sign in sign up
<<
Home , Slag

ASSIGNMENT BY:- PRANTIK BANERJEE (2019UGMM027) NISHANT (2019UGMM083)

REFINING OF METALS

In metallurgy, consists of purifying an impure metal. It is to be distinguished from other processes such as and calcining in that those two involve a chemical change to the raw material, whereas in refining, the final material is usually identical chemically to the original one, only it is purer. The processes used are of many types, including pyrometallurgical and hydrometallurgical techniques. METHODS OF METAL REFINING Refining plays a crucial role in metallurgy. Any metal which has been extracted from its is usually impure in nature. This impure metal which is extracted is called crude metal. Refining is a method of removing impurities in order to obtain metals of high purity. The impurities are removed from crude metal by various methods based on the properties of the metal and the properties of impurities. Some methods involved in the purification of crude metal are:

• Distillation • • • Zone refining • Vapour phase refining • Chromatographic methods Distillation: This method is used for the purification of metals which possess a low boiling point such as mercury and . In this process the impure metal is heated above its boiling point so that it can form vapours. The impurities do not vaporize and hence they are separated. The vapours of the pure metal are then condensed leaving the impurities behind. Liquation: In this method, melting point of the metals are taken into consideration. Metals with low melting points are purified using this process. The melting point of the impurities is higher than the metal. The metals are converted into liquid state by supplying heat at a temperature slightly above their melting point. Pure metal melts and flows down from the furnace leaving the impurities behind. Electrolysis: In this process the impure metal and a strip of pure metal is used. The crude metal is made to act as an anode and pure metal is made to act as a cathode. They are dipped in an electrolytic bath which contains the soluble salt of the same metal. As electricity is passed through the solution, the less basic metal moves towards the anode mud leaving the more basic metal in the solution. For example is purified using this method. Zone refining: Metals such as germanium, , gallium, indium and boron are made free from impurities using this method. In this process the impure metal is attached to a circular mobile heater at one end. As the heater is moved, the pure metal crystallizes out and the impurities pass on to the adjacent part of the metal. This way the impurities get accumulated at the other end of the rod which is cut in order to obtain the pure metal. Vapour phase refining: In this type of purification, the metal should form a volatile compound in the presence of a reagent and it should easily decompose in order to recover the metal. The metal is transformed into its volatile compound. This volatile compound then undergoes decomposition in order to give pure metal. For example: is purified in this manner. Chromatographic method: In this method the crude mixture is put into liquid or gaseous medium. This medium is moved through an adsorbent. Different components of the mixture are adsorbed at different levels of the column. These components of the mixture are removed by using suitable solvents.

DISTILLATION OF METALS

LIQUATION

REFINING OF COPPER

The purpose of refining to get Cu extraction is twofold as • First, to obtain metal in pure form. • Second, to recover precious metals containing in blister Cu produced.

(i)Fire Refining: The purpose is to remove S from liquid blister Cu as SO2 by oxidation with air and eliminates O2 by introducing hydrocarbons. In this case, the refining is done in reverberatory furnace of 400 ton of capacity contains blister Cu get oxidized to recover Cu removing the impurities such as S, Fe, Se, Zn by converting its corresponding and then skimmed off. But, some Cu also in the form of oxides. To prevent Cu loss with green branches used to reduce Cu2O using hydrocarbon or some other reducing gases. In this case, the purity of Cu obtained 99.97%. Fire refining is also done in rotary type refining furnace, where blister Cu directly treated by blowing air.

(ii) Electrolytic Refining: The purpose is to further refined the fire-refined Cu by electrolysis. The electrolysis done in a electrolytic refining tank made of concrete or wood of 3-5 m deep and utilization minimum space with maximum cathode and anode area. The electrolyte is CuSO4, H2SO4, some glue and alcohol at temperature 50-600 C. Cu transferred from crude anode to pure cathode. Impurities in blister Cu such as Fe, Co, Ni, Se, Te etc. go into the solution and precious metals get collected below anode known as anode slime.

