain minerals of the platinum group (Ir, Ru, Pt, Pd, and Rh), especially in South Africa and Zimbabwe, often found as inclusions in chromite silicate matrix.

The mineral and gangue components have different magnetic susceptibilities, which makes it possible to use various magnetic separation methods. Typical values of magnetic susceptibility of chromium spinelides in Kazakhstan deposits are 35 to 70. Gangue minerals might have much higher values (asbestos with fine magnetite inclusions has 2260) or much lower values (chlorides 10 to 11, magnesite 2.5, calcite 5).

Depending on the concentration of chromium spinelides and overall composition of chromium ore (i.e., the content of Cr2O3), the deposits are subdivided by way of convention into commercial and noncommercial types (Table 8.3). High grades might be essentially processed without or with a little dressing procedure; other types require proper dressing technologies.

Chromium ores are also subdivided according to their physical state (strong, friable, pulverized, and silicified) and the chromites grain size (coarse 7 to 15 mm, medium 1 to 7, and fine<1 mm). So-called strong ores occupy ~80% of all chromite ores, friable ores occupy ~10%, and pulverized and silicified ores occupy 5% each (Lyakishev and Gasik, 1998).

The ores of certain sites might be enriched with or depleted in Cr2O3, as well as complementary SiO2, A12O3, MgO, Fe2O3, and CaO, even though they retained their original textural and structural characteristics. That makes it difficult to carry out selective mining and beneficiation. Therefore, it is essential to carry out the classification of chromium ores from commercial deposits, so as to be able to choose the most effective methods and parameters for dressing processes, to utilize ores and concentrates for smelting specific types of chromium ferroalloys, and to obtain chromium compounds and refractory materials.

This process has been operated at Chiba Steelworks, Kawasaki Steel Corporation (currently JFE Steel Corporation) since 1994. In this steelworks, stainless steel is produced by smelting reduction process of chromium ore with hot metal produced at a BF in a specialized converter-type furnace. Stainless steel dust containing Cr and Ni is generated from this smelting process. STAR process treats such dust and FeCrNi metal is recovered and recycled at stainless steel production. Figure 4.4.21 shows the schematic diagram of the process. The furnace is a coke-packed shaft furnace with upper and lower tuyeres. Collected wet dust is conveyed as slurry. After drying, dust in powder form is injected with oxygen-enriched hot blast air at 1123K through upper tuyeres. Dust is melted and metal oxides are reduced during dripping down in packed coke between upper and lower tuyeres. The yield of metallic Cr reaches 98% due to its highly reducing condition.

Chromium is the 21st most abundant element in the earths crust with an average concentration of 100g/g. Although approximately 40 chromium-containing minerals are known, the only one of economic importance is chromite (FeOCr2C3). Chromium ores are often classified based on their elemental composition and industrial use into: high-chromium (4655% Cr2O3, Cr:Fe >2.1) used for metallurgical processes, high-iron (4046% Cr2O3, Cr:Fe 1.52.1) used for chemical and metallurgical purposes, and high-aluminum (3338% Cr2O3, 2234% Al2O3) used widely as refractory material [1]. More than 1 billion tons (430 million tons of chromium metal) of the worlds chromite reserves are located in South Africa. The world mine production of chromium in 1983 totaled 8.1 million tons [15]. Because more than 90% of the chromium reserves are in developing countries, and East-Europe and the U.S.S.R. countries and the principal consumer nations produce very little, chromium is a classic example of a strategic mineral as evidenced by a review of the historical trend of chromite production. Chromium is stockpiled by industrialized countries with about 5 million tons being stored in the U.S.A. [16].

