Chapter 1

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PbCrO4. 16.09 Adelaide Mine, Dundas, Tasmania. Daubreelite. Fe2+Cr2S4. 36.1 Bolsonde Mapimi meteorite, Mexico. Eskolaite. Cr2O3. 68.42 Outokumpu ...
Chapter 1

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1.1Introduction World chromite supply has come under severe pressure over the past year driven by robust demand for ferrochrome used in ferroalloy production, ultimately used to make stainless steel. With the metal industry hungry for raw material, residual supplies that typically serve non-metallurgical markets (known as “special grades” for the refractory, chemical, and foundry sectors) are selling out. Over 90% of the world’s chromite production is converted into ferrochrome for metallurgical applications, a figure that dwarfs supply to non-metallurgical markets. Out of a total global chromite annual output of about 19 Mt per annum, the refractory industry accounts for about 1% while 3% each is consumed in the foundry and chemical industries. The smelting of chromite produces tailings with a high concentration of chromium.

1.2General Objectives To find an effective technology for extraction of chromium from chrome tailings.

1.3Specific Objectives  To study the occurrence of chromium.  To investigate the chemistry of chromium.  To study the technologies in existence that can be used to extract chromium from chromium tailings (slag).  To evaluate the technologies used in the beneficiation of chromium from chromium tailings.  To recommend the best technology that can be used to extract chromium from chromium tailings.

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Chapter 2 CHROMIUM

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World chromite supply has come under severe pressure over the past year driven by robust demand for ferrochrome used in ferroalloy production, ultimately used to make stainless steel. With the metal industry hungry for raw material, residual supplies that typically serve non-metallurgical markets (known as “special grades” for the refractory, chemical, and foundry sectors) are selling out. Over 90% of the world’s chromite production is converted into ferrochrome for metallurgical applications, a figure that dwarfs supply to non-metallurgical markets. Out of a total global chromite annual output of about 19 Mt per annum, the refractory industry accounts for about 1% while 3% each is consumed in the foundry and chemical industries. Given that most chromite is produced by vertically integrated ferrochrome producers, the amount of material available to supply non-metallurgical markets is dictated by the fluctuating requirements of the metallurgical industry. This chapter will consider the geology of chromium, its properties and its applications and uses.

2.1 Sources of Chromium

Chromium (Cr), a steely-gray metal which is rarely found as a pure metal in nature. It occurs mostly as different types of ore depending on their mineralogy and chromium composition. There are three main sources of chromium ores i.e. stratiform deposits, alphine deposits and rock deposits.

2.11Stratiform Mafic–Ultramafic Chromite Deposits

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These deposits are also known as chromium (Cr)-platinum (Pt) mafic–ultramafic complexes. Two examples are: i). The Precambrian Stillwater Complex in Montana, which was originally intruded as a sill. This basic layered intrusion composed of approximately 15 repetitious layers of hartzburgites, chromatite, bronzitite and is exposed only over a 48.28 km length because it is bounded by faults at either end. The complexes’ thickness has been measured from 4,876.8 m to 5,486.4 m. Thirteen chromatite zones have been recognized but only one zone has been mined. PGMs and disseminated nickel–copper sulde minerals also occur within the chromite ore zones. ii). The Bushveld Igneous Complex of the Republic of South Africa which covers an area that averages 280 km long by 160 km wide (with the greatest width approximately 400 km) and a thickness of 9 km centered on the town of Rustenburg in the Transvaal State. The Bushveld complex contains cumulus ferrogabbro to diorite, which has vanadium–titanium magnetite layers. Chromite layers occur in cumulate hartzburgite, dunite, and pyroxinite. The ores contain varying amounts of the minerals chromite, ilmenite, magnetite, pyrrhotite, pentlandite, chalcopyrite, and PGMs. Chromite ore thicknesses increase in basal depressions within the layers. Ore reserves may be several billion metric tons.