(ii) Newer Process of Cu refining: :

Fig. 2.6: Flash Smelting Process:

Enriched preheated air or pure O2 used to increase combustion rate and autogeneous smelting. The gases coming out rich of SO2 due to high combustion rate and used for H2SO4 manufacturing. Process is autogeneous provide exothermic heat. Air used as oxidant to preheated. The composition of concentrate used in flash smelting has Chalcopyrite (CuFeS2) 66%, Pyrite (FeS2) 24%, (SiO2) 10%. Whereas the Cu matte contains 70% Cu, 8% Fe, 22% S, slag contains Fe 40% at furnace temperature 13000 C.

Main reactions of flash smelting of Cu concentrate are mentioned below

Continuous Smelting: It encompass smelting and converting in a single vessel i.e. Cu concentrate charged at one end and Cu metal withdrawn continuously at the other end. Mainly three are three processes, given below (a) WORCRA. (b) Noranda. (c) Mitsubishi.

(a) WORCRA Process: This name divided as the first 3 alphabets stands for the developers and last 3 alphabets for the place. Features: • Counter current movement of gas and concentrate. So, continuous production of blister Cu. • Directly blister Cu i.e. metal instead of matte form. • Combine smelting and converting. • The heat required for reaction directly obtained as the reaction is exothermic. • Counter movement cause continuous production of H2SO4 due to continuous extraction of gas. • Cu% continuously obtains from slag by means of cleaning operation. Process: The process combines 3 different operations in a single furnace as • Continuously smelting • Continuously converting • Continuously slag cleaning by conditioning and settling Efficiency: It increases by means of counter current movement increase the reaction surface area in the smelting and converting zone. Hence, effective removal of impurity occur i.e. mainly Fe due to counter movement of slag and matte. As a result, Cu gets reverted back to matte and obtain. Advantage : • Continuous process • Capital cost low • Concentrate passes large surface area. Hence accelerate the reaction Disadvantage: • Not durable • Operating cost is high

Noranda process: The Noranda reactor is a horizontal cylindrical furnace with a depression in the centre where the metal collects and a raised hearth at one end where the slag is run off. Pelletized unroasted concentrate is poured into the molten bath at one end, where tuyeres inject an air-oxygen mixture. This causes an intense mixing action that aids the melting, smelting, and oxidation steps, which follow one another in sequence, by taking advantage of the exothermic heat. The TBRC also is cylindrical in shape but is inclined at 17° to the horizontal, has an open mouth at the high end for charging and pouring, and revolves at 5 to 40 rotations per minute. A lance inserted through the mouth can give any combination of oxygen, air, or natural gas to impinge on the molten bath and create the conditions required for smelting and oxidizing. The combination of surface blowing and bath rotation improves the performance of the converter.

In this process, the slag contains high% Cu compare to WORCRA process. Mainly 3 layers present in the product as • Cu – First layer. • Matte – Second layer. • Slag – Third layer. Condition: • If given air is more than the stoichiometric amount of air required for oxidation, then matte level decreases and Cu level increases. • If insufficient air required for stoichiometric amount then unoxidised and S tends to combine with Cu to form matte. So matte level increases and Cu level decreases. • If air supply is equal to the stoichiometric amount then both matte and Cu level get increases.

NORANDA PROCESS REACTOR

NORANDA PROCESS FLOWCHART

The Mitsubishi process: The Mitsubishi process is a continuous smelting-converting operation that uses three stationary furnaces in series. The first furnace is for smelting, with oxygen lances and a fuel-fired burner inserted through the roof. Slag and matte flow from here to a slag-cleaning furnace (heated by electric arc), and high-grade matte flows from this to the converting furnace, where oxygen-enriched air is blown into the bath through roof lances. Exothermic heat produced here is sufficient to keep the bath up to reaction temperature.

Process: First, in smelting furnace (wet concentrate + + air + O2) is smelted to produced matte of 60-65% Cu and rest is slag. Secondly, both matte and slag goes into slag cleaning furnace where slag get discarded and matte goes to next furnace operation. Thirdly, in converting furnace matte oxidized to blister Cu by blow of O2 enriched air and add as slag. So slag discarded as lime ferrite. Blister Cu produced of low % S and hence, obstruct the transfer of Cu to lime ferrite slag.