The demand for chromium is dictated by the economic conditions of the steel industry which dominates the use of this metal. The three principal industrial uses for chromium each year are 76% for metallurgical, 13% for refractory, and 11% for chemical applications [17]. The metallurgical industry uses 44% of the total chromium it utilizes for the production of stainless and heat resistant steels [18]. The use of chromium for the production of refractories is the result of the high melting point (2040C) and the chemical inertness of chromite. Chromite is, therefore, useful in the production of refractory bricks, and mortars used in high-temperature furnaces [2]. The chemical industry produces more than 70 different chromium compounds for commercial use. Those produced in the largest quantities include: sodium chromate, potassium chromate, potassium dichromate, ammonium dichromate, chromic acid, and basic chromic sulfate used primarily for tanning leather [1,19,20]. Chromium compounds are used to prevent corrosion, improve product durability, and to provide high quality paints. Pigments for paints are often made from chromate and are used to provide color (lead chromates) or for corrosion inhibition (zincchromates) [15,21]. Chromates are also used as rust and corrosion inhibitors in engines and chromium compounds are added to antifreeze to inhibit corrosion and stop the growth of algae [1]. Chromic acid is used primarily for plating and is also used for photoengraving and offset printing. Many chromium compounds play a significant role in research serving as catalysts for chemical reactions and are commonly found in research laboratories.

The Morensky-type deposits can be found in very large bodies of basaltic magma, which were intruded into stable continental rock. An example includes the Busheld Complex in South Africa and the Great Dyke of Zimbabwe. Mineralization similar to the above is also found in the Stillwater Complex in Montana, USA.

The Busheld Complex consists of varieties of ore types, including high-chromium ores, ore with floatable gangue minerals and small but significant quantities of ultrafine slimes that are important from a processing point of view.

The Stillwater Complex consists of a sequence of differential layers of mafic and ultramafic rocks, which extend for a strike length of up to 40km and has a maximum exposed thickness of about 7.4m [3]. There are several mineralization zones at the Stillwater Complex, including a PGM-rich zone and a low-grade zone. The Stillwater ore that is processed nowadays contains olivine, plagioclase, as well as plagioclase-brauzite, all of which are naturally hydrophobic gangue minerals.

Another similar origin deposit is Lac des Illes in Canada. This complex is apparently contrary to a somewhat general rule in that of intrusion and is regarded as Archean age and may be therefore intruded prior to the Kenora origin into a technically unstable environment.

Chromium was discovered in 1797 by the French chemist Louis Nicolas Vauquelin, in a mineral that became known as crocoite, PbCrO4. A year later, he isolated chromium metal by reducing the oxide with carbon. Soon after, the commercial process for manufacturing chromates by roasting chromite with soda ash was developed. A process was developed in 1893 by Henri Moissan to produce ferrochromium in an electric furnace by the reaction of chromium oxide and carbon. In 1898, the German chemist Hans Goldschmidt produced chromium by the aluminothermic reduction of Cr2O3. Other advances have included the production of chromium by aqueous electrolysis in 1954.

Chromite is the only commercial source of chromium. Large deposits are found in Turkey and Albania. The most important applications of chromium ores are in the manufacture of stainless steel, gray cast iron, iron-free high-temperature alloys, and chromium plating for surface protection. The chemical industry uses chromium ores in the production of chromium compounds for the tanning of leather.

Chromium is extracted from its ores by two processes. In the alkaline process, the finely ground ore is roasted with Na2CO3 under oxidizing conditions at 1100 C. The sodium chromate is leached from the calcine. The solution is reduced with SO2 and used for electrowinning, or Na2Cr2O7 is crystallized then converted to Cr2O3 for chromium production by aluminothermic reaction. In the acidic process, the ore is either leached with sulfuric acid or reduced to form ferrochromium, which is ground and dissolved in sulfuric acid. Iron is removed by crystallization as ferrous ammonium sulfate. The chromium in the solution is used to produce electrolytic chromium.

Aluminum is the most important reducing agent for producing chromium from Cr2O3. Aluminum powder and Cr2O3 are blended and charged into a vessel lined with Al2O3 refractory. The charge is ignited electrically. The exothermic reaction results in a temperature >2000 C, which leads to a clean separation of slag from metal. Chromium is also produced by carbon reduction in a furnace at 1400 C. The carbon and oxygen contents are 1.5%.

In the electrolytic process, a diaphragm is necessary to prevent migration of Cr(VI) into the cathode compartment. Flow is maintained into the anode compartment from the cathode compartment by a higher level of solution in the latter. The pH of the catholyte must be controlled. At too low a value, H2 evolution increases; at too high a value, precipitation of Cr(OH)3 occurs. Chromium is plated onto stainless steel cathodes until it attains a thickness of ca. 3 mm. The plate is stripped from the cathode and degassed by heating at 420 C.