2.12Podiform- or Alpine-Type Chromite Deposits Podiform-type deposits occur as podlike masses in the ultramac portions of ophiolite complexes. Local rock types include highly deformed dunite and hartzburgite, which may be locally serpentinized (Schofield, D.I., Thomas, R.J., Goodenough, K.M., De Waele, B., Pitfield, P.E.J., Key, R.M., Bauer, W., Walsh, G., Lidke, D., Ralison, A.V., Rabarimanana, M., Rafahatelo, J.M., Randriamananjara, T. (2010)) The deposits generally are highly deformed, having formed throughout the Phanerozoic in the lower part of the oceanic lithosphere along spreading plate boundaries. They can be divided into minor podiform chromites, most of which are found in California and Oregon, and major podiform chromite, occurring mostly in Iran, Turkey, and Cuba. Ore mineralogy generally consists of chromite with possible ferrichromite, magnetites, and PGMs (ruthenium, osmium, and iridium). Minor podiform deposits range from about 0.16 metric tons to 10,000 5|P a g e Jeremia Mupamba

metric tons with a median of about 100 metric tons. Ore grades range from 10% to 56% 𝐶𝑟2 𝑂3 with a median grade of 44% 𝐶𝑟2 𝑂3 .( Shallo, M., Kodra, A., Gjata, K. (1990))

2.13 Rock

Elemental Cr concentrations in crustal rocks range from 20 mg/kg in felsic igneous rocks such as granites to more than 2,000 mg/kg in ultramac igneous rocks (and their metamorphosed equivalents). The crustal average (CA) is reported at approximately 100 mg/kg. The CA is the basis fordening the Cr enrichment factor (EF) in rocks and soils, which ranges from 2 for felsic rock types, primarily granites, to 20 for ultramac rocks. Cr(III) is also a trace element constituent of igneous rocks, probably occurring as separate phase minerals such as chromite, chromium-bearing magnetite, and/or ilmentite. It ranges from 100 mg/kg in amphiboles, pyroxenes (where it is actually dispersed in augite), biotite, magnetite, and olivine to 1.0 mg/kg in plagioclase and potassium feldspars. Therefore, the largest source of Cr in minerals in the crustal rocks is in the rockforming mineral. (Stowe, C.W. (1994))

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Table 1.0 Summary of the ores of chromium Mineral Name Ankangite Barbatonite Bracewellite Brezinaite Cassendanneite Caswellsilverite Chromatite Chromdravite Chromite Cocroite Daubreelite Eskolaite Formacite Magnesiochromite Nichromite Tarapacaite Tongbaite Zino Chromite

Formulae Ba(Ti, V3+, Cr3+)8 O16 Mg 6 Cr2 (CO3 )(OH)1 Cr3+O(OH) Cr3 S4 Pb 5 (VO4 )2 (CrO4 )2 NaCrS2 CaCrO4 NaMg 3 (Cr, Fe3+)6 (BO)3 Fe2+Cr2 O4 PbCrO4 Fe2+Cr2 S4 Cr2 O3 (Pb, Cu)3 [(Cr, As)O4 ]2 MgCr2 O4 (Ni, Co, Fe2+)(Cr, Fe3+ K2 CrO4 Cr3 C2 ZnCr2 O4

Cr conc % Type Locality 1.29 Ankang Country Shaanxi Province , China 6.03 Kaapche, Barberton , Transvaal, RSA 61.17 Merume River, Kamakusa, Guyana 54.88 Tucson Meteorite, Pima 6.86 Ural Mountains Russia 37.38 Norton County meteorite 33.32 Jerualem-Jerico Highway 20.99 Karelia , Russia 46.46 Batid der la Carrade, Gassin, Var , France 16.09 Adelaide Mine, Dundas, Tasmania 36.1 Bolsonde Mapimi meteorite, Mexico 68.42 Outokumpu, Karelia, Finland 6.93 El Khun Mine, Anarak, Iran 54.09 Schwarzenberg, Silesia, Germany 30.65 Transvaaal RSA 26.78 Santa Ana mine , Chile 86.66 Liu Zhuang China 44.56 Karelia Rusia

Source Hurlbut (1963); Martin and Blackburn (1999 and 2001); Perroud (2001); Webmineral (2002).: The properties of chromium are discussed in the next section.