REFINING OF NICKEL

1. Carbonyl Process for Refining Ni:

(i) Mond’s Process: In 1889, this refining process of Ni recovered by Carl Langer and Ludwig Mond. In this process, at temperature (40-90)0 C metallic Ni combine with CO to give gaseous nickel carbonyl [Ni(CO)4]. At higher temperature (150-300)0 C Ni(CO)4 decomposes to give Ni and CO gas. The reactions are

Other forms of Carbonyls are volatile carbonyl [Fe(CO)5], Co carbonyl in tetracarbonyl [Co2(CO)8] tricarbonyl [Co4(CO)12] form. Cu and other major elements are not form carbonyls.

(ii) INCO Process:

INCO Atmospheric Carbonylation Process: The first reduces to active Ni in the presence of H2 at about 4000 C. Then active Ni undergoes for carbonylation at 500 C to form Ni(CO)4 then at 2300 C goes for decomposed to Ni either in pellets about 1 cm dia or powder form about 3.5 µm size.

INCO Pressure Carbonylation Process: The carbonylation reaction has 4 to 1 volume change permits at about 1800 C and 70 atm pressure carbonyls of Ni, Fe, and Co formed. From which Ni(CO)4 recovered by fractional distillation and converted to metallic Ni in pellet decomposer or a powder decomposer. 2. Electrolytic Refining of Ni:

Fig. 2.21: Electrolytic Refining of Ni The Ni oxide reduce by coke in fuel fired furnace or electric furnace, and then cast into Ni metal anode. These anodes are electrolytically refined in a bath contains 60 gm/lit Ni+2, 95 gm/lit SO4 2- , 35 gm/lit Na+ , 55 gm/lit Cl- , 16 gm/lit H3BO3. This electrolysis carried out at 600 C. Cu remove by cementation with active Ni powder, Fe and other impurities remove by aeration of electrolyte, Co remove by Cobaltic Hydroxide for further Chlorine oxidation. Electrolyzed Ni analyzes about 99.93% Ni. 3. Flash Smelting of Ni Flash smelting combines and melting in one furnace. Most of its heat comes from oxidizing Fe and S in the concentrate feed. It requires very little hydrocarbon fuel. It accounts for ~ 3/4 of Ni sulfide smelting [28]. Flash smelting entails continuously blowing oxygen, air, dried particulate sulfide concentrate and silica flux into a 1300 °C furnace. The principal objective of flash smelting is to produce molten matte that is richer in Ni (and other metals) than the original concentrate. The enrichment is obtained by: (1) oxidizing much of the concentrate’s S and removing it as SO2 gas, (2) oxidizing much of the concentrate’s Fe and removing it in molten Fe slag (with SiO2 flux), and (3) removing the concentrate’s “oxide” rock by dissolving it into the molten slag. Thus, its products are molten sulfide matte, molten Ni-lean slag, and hot, dust-laden off-gas containing 20–50 vol.% SO2. The advantages of flash smelting over roasting/electric furnace melting are that flash smelting: (1) requires much less energy than electric smelting and (2) avoids the electric furnace’s emission of weak SO2 gas. It is also a high production rate process, perfectly matched to the 50–100-μm particles in concentrates. Most of the heat needed for smelting comes from concentrate oxidation, i.e., from reactions like: Flash smelting’s main disadvantage is that it loses more Ni to slag than electric furnace smelting because it is more oxidizing. It almost always needs a subsequent electric settling furnace matte-from-slag recovery step. The electric settling furnace is sometimes included as an appendage to the flash furnace. Nickel flash smelting furnaces are either ’s or Inco’s design Outotec flash smelting oxidizes its concentrate by mixing dried concentrate and flux with oxygen- enriched air and blowing the mixture downward into a hot furnace. Inco flash smelting mixes dried concentrate and flux with oxygen and blows it horizontally into a hot furnace. Otherwise, the processes are similar. Outotec flash smelting (seven Ni smelting furnaces worldwide) accounts for about 70% of Ni flash smelting. Inco flash smelting (two furnaces, one smelter worldwide) accounts for ~ 30% .

(a) Outotec Flash smelting furnace (b) Inco Flash smelting furnace

REFERENCES

• WIKIPEDIA • LECTURE NOTES FROM IIT • ENCYCLOPEDIA BRITANNICA • WILLS PROCESSING TECHNOLOGY BOOK