For many applications the oxygen content of the electrolytic chromium is too high. Deoxidation is carried out on a commercial scale by two techniques. In the first, either flake or briquettes of powdered flake are contacted with H2 at elevated temperature. The second technique involves heating briquettes of ground electrolytic flake and carefully controlled trace amounts of C in a vacuum furnace at 1400 C to form CO. The product is cooled in helium to prevent contamination.

Chromium occurs in nature primarily as ferrous chromite (FeCr2O4), where chromium is in the chromic (Cr(III)) state. Rivers and lakes contain 110gl1 Cr. Outdoor rural air contains <10ngm3 on average, and urban areas can contain up to 30ngm3 Cr. Soil concentrations range from trace to 250mgkg1. On average, 200000 tons of chromium is released into the environment annually by the natural weathering processes.

The first uses of Cr were in plating of swords, both in ancient China and by the Hittites in Egypt (1300 BC). The metallurgy industry is the predominant user of Cr ores today. Ferrochromium is manufactured by the reduction of the ore. In 2007, the world production of chromite was 20 million tons (U.S. Geological Survey 2008). The world resources of chromite are greater than 12 billion tons, and this is, according to the U.S. Geological Survey governmental agency, sufficient to meet conceivable demand for centuries. Ferrochromium and Cr metal are the most significant classes of Cr used in the alloy industry, as the anticorrosive property of chromium is highly desirable in steel alloys, stainless steel, and electroplating. Other applications of chromium agents are in wood preservatives, agricultural antifungicides, antifreeze, porcelain and glassmaking, anticorrosive paints, and photoengraving (Langard and Costa 2007).

The latest assessment of occupational exposure by the U.S. Department of Public Health and Human Services (PHHS) estimated the number of workers exposed to Cr to range from 300000 to 550000 (ATSDR 2000). The Occupational Safety and Health Administration (OSHA) has determined that the toxicological effect on the human body is similar from Cr(VI) in any of the substances covered under the scope of this standard, regardless of the form or compound in which it occurs (OSHA 2004). Many states have set permissible exposure limits (PELs) considerably lower than those set by federal agencies. Steel mills can reach levels of 1220gm3 of ambient chromium indoors. Settling of fly ash and chromium-containing industrial sewage deposition contaminate water sources. Drinking water standards are 100gl1 as set by the US EPA.

Chromium is found in all human organs of adults as well as newborns. Uptake of Cr(VI) through inhalation or ingestion is much quicker than that of Cr(III). The reason for the difference in cellular uptake has to do with the geometry of the two species: Cr(VI) is a tetrahedral structure whereas Cr(III) is octahedral, for which cellular uptake mechanisms do not exist.

Animal experiments indicate that Cr(III) is poorly absorbed from the digestive tract. Human absorption of Cr(VI) is quite variable between individuals and within an individual at different times. Cr concentrations are generally highest in the lung tissue. Cr is excreted predominantly through the urine. Because Cr(VI) compounds are more soluble and more toxic than Cr(III) compounds, and because the stomach has a greater capacity to reduce Cr(VI) to Cr(III), absorption of chromium in the lung tissue (by inhalation) is more dangerous to health than through the diet or drinking water.

Most of the reduction reactions taking place in the submerged arc furnace are highly endothermic and are listed in Table 1.10.1 along with standard Gibbs free energies as a function of temperature. Table 1.10.1 also lists for various reactions the temperature at which the CO pressure is one atmosphere, under standard state conditions for the reactants and products. In actual smelting practice, the reduction temperature will be raised if the oxide is chemically combined in the gangue, and lowered if the reaction product can dissolve in the ferrochromium alloy.

The data in Table 1.10.1 indicate that at these smelting temperatures, iron, chromium, and silicon form stable carbides. In the case of Cr2O3, it first reacts with carbon to form higher carbide, Cr7C3, which reacts back at higher temperatures with Cr2O3 to form lower carbide Cr23C6. This reaction occurs at approximately the same temperature as that required for reducing silica to SiC.