2.2 Properties of chromium The properties of chromium are divided into two i.e. physical and chemical properties. 2.2.1 Physical Properties

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Chromium is a metal which exhibits some remarkable physical properties which are responsible for its wide range of application in industry. The physical properties of chromium are summarized in table 1.1 below. Table 1.1

PROPERTIES Density (@20°C/68°F) Melting point Boiling point Young modulus (GPa) Rigidity Modulus(GPa)

METRIC 7.19 g/cm3 1907°C 2672°C 279 115

2.22Chemical Properties Chromium is a member of group VIB of the periodic table. It has oxidation states ranging from 𝐶𝑟 +2 to 𝐶𝑟 +6 , but it most commonly occurs as 𝐶𝑟 0, 𝐶𝑟 +2 , 𝐶𝑟 +3 , and 𝐶𝑟 +6. Divalent chromium, however, is relatively unstable, being rapidly oxidized to the trivalent form; thus, only two forms-trivalent and hexavalent-are found in nature. The oxidation potential of hexavalent to trivalent chromium is strong, and it is highly unlikely that oxidation of the trivalent form could occur in vivo1.[8] The hexavalent form of chromium, almost always linked to oxygen, is a strong oxidizing agent. The trivalent state is the most stable and important oxidation state of chromium. In this state, it has a strong tendency to form complexes whose ligand rates of exchange are low (half-times of several hours). Trivalent chromium forms octahedral complexes of coordination number 6, and a large number of complexes are known-water, ammonia, urea,

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ethylenediamine, halides, sulfate, and organic acids. It is largely because of this kinetic inertness that these compounds persist for relatively long periods in solution, even in conditions in which they are thermodynamically very unstable. Chromic ion does not exist in solution. It forms complexes with water and other anions in an acid solution. In an alkaline solution, it olates by forming polynuclear com-1 pounds, which precipitate in time. Olation is enhanced by alkali and heat to 1200 𝐶.

2.3 Application and uses of chromium Because of its remarkable properties, chromium has wide range of application and uses. Cr is used in the manufacturing of stainless steel, numerous alloys, Cr plating, pigments, catalysts, dye, tanning, wood impregnation, refractory bricks, magnetic tapes, and more. 2.3.1 Paint manufacturing Chromium compounds are used in paint pigments. Chromates of barium (Ba), lead (Pb), and zinc (Zn) give us the pigments of lemon Cr, Cr yellow, Cr red, Cr orange, zinc yellow, and zinc green. Cr green is used in the making of green glass. Cr chemicals enhance the colors of fabrics and are used to achieve the brightly colored Cr-based paints for automobiles and buildings. 2.3.2 Stainless Steel Manufacture As an alloy, Cr has been referred to as the “guardian metal.” With as little as 10% Cr, an alloy made with steel or Fe protects these materials from corrosion, yielding the stainless steel and rust-less iron which are common household items, such as stainless steel knives, ball bearings, watch cases, and chrome front and rear vehicle bumpers. The ball bearings of chrome 9|P a g e Jeremia Mupamba

steel have been subject to more than 1,000,000 lb/in.2 or 6.895 × 109 Pa (N/m2). 2.3.3 Furnace linings Chromium plating has replaced Ni-plating owing to Cr’s superior hardness and resistance to chemical action. Heat-resistant Cr oxides are used for high temperature applications, such as the bricks used in lining furnaces. 2.3.4 Photography When potassium dichromate (𝐾2 𝐶𝑟2 𝑂7 ) is mixed with water and the solution is dried and exposed to light, it becomes solid again. This property is applied to the manufacture of waterproof glues and in photography and photo engravings.[8] 2.3.5 Tanning and drying process Chromium alum and chromic acid are used in the tanning and dyeing processes.

These are some of the uses of chromium, other uses are listed below.

