Uses <ul><li>Protective coating for steel (Galvanizing) </li></ul><ul><li>Fabrication of alloys such as Cu-Zn brasses </li></ul><ul><li>Ideal for die- casting </li></ul><ul><li>Rolled Zinc plates are used in dry cell batteries </li></ul><ul><li>Formability of the metal makes it ideal for forging and extrusion </li></ul><ul><li>Also used with other compounds in manufacture of paints and pigments. </li></ul>
Major ores of Zinc <ul><li>Sphalerite (ZnS) </li></ul><ul><li>Zincite (ZnO) </li></ul><ul><li>Franklinite [ZnO(Fe, Mn) 2 O 3 ] </li></ul><ul><li>Calamine[Zn 2 (OH) 2 SiO 3 ] </li></ul><ul><li>Smithsone (ZnCO 3 ) </li></ul>
Processes of Zinc extraction Name of process Horizontal Retort Vertical Retort Electrothermic Electrolytic Imperial Smelting Year of commercial adoption 1800 1930 1936 1915 1950
The principal processes by which zinc is extracted from its ores can be categorized under pyro-metallurgical processes and hydro-metallurgical processes. <ul><li>Pyro- metallurgical processes </li></ul><ul><li>Horizontal Retort </li></ul><ul><li>Vertical Retort </li></ul><ul><li>Electro- thermal </li></ul><ul><li>Imperial Smelting </li></ul><ul><li>Hydro- metallurgical processes </li></ul><ul><li>Roast leach electrowin </li></ul><ul><li>Pressure leaching </li></ul><ul><li>Electrolytic </li></ul>
A common flowsheet of both pyro- metallurgical and Electrolytic (Hydro- metallurgical) processes may be depicted as
Presently about 15 - 20% of the world's zinc production comes from pyrometallurgical route.
ROASTING Both of the processes (pyro+ hydro- metallurgy) share the same first step: roasting. Roasting is a process of oxidizing zinc sulfide concentrates at high temperatures into an impure zinc oxide, called " Zinc Calcine ". The chemical reactions taking place during the process are: Approximately 90% of zinc in concentrates are oxidized to zinc oxide, but at the roasting temperatures around 10% of the zinc reacts with the iron impurities of the zinc sulfide concentrates to form zinc ferrite .
Types of roasting Multiple Hearth Roaster Suspension Roaster Fluidized Bed Roaster
Engineering Design: In a multiple-hearth roaster, the concentrate drops through a series of 9 or more hearths stacked inside a brick-lined cylindrical column. Working: As the feed concentrate drops through the furnace, it is first dried by the hot gases passing through the hearths and then oxidized to produce calcine. Reaction Conditions: The reactions are slow and can be sustained only by the addition of fuel. Pressure and Time requirements: Multiple hearth roasters are unpressurized and operate at about 690 °C (1,270 °F). Operating time depends upon the composition of concentrate and the amount of the sulfur removal required. Limitations/ Advantages: Multiple hearth roasters have the capability of producing a high-purity calcine
Suspension Roaster Engineering Design: The roaster consists of a refractory-lined cylindrical steel shell, with a large combustion space at the top and 2 to 4 hearths in the lower portion, similar to those of a multiple hearth furnace. Working: In a suspension roaster, the concentrates are blown into a combustion chamber very similar to that of a pulverized coal furnace. Additional grinding, beyond that required for a multiple hearth furnace, is normally required to ensure that heat transfer to the material is sufficiently rapid for the desulfurization and oxidation reactions to occur in the furnace chamber. Pressure and Temperature requirements: Suspension roasters are unpressurized and operate at about 980 °C (1,800 °F)
Fluidized Bed Roaster Schematic diagram depicting a Fluidized Bed Roaster
Working: In a fluidized-bed roaster, finely ground sulfide concentrates are suspended and oxidized in a feedstock bed supported on an air column. Reaction Conditions: As in the suspension roaster, the reaction rates for desulfurization are more rapid than in the older multiple-hearth processes. Pressure and Temperature requirements: Fluidized-bed roasters operate under a pressure slightly lower than atmospheric and at temperatures averaging 1,000 °C (1,830 °F). In the fluidized-bed process, no additional fuel is required after ignition has been achieved. Advantages: The major advantages of this roaster are greater throughput capacities, greater sulfur removal capabilities, and lower maintenance.
Hydro-metallurgical exraction of Zinc About 80% of world’s total zinc output is produced through conventional hydrometallurgical route i.e. Roast-leach-electrowin (RLE) route. The three out of the four plants installed in India are operating on hydrometallurgical process route. The pre-requisite condition for zinc metal extraction from sulphide concentrate through a hydro-metallurgical route is the elimination of its sulphur content in order to make it amenable to further treatment by leaching.