In submerged arc smelting, the carbon content of the ferrochromium is generally high and varies significantly because of the formation of various chromium carbides such as Cr3C2, Cr7C3, and Cr23C6. Table 1.10.2 shows the stability ranges for the reduction products and also the carbon contents of the carbides. The data indicate that temperatures over 1600C are necessary to achieve carbon content under 9%.

As mentioned earlier, chromite ores contain iron oxides and gangue that have significant effects on the reduction reactions in submerged arc smelting. Studies on solid-state reduction of chromite have shown that the iron oxides get reduced more readily than the chrome oxides. Under such conditions, iron forms only one carbide Fe3C, which can dissolve chromium to form a complex carbide (Cr,Fe)7C3. Thus, an ore rich in iron oxide will have high reducibility at relatively low temperatures. Accordingly, the reduction of chromic oxide in such ores will also occur at relatively lower temperatures with the likely formation of a carbon-rich chrome carbide (Cr3C2 or Cr7C3) known to be stable at lower temperatures.

The gangue present in a chromium ore has a significant effect on the temperature of the smelting zone, thereby influencing the carbon content of the ferroalloy. An ore having a relatively high MgO content will require a higher smelting temperature. Further, if silica flux addition is reduced or lime is added, the liquidus temperature of the slag will increase thus promoting high-smelting temperatures.

It has been argued that, because of their high surface-to-volume ratio, fine ores react readily at low temperatures forming a product high in carbon. The use of chromitecarbon agglomerates is reported to have produced high-carbon ferrochromium since they start reacting at lower temperatures. On the other hand, coarse-sized ores will not be as reactive and thus can survive to a lower depth in the burden and react with high carbon alloy, thereby promoting an LC ferrochromium.

One method proposed for minimizing the carbon content of the ferrochromium is to create a region in the furnace that is oxidizing to the chromium carbide with the use of a high-density high-smelting ore. The high density (~4g/cm3) will allow it to pass through the low density slag (~3g/cm3) and form a chromic oxide-rich layer above the molten ferrochromium. Such an ore layer can oxidize the chromium carbides. Further, due to its viscosity it will remain in the furnace even after tapping. This explains how relatively LC levels can be obtained with the use of dense, lumpy, low reducibility ores.

The silicon content of the ferrochromium should be low as it is an undesirable element in stainless steels. Low silicon content is favored by a low operating temperature, high carbon content in the ferroalloy and a basic slag. Other impurities are not known to affect the silicon content significantly. In general, the specifications for ferrochromium call for a silicon level below 3%. This can be achieved by utilizing the idea, mentioned earlier, of forming a layer of chromic oxide ore in the lower portion of the slag, which oxidizes some silicon in addition to carbon.

Thus, with increase in coke rate, the relative amount of SiO(g), and consequently, silicon in the metal will also increase. It is believed that under the highly reducing conditions in the submerged arc furnace, the silicon content of the ferrochromium would also depend on the silica content of the ash in the carbonaceous reductant used.

Phosphorous is detrimental to both the mechanical properties and corrosion resistance of stainless steels. In the submerged arc smelting of chromite ore, a portion of the phosphorus contained in the charge is vaporized and removed with the off-gas; however, up to 60% can be retained in the alloy, For low phosphorous levels (<0.02%) in ferrochromium, the phosphorus content of the raw materials should be as low as possible. Also a relatively low operating temperature will promote removal of phosphorus into the slag phase, especially under oxidizing conditions. However, due to the highly reducing and hot conditions in the submerged arc furnace, there are no easy ways to produce a low-phosphorus ferrochromium from high phosphorus ore/coke.

Even though several studies have been made on removal of phosphorus from liquid ferrochromium using fluxes such as CaF2CaC2 and CaCaF2, industrial practices mostly rely upon the use of imported low-phosphorus metallurgical coke as a preventive measure.

10 best chromium crusher grinders available now (2018)

10 best chromium crusher grinders available now (2018)

If youre going to go big with a Chromium Crusher grinder, you might as well head straight to the 2.5 4-piece grinder with a hand crank. While a hand crank grinder may look like something your grandma may purchase for her dry herb garden (which she definitely could do), its way cooler than that.