        

Antifouling pigments Antifouling pigments High-temperature batteries Antiknock compounds Human joint replacement (hip) Alloy manufacturing Magnetic tape manufacture Catalysts Metal finishing

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                   

Ceramics Metal primers Corrosion inhibitors Dental constructions Phosphate coatings Drilling muds Photosensitization Electroplating (decorative finishes, hard-wearing surfaces) Pyrotechnics Electronics Emulsion hardeners Tanning Flexible printing Textile preservatives Fungicides Textile printing and dyeing Gas absorbers Wash primers Harden steel (armor plating, armor piercing projectiles) Wood preservatives

Because of the wide application of chromium there has been a growing need to extract chromium from the ground. The next chapter will look at the method used to extract chromium from chromite.

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Chapter 3

Extraction of chromium

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This section will consider the extraction of chromium from chromite. Chromite, also called iron(II)-Cr(III) oxide (𝐹𝑒𝐶𝑟2 𝑂4), is the principal ore of Chromium. 𝐹𝑒𝐶𝑟2 𝑂4 is a weakly magnetic,[7] Fe-black, brownish black to silvery white metal. 𝐹𝑒𝐶𝑟2 𝑂4 is of igneous origin and forms in peridotite of plutonic rocks. 𝐹𝑒𝐶𝑟2 𝑂4 occurs exclusively in mafic and ultramafic rocks as a crystal accumulated in the early stages of magmatic crystallization.[6] 𝐹𝑒𝐶𝑟2 𝑂4 has also been identified in serpentinites, which may be developed through hydrothermal alteration of a peridotite. Uvarovite, the Cr garnet, is commonly associated in the field with 𝐹𝑒𝐶𝑟2 𝑂4 . The Moh’s hardness of 𝐹𝑒𝐶𝑟2 𝑂4 is 5.5 and the specific gravity is 4.3 to 5.0 and because of these physical characteristics of 𝐹𝑒𝐶𝑟2 𝑂4, the metal is occasionally concentrated in placer deposits.

Fig 1.0 Chromite

Several methods are used to extract chrome chromite depending on the available equipment, a generalized method of the extraction of chrome ore will be discussed. Chrome ore (chromite) is extracted from the ground using excavators, thus run off mine is produced.[3] 13 | P a g e Jeremia Mupamba

Run off mine(ROM)

COMMINUTION

SEPERATION

PRODUCT HANDLING

Fig 2.0 Chromium mining and extraction

The ROM is transported from the mine site to where it is processed, this is usually done by conveyors or trucks depending on the particle size of the ore and the distance from the mine to where the processing is carried out.

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This is when the particle size of the ore is progressively reduced until the clean particles of mineral(chromium) can be separated by such methods as are available. Comminution in its earliest stages is carried out in order to make the freshly excavated material easier to handle by scrapers, conveyors, and ore carriers.

FEED (ROM)

SECONDARY CRUSHER PRIMARY CRUSHER

OVERSIZED

SCREEN

UNDERSIZED

PRODUCT

Fig 3.0 Comminution circuit

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The first stage of comminution is primary grinding. In this stage heavy duty machinery is used for example gyratory crushers. This stage reduces the ore particle size from around 2meter to around 2cm. The products from primary grinding are fed into the screen where the undersized materials proceed to the next stage i.e. grinding whereas the oversized materials are taken to secondary crusher for re-crushing. In secondary crushing, light duty machinery is used for example cone crushers. The last stage in comminution is grinding whereby the particles are reduced in size by a combination of impact and abrasion, either dry or in suspension in water. In this stage particle size of the ore is significantly reduced (from diameter of 2cm to a diameter of 100 μm).

FEED FROM CRUSHING

MILL

Oversized

Classifier

Fig 4.0 Grinding circuit

Machinery used here is light duty for example tumbling mills and ball mills.

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SEPERATION

3.2 Separation(Concentration) The object of mineral processing, regardless of the methods used, is to separate the minerals into two or more products with the values in the concentrates, the gangue in the tailings, and the "locked" particles in the middlings. Such separations are, of course, never perfect, so that much of the middlings produced are, in fact, misplaced particles, i.e. those particles which ideally should have reported to the concentrate or the tailings.

This section will consider the concentration of chromite ore using the chromothermic reduction of chromite, based on the use of chromium metal as a reducing agent.