Leaching The basic leaching chemical formula that drives this process is: This is achieved in practice though a process called double leaching. Double Leaching: The calcine is first leached in a neutral or slightly acidic solution (of sulfuric acid) in order to leach the zinc out of the zinc oxide. The remaining calcine is then leached in strong sulfuric acid to leach the rest of the zinc out of the zinc oxide and zinc ferrite. The result of this process is a solid and a liquid; the liquid contains the zinc and is often called leach product. Economic considerations: The solid obtained after double leaching is called leach residue and contains precious metals (usually lead and silver) which are sold as a by-product. Iron removal: There is also iron in the leach product from the strong acid leach, which is removed in an intermediate step.
Roast Leach Electrowin Process <ul><li>Why roasting? </li></ul><ul><li>The main purpose of roasting of zinc sulphide concentrate is to convert it into a product, which is amenable to further treatment through hydrometallurgical process for extraction of zinc. </li></ul><ul><li>Secondly, to fix the sulphide contents into sulphur dioxide gases for subsequent economical recovery as sulphuric acid. </li></ul>
<ul><li>Roasting reaction: </li></ul><ul><li>2ZnS + 3O 2 = 2ZnO + 2SO 2 </li></ul><ul><li>These rich gases are cleaned and cooled to recover dust content as zinc calcine and to remove the various harmful impurities such as Hg, Se, F, Cl, As, etc. </li></ul><ul><li>The dead roasted product, zinc calcine, is subjected to leaching with recycled electrolyte to extract zinc content. </li></ul><ul><li>The enriched zinc sulphate solution is further subjected to purification with zinc dust to eliminate impurities like copper, cadmium, cobalt, nickel etc. before being subjected to electrolysis. </li></ul>
Electrolysis Zinc is extracted from the purified zinc sulfate solution by electrowinning, which is a specialized form of electrolysis. Working: The process works by passing an electric current through the solution in a series of cells. This causes the zinc to deposits on the cathodes (aluminum sheets) and oxygen to form at the anodes. By Products: Sulfuric acid is also formed in the process and reused in the leaching process. Limitations in maintainence: Every 24 to 48 hours, each cell is shut down, the zinc-coated cathodes are removed and rinsed, and the zinc is mechanically stripped from the aluminum plates
Pressure Leach Process <ul><li>History: </li></ul><ul><li>The pressure leach technique was first successfully commercially applied for zinc extraction with the commissioning of first plant in 1981 at Cominco, Trail, Canada. </li></ul><ul><li>There are presently three electrolytic zinc plants in the world where this technique has been integrated into the existing facilities. </li></ul><ul><li>Process: </li></ul><ul><li>In this process zinc sulphide or bulk zinc concentrates are oxidized under oxygen overpressures of 1200 kpa abs. at a temperature of 150 0 C in sulphuric acid medium to produce zinc sulphate solution directly and the sulphide content is precipitated as elemental sulphur. </li></ul><ul><li>Chemical Equation: </li></ul><ul><li>ZnS + H 2 S0 4 + 0.5 0 2 = ZnSO 4 + H 2 0 + S </li></ul><ul><li>Factors affecting kinetics: </li></ul><ul><li>Particle size </li></ul><ul><li>Mineralogy </li></ul><ul><li>Surface active additives </li></ul><ul><li>Acidities </li></ul><ul><li>Reaction time </li></ul><ul><li>Temperature </li></ul>
Alternative pyrometallurgical technologies <ul><li>In the recent past, an alternative technology to conventional roasting followed by leaching, has been developed by Sherritt-Gordon of Canada , which eliminates the need of roasting step prior to leaching. </li></ul><ul><li>The process fixes the sulphide content of concentrate as elemental sulphur, thus eliminates the need for a separate sulphuric acid production facility, whereas the zinc metal content of the concentrate is converted into a zinc sulphate solution, thus combining both roast-leach steps of conventional process into a single unit operation. </li></ul>
Pyro- metallurgical extraction of Zinc Presently about 15 - 20% of the world's zinc production comes from pyrometallurgical route. The horizontal and vertical retort processes and electrothermal process were used in the past for zinc production but have become obsolete due to high power consumption and low recovery. The only pyrometallurgical process of importance presently is Imperial Smelting Process (ISP).