It comes in a typical gun metal color, which looks like a regular silver when you receive it. When you place the bud in the top chamber, put the cover on, then crank the bud down into the holding chamber.

From there, the big pieces of weed will stay, while tiny specs called trichomes, will fall into the kief catch. Kief, of course, is a super potent form of bud, which definitely ups your smoking experience. If youre looking for a grinder few can boast about, you definitely need this Chromium Crusher Grinder.

When it comes to fun grinder colors, Chromium Crusher is a huge proponent of pink. While many people associate pink as a girly/feminine color, its simply not true. Any person (guy or girl) is completely entitled to their love of the light-red hue.

And, grinders usually come in boring black, gun mental, or dark gray. So, investing in one of the fun-colored Chromium Crusher grinders will make your weed accessory really pop amongst the rest of your set-up.

The design is contoured to ergonomically fit your hand, making the grind smooth and easy. Plus, it includes a kief catch, in addition to a kief scraper (snag some here, if youre a fan of extras.) Youll be riding that pink passion to the green Cloud 9 in no time.

Plus, its available in a wide variety of colors. So if youre not digging on the black, check out their green, gold, purple, and more. This bad boy comes with a kief scraper, so you can indulge in extra stoniness whenever you please. Snag the Chromium Crusher 1.6 grinder for all of your traveling needs.

Crushing up nugs constantly can be annoying. So, a larger grinder is a solid choice for those who smoke on the reg. This Chromium Crusher is gun metal, which is basically that typical grinder-silver color.

Its a no-frills grinder that grinds up your bud perfectly, and sifts out kief really well. If youre looking for a Chromium Crusher grinder for your weed set-up at home, youve found it in the 3 gun metal grinder.

While a grinder is great, it can be a pain in the butt to figure out if youve ground up your bud enough. How many times have you ground up what you thought was all of a nug, only to find a sliver had come through? Its pretty annoying.

Like many Chromium Crusher grinders, this bad boy is 2.5 in diameter. Its made of a super heavy-duty aluminum alloy, so its extremely durable. This particular Chromium Crusher features a new, sleeker design, which ultimately makes your grinding experience even easier.

This bad boy is considered the 2.0 version of the OG Chromium Crushers. In addition to its increased grip strength, it also boasts a larger grind-able volume. In other words, you can grind more at once.

As a four-piece grinder, it has a three-chamber design, ensuring youll collect plenty of kief. And, a kief scraper is included. Measuring 2.5 in diameter and two-inches in height, its the perfect in-between size. And, its made of a super durable aluminum alloy.

If you know anything about grinders (and its ok if you dont!), then you know that each one grinds your bud differently. Some produce super chunky material, while others create bud thats almost as fine as kief.

I know, youre probably thinking the same thing as me. Ummmmm why would they make a grinder into a fidget spinner? On the surface, it seems stupid. Peep behind the curtains, though, and it makes sense.

Unlike other models on this list, though, its actually a three-piece grinder. So, it has a grinder chamber and a holding chamberbut no kief chamber. Dont worry about collecting that kief-y goodness: It comes with a scraper. Toke up and fidget with the same device.

Right on the side sits a window, allowing you to peep the grinders contents. The grinder itself measures 2.5 in-diameter by 1.75 in-height. Constructed with a heavy-duty, durable alloy, so you dont need to worry about busting it.

As a four-piece grinder, it has a three-chamber design, including a kief catch. To make your life easier, it also comes with a kief scraper. Especially because its designed with a 500-micron mesh screen, itll bring your kief game to the next level. And, did I mention this babys rainbow? Yeah. Rainbow.

Most Chromium Crushers measure two-to-two-and-a-half-inches. Well, this guy doubles some of them, with a diameter of four-inches (3.88, if youre being precise), and a height of 2.25. And, it weighs a hefty 21-ounces.

As a four-piece grinder, it collects kief at a steady rate. And, it comes with a kief scraper for your convenience. 54 diamond-shaped teeth grind your bud into the perfect, fluffy texture. Whether you pop it into a bowl or blunt, youll have plenty of leftovers for later.

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