3 𝐹𝑒𝐶𝑟2 0 + 2 𝐶𝑟 = 3 𝐹𝑒 + 4 𝐶𝑟2 03

This method is carried out vin a furnace as it requires high temperatures to take place. This method produces tailings with a percentage concentration of about 14-16%, thus the disposal of these tailings raises a lot of environmental concerns.

3.3Poisonous Nature of Chromium

The tailing obtained from this process usually contains 14-16% chromium, thus this presents a lot of problems when it comes to the disposal of chromium ore tailings. The effects of this chromium from the tailings are:  Results in algal bloom when the chromium waste is washed into rivers and other water sources.  The chromium in the tailings is oxidized to 𝐶𝑟 +6 which is carcinogenic. The human carcinogenicity of chromium (VI) is well established. It is classified by the International Agency for Research on Cancer (IARC) as a Group 1 agent, or “Carcinogenic to humans”.[12] IARC’s assessment was based on 17 | P a g e Jeremia Mupamba

many studies that indicate a risk of lung cancer in workers exposed to chromium (VI) through inhalation, especially those involved in chromate and chromate pigment production and electroplating. A possible risk of nose and nasal sinus cancers was found to have weaker grounding in evidence.6 IARC’s Group 1 includes 113 different hazards; among these are tobacco smoke, asbestos, sunlight, and wood dust. IARC classifies compounds of chromium (III) and metallic chromium as a Group 3 agent, “Not classifiable as to its carcinogenicity”.[13]

The next chapter will consider methods which can be used to treat chromium ore tailings so as to reduce the concentration of the chromium to levels below 2 %. the methods to be considered ,[4]are-:     

Froth floatation Convectional jigs Leaching Spiral concentrators Shaking tables

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CHAPTER 4

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This chapter will discuss the methods used to concentrate low grade ores i.e. process tailings so as to progressively reduce the amount of concentration of chromium to manageable levels.

The recovery, in the case of the concentration of a metallic ore tailing, is the percentage of the total metal contained in the ore that is recovered from the concentrate; a recovery of 90% means that 90% of the metal in the ore is recovered in the concentrate and 10% is lost in the tailings. The recovery, when dealing with non-metallic ores, refers to the percentage of the total mineral contained in the ore that is recovered into the concentrate, i.e. recovery is usually expressed in terms of the valuable end product. The ratio of concentration is the ratio of the weight of the feed to the weight of the concentrates. It is a measure of the efficiency of the concentration process, and it is closely related to the grade or assay of the concentrate; the value of the ratio of concentration will generally increase with the grade of concentrate. The enrichment ratio is the ratio of the grade of the concentrate to the grade of the feed, and again is related to the efficiency of the process

4.1 Froth floatation

Froth flotation is a process whereby valuable minerals are selectively separated from the invaluable material (gangue), based on the surface properties of the finely ground mineral particles (Banford, A. W., Aktas, Z., and Woodburn, E. T., 1998) Flotation is achieved by selectively imparting hydrophobicity to the desired mineral particles and bubbling air bubbles through the slurry. The mineral is rendered hydrophobic (Du Plessis, R., Miller, J. D., and Davidtz, J. C., 2003) by adding a surfactant or collector chemical to the slurry in a conditioning tank before the pulp is pumped into the flotation cell (Figure 5) where flotation takes place.

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Fig 5

Below is a froth floatation circuit that can be used to concentrate chromium from chrome tailings.

Additional reagents used are-:

Collectors

SIBX

Depressant

CMC Norilose

Frother Activator

Dow 200 Copper Sulphate

Table 2 Froth floatation is one of the most popular methods used, but however it is accompanied by numerous demerits.

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MERITS DEMERITS  VERY EFFECTIVE IN SEPARATING  It is expensive to operate since VERY FINE PARTICLES. froathers and regulators are used  EXCELLENT FOR SEPARATION  There is need for dewatering LOW GRADE FEEDS I.E. PRODUCE since the concentrate produced TAILINGS WITH LESS THAN 3% is in slurry form OF THE CHROMIUM.