Imperial Smelting Process (ISP) Schematic representation of an ISP furnace
Contribution in total Zinc production: Currently about 8 -10% of the world's primary - zinc production are through the Imperial Smelting Process. Working: The Imperial Smelting Process is similar to blast furnace processes except that it is operated with hot top whereby preventing reoxidation of zinc vapours. The process consists of basic two operations namely; sintering and blast furnace smelting of sintered lumps to extract lead and zinc simultaneously. Reaction: C + 0.5 O 2 = CO C + O 2 = CO 2 CO 2 + C = 2CO ZnO + CO = Zn + CO 2
Merits <ul><li>It is possible to simultaneously smelt low-grade complex mixed charges of Zinc and Lead ores and concentrates in order to recover both Zinc and Lead. </li></ul><ul><li>Since the overall efficiency is higher, the recovery of Zinc becomes less expensive. </li></ul><ul><li>A widevariety of furnace sizes are available, the trend being towards units with larger capacities at lower operational costs. </li></ul><ul><li>The furnace function is fully automated. </li></ul><ul><li>The mechanism is highly robust. </li></ul>
Demerits <ul><li>Labour intensive process. </li></ul><ul><li>Process requires mix of zinc and lead concentrates. </li></ul><ul><li>It is a Present demand scenario does not call for addition of lead smelting capacity in the country. </li></ul><ul><li>Because of high temperature involved in maintainability of the plant. </li></ul><ul><li>Low plant availability. </li></ul>
St. Joseph Mineral Company (electrothermic) process <ul><li>Working: </li></ul><ul><li>The process begins with a downdraft sintering operation. </li></ul><ul><li>The sinter, which is a mixture of roaster calcine and EAF calcine, is loaded onto a gate type conveyor and then combustions gasses are pumped through the sinter. The carbon in the combustion gases react with some the impurities, such as lead, cadmium, and halides. </li></ul><ul><li>These impurities are driven off into filtration bags. </li></ul><ul><li>The sinter after this process, called product sinter, usually has a composition of 48% zinc, 8% iron, 5% aluminum, 4% silicon, 2.5% calcium, and smaller quantities of magnesium, lead, and other metals. </li></ul><ul><li>The sinter product is then charged with coke into an electric retort furnace. A pair of graphite electrodes from the top and bottom furnace produce current flow through the mixture. </li></ul><ul><li>The coke provides electrical resistance to the mixture in order to heat the mixture to 1,400 °C (2,550 °F) and produce carbon monoxide. </li></ul><ul><li>Advantages: </li></ul><ul><li>It is able to smelt a wide variety of zinc-bearing materials, including electric arc furnace dust. </li></ul><ul><li>Disadvantages: </li></ul><ul><li>It is less efficient than the electrolysis process. </li></ul>
New Jersey Zinc continuous vertical retort <ul><li>This processes peaked in 1960, when 5% of the world production was done by this process. </li></ul><ul><li>Working: </li></ul><ul><li>This process begins by roasting concentrates that are mixed with coal and briquetted in two stages. The briquettes are then heated in a autogenous coker at 700 °C (1,292 °F) and then charged into the retort. </li></ul><ul><li>There are three reasons to briquette the calcine: </li></ul><ul><li>To ensure free downward movement of the charge. </li></ul><ul><li>To permit heat transfer across a practical size cross-section. </li></ul><ul><li>To allow adequate porosity for the passage of reduced zinc vapor to the top of the retort. </li></ul><ul><li>The reduced zinc vapor that is collected at the top of the retort is then condensed to a liquid. </li></ul><ul><li>Design Improvement: </li></ul><ul><li>Overpelt improved upon this design by using only one large condensation chamber, instead of many small ones, as it was originally designed. This allowed for the carbon monoxide to be recirculated into the furnaces for heating the retorts. </li></ul>
Belgian-type horizontal retort process This process was the main process used in Britain from the mid-19th century until 1951 Disadvantages: The process was very inefficient as it was designed as a small scale batch operation. Design Modification: Each retort only produced 40 kilograms (88 lb) so companies would put them together in banks and used one large gas burner to heat all of them.
Conclusion The metallurgy of Zinc is a complex process with its own share of design constraints. Of the two techniques (pyro-metallurgy and hydro-metallurgy), hydro-metallurgy has the lion’s share of the total Zinc production. Imperial Smelting Process is the only commercially feasible pyro-metallurgical process. Hydro-metallurgical processes stand out basically due to their relatively much higher energy efficiency and the relatively lower running costs. As the modern industrial norms around the world get stringent by the various governments, metallurgists around the world direct themselves towards better harmony with the planet, at the same time optimizing the Zinc outputs.