Table 3

Floatation circuit

Concentrate

Fig 6

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Tailings

4.2 Convectional Jigs

Convectional Jigs can be used for the concentration of chromium from chrome sludge (] Pascoe, R.D., Power, M.R., Simpson, B. (2007) In principle, separation of particles of differing specific gravity is effected in a bed resting on a ragging screen (Papp, J.F. (2005)) The bed is fluidized by a vertical pulsating motion created by a diaphragm and an incoming flow of hutch water, coupled with a bed of intermediate specific gravity particles or ‘‘ragging’’. The pulsating and dilating action of this motion on the bed causes the heavier particles (high specific gravity and size) to sink into and through the ragging to form a concentrate underflow, and lighter and smaller particles to form a tailing overflow. (Ports and Dredging, 1991)

The jigging circuit for the separation of chrome tailings is shown below.

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Feed

Pump Coarse jig

Pump Middle Jig

Fine Jig

Chromium in slurry

Tailings

Dryer

Tailings

Fig 7

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ADVANTAGES  NO SLIME GENERATION

DISADVANTAGES  Operation an ‘‘art’’, largely based on experience, and is subjective  OPEN FOR EYE INSPECTION  Recovery of fines is difficult as it DURING CONCENTRATION, THUS requires use of multiple jigs PROBLEMS DURING PROCESS CAN BE EASILY IDENTIFIED AND CORRECTED.  Uses a lot of water

4.3 Leaching

This method uses chemicals like Potassium hydroxide to reduce the chromium tailings thus producing Potassium dichromate, which goers for further processing so that we get pure chromium. This process is accompanied by the use of high temperatures, thus is an energy consuming process.

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Leaching Circuit

RECYCLE OF KOH

AIR BLOWN COOLER

MIXING OF FEED WITH KOH

PRESSURE LEACHING VESSEL

FILTER CAKE

FILTER

WATER

PRODUCT (POTASSIUM DICHROMATE)

RESIDUE

ROTARY FILTER

TAILINGS

Fig 8

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MERITS DEMERITS  EXCELLENT REAGENT  Product is produced as RECOVERY THUS REDUCES potassium chromate thus there OPERATIONAL COSTS. is need for further processing so as to get pure chromium.  IT IS POLLUTION FREE  High energy consumption as high temperatures are used  Cannot handle high tonnages of feed.  Takes a lot of time to process small amounts of feed.

4.4 Spiral Separators

It is composed of a helical conduit of modified semi-circular cross-section. Feed pulp of between 15 and 45 % solids by weight and in the size range 3 mm to ~75 µm is introduced at the top of the spiral and, as it flows spirally downwards, the particles stratify due to the combined effect of centrifugal force, the differential settling rates of the particles, and the effect of interstitial trickling through the flowing particle bed (Wills, B.A. & Napier-Munn (2006).

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Fig 9 Cross-section of a spiral separator

The wash waterless spirals are used during the concentration of chromium and have proven very economic and energy efficient despite the fact that they cannot handle high capacity feed and their low recovery ration. The merits and the demerits of using spiral concentrators are summarized below.

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MERITS DEMERITS  USES LESS WATER COMPARED  Separation between middling’s TO OTHER METHODS SUCH AS and concentrate is difficult thus JIGS causing a reduction in the efficiency of separation.  LOW COST OF MAINTENANCE AS  Low capacity IT DOES NOT HAVE MOVING PARTS.  EASY OPERATION  Operation is batch wise not continuous  NO ADDITIONAL REAGENTS  Concentrate produced needs REQUIRED drying as it contains moisture.

4.5 Shaking tables

Fig 10 shaking table

It consists of a slightly inclined deck (A), onto which feed, at about 25% solids by weight, is introduced at the feed box and is distributed along a duct (C); wash 29 | P a g e Jeremia Mupamba

water is distributed along the balance of the feed side from launder (D). The table is vibrated longitudinally, by a mechanism (B), using a slow forward stroke and a rapid return, which causes the mineral particles to "crawl" along the deck parallel to the direction of motion. The minerals are thus subjected to two forces, that due to the table motion and that, at right angles to it, due to the flowing film of water. The net effect is that the particles move diagonally across the deck from the feed end and, since the effect of the flowing film depends on the size and density of the particles, they will fan out on the table, the smaller, denser particles tiding highest towards the concentrate launder at the far end, while the larger lighter particles are washed into the tailings launder, which runs along the length of the table. The merits and the demerits are summarized below.

MERITS DEMERITS  LOW OPERATIONAL COST AS  High maintenance cost of THERE IS NO NEED FOR machinery ADDITIONAL REAGENTS  CAN HANDLE DRY FEEDS OR  Uses a lot of electricity FEED IN SLURRY FORM  Cannot handle high feed rates for example 90t/hr

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The methods listed above are some of the most popular methods used for concentration of chromium tailings. The next chapter will evaluate these methods using the K-T analysis.

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CHAPTER 5 Evaluation of the technologies.

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In this chapter the different methods used to concentrate chrome slag are evaluated, thus finding a suitable method that can be used in concentration. Kenper-Technology (K-T) analysis method will be used.

5.1 K-T Analysis

The KT problem analysis is a systematic, structured methodology that is used for analyzing information, prioritizing and evaluating it. It evaluates courses of action to utilize the ultimate results based on the explicit objective.[10] It is a step by step process for systematically solving problems and analyzing risks and opportunities. This analysis gibes the best possible choice based on achieving the outcome with minimal negative consequences.[11]

5.2 KT procedure

 State the Problem  Specify the Problem (What, When, Where, Extent, ‘Is’ vs ‘Is Not’)  Develop possible causes from knowledge and experience or distinctions and changes (Kepner, C. H. and Tregoe)  Test possible causes against the specification  Determine the most probable cause  Verify assumptions, observe, experiment, or try a fix and monitor

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5.3Categories for evaluation CATEGORYOF EVALUATION COST

DESCRIPTION

ASSIGNED SYMBOL

Cost of equipment thus the capital investment for buying the equipment to be used.

A

Operational cost thus the costs of the reagents used during the process

B

Maintenance cost thus the cost of maintaining and repairing the machinery.

C

OF This is how much feed the technology can handle per hour. REAGENT RECOVERY How well the technology recovers the reagents used for reuse. GRADE QUALITY The quality (purity) of the products produced. ENVIRONMENTAL The technology’s effect SUSTAINABILITY on the environment in terms of production of toxic waste. ENERGY EFFICIENCY Power requirements for operating the machinery.

D

CAPACITY EQUIPMENT

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E

F G

H

5.4Degree of effectiveness

EFFECTIVENESS OF TECHNOLOGY GOOD REGULAR POOR

SCORE 3 2 1

5.5 Evaluation of Technologies

CRITERIA EVALUATION A B C D E F G H TOTAL

OF 4.1 1 1 2 1 2 3 3 1

4.2 1 1 1 3 3 2 1 2

14

14

TECHNOLOGIES 4.3 3 3 2 1 3 1 1 1 15

4.4 3 3 3 2 2 1 2 3 19

From the above analysis, technology 4.4 has the highest score, but using it presents problem such as grade quality. This can be improved by combining several separation technologies to make an ultimate technology. The next section will consider methods of optimizing the separation stage.

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5.6Improvement of the separation process.

Combination of different technology can result to a sharper separation. Below is a diagram to show the separation circuits.

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Fig 11 combinational circuit This combined circuit results in:  Improved grade efficiency  Lower energy requirements

These are some of the advantages of combining the circuit.

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5.7 Conclusion

The concentration of chromium sludge(tailings) is carried out so that the concentration of chromium in discarded tailings is reduced to relatively harmless and environmentally friendly concentrations i.e. less than 1% by weight. This can only be achieved by combination of different technologies. The combination of these technologies at times becomes very problematic as they require large initial investments and they consume a lot of energy. Efforts are underway to remedy the situation.

References

[1]Banford, A. W., Aktas, Z., and Woodburn, E. T., 1998, Interpretation of the effect of froth structure on the performance of froth flotation using image analysis, Powder Technology, vol. 98, pp. 61-73.

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[2] Du Plessis, R., Miller, J. D., and Davidtz, J. C., 2003, Thiocarbonate collectors in pyrite flotation fundamentals and applications, Accepted for presentation: XXII International mineral processing congress, Cape Town South Africa

[3] Papp, J.F. (2005) Chromium. Minerals Yearbook 2003. Volume 1.

[4] Nafziger, R.H. (1982) A review of the deposits and beneficiation of lower-grade chromite. J. South African Inst. Mining and Metallurgy, 205–226.

[5] Pascoe, R.D., Power, M.R., Simpson, B. (2007) QUEMSCAN analysis as a tool for improved understanding of gravity separator performance. Miner. Eng. 20, 487–495.

[6] Pascoe, R.D., Power, M.R., Simpson, B. (2007) QUEMSCAN analysis as a tool for improved understanding of gravity separator performance. Miner. Eng. 20, 487–495.

[7] Nafziger, R.H. (1982) A review of the deposits and beneficiation of lower-grade chromite. J. South African Inst. Mining and Metallurgy, 205–226.

[8] Schroeder, H. A., J. J. Balassa, and I. H. Tipton. Abnormal trace metals in man-chromium. J. Chron. Dis. 15:941-964, 1962.

[9] Malitch, K.N., Thalhammer, O.A.R., Knauf, V.V., Melcher, F. (2003) Diversity of platinum-group mineral assemblages in banded and podiform chromitite from the Kraubath ultramafic massif, Austria: evidence for an ophiolitic transition zone? Mineralium Deposita, 38, 282–297. 39 | P a g e Jeremia Mupamba

[10] Kepner, C. H. and Tregoe, B.B. 1997. The New Rational ManagerPrinceton: Princeton Research Press

[11] Fogler, H. S. and LeBlanc, S. E. 2009. Strategies for Creative Problem SolvingUpper Saddle River: Prentice Hall

[12] International Agency for Research on Cancer. IARC monographs on the evaluation of carcinogenic risks to humans: Arsenic, metals, fibres, and dusts. Volume 100 C: A review of human carcinogens. Lyon, France: International Agency for Research on Cancer; 2012. Available from: http://monographs.iarc.fr/ENG/Monographs/vol100C/mono100C.pdf

[13] International Agency for Research on Cancer. IARC monographs on the evaluation of carcinogenic risks to humans: Chromium, nickel and welding. Volume 49. Lyon, France: International Agency for Research on Cancer; 1990. Available from: http://monographs.iarc.fr/ENG/Monographs/vol49/mono49.pdf. [14] NAIKER, O. and RILEY, T. Xstrata Alloys in Profile. Johannesburg: South African Institute of Mining and Metallurgy, South African Pyrometallurgy 2006, 5–8 March, 2006.

[15] BARNES, A.R., FINN, C.W.P., and ALGIE, S.H. The prereduction and smelting of chromite concentrate of low chromium-to-iron ratio. Johannesburg, Journal of the South African Institute of Mining and Metallurgy, March 1983

[16] IHC Jig versus other gravity separators—reprinted from Ports and Dredging. IHC Holland. Sliedrecht, Holland: Ports and Dredging, 1991, vol. 128

40 | P a g e Jeremia Mupamba

[17] Wills, B.A. & Napier-Munn (2006) Wills’ mineral processing technology. Butterworth-Heinemann, Oxford, 444 pp.

[18] Stowe, C.W. (1994) Compositions and tectonic settings of chromite deposits through time. Economic Geol. 89, 528-546.

[19] Shallo, M., Kodra, A., Gjata, K. (1990) Geotectonics of the Albanian ophiolites. In: Malpas, J., Moores, E.M., Panayiotou, A., Xenophontos, C. (Eds.), Ophiolites: Oceanic Crustal Analogues: The Geological Survey Department, Cyprus, pp. 265– 270.

[20] Schofield, D.I., Thomas, R.J., Goodenough, K.M., De Waele, B., Pitfield, P.E.J., Key, R.M., Bauer, W., Walsh, G., Lidke, D., Ralison, A.V., Rabarimanana, M., Rafahatelo, J.M., Randriamananjara, T. (2010) Geological evolution of the Antongil Craton, NE Madagascar. Precambrian Research, 182, 187–

41 | P a g e Jeremia Mupamba