Tutorial on roasting furnaces final
- 1. TUTORIAL on ROASTING
- 2. Roasting of an ore or concentrate is to convert it into another chemical form. An oxide is more easily reduced to metal than a sulphide and leaching is easier if the metal is present as sulphate, chloride or oxide. Some typical roasting operations are: Oxidizing Roasting - Metal sulphide Metal oxide MS + 3 /2 O 2 MO + SO 2 Volatilizing Roasting - Elimination of volatile oxides such as As 2 O 3, Sb 2 O 3, ZnO from the ore Roasting
- 3. Chloridizing Roasting - Metal compounds to chlorides under oxidizing or reducing conditions 2NaCl + MS +2O 2 Na 2 SO 4 + MCl 2 Sulphatizing Roasting - Sulphide ores to sulphates prior to leaching Magnetic Roasting - Controlled reduction of hematite (Fe 2 O 3 ) magnetite (Fe 3 O 4 ) Reduction Roasting - Partial reduction of an oxide ore prior to actual reduction smelting Blast Roasting or Sinter Roasting - Partial oxidization as well as physical condition ore is changed.
- 4. Roasting depends on following factors: 1. Time 2. Temperature 3. Avaibility of O 2 or air 4. physical condition Criteria of selection of roasting process 1. Physical condition of product blast furnace smelting – product should coarse or cellular reverberatory furnace – product should be fine leaching - product should be porous 2. Chemical composition of product For copper – retain some sulphur For Lead & Zinc - complete elimination of sulphur
- 5. Multiple Hearth Roasting MacDougall in England (19 th Century) Basic principle – Counter current flow of solid ore & the oxidizing gases Construction – - It consists of several (about 10) circular brick hearths superimposed on each other - Cylindrical steel shell lined with brick - Revolving mechanical rabbles attached to arms move over the surface of each hearth to continuously shift the ore - Arms are attached to a rotating central shaft that passes through the center of the roaster
- 6. Working: - The hearth at the top dry and heat the charge Ore is discharged automatically at the top hearth It gradually moves downwards through alternate passages around the shaft and periphery and finally emerges at the bottom The oxidizing gases flow upwards External heating of charge is unnecessary except when charge contain moisture Drawbacks: Roasting is slow Gases are unsuitable for production of H 2 SO 4 because they do not contain sufficient SO 2 and SO 3
- 7. Pictorial view of multiple hearth roasting unit
- 8. Roasting of Zinc sulfide Roasting is a high-temperature process that converts zinc sulfide concentrate to an impure zinc oxide called calcine. The following reactions occur during roasting: 2ZnS +3O2 2ZnO + SO2 2SO2+ O2 2SO3 In a multiple-hearth roaster, the concentrate drops through a series of 9 or more hearths stacked inside a brick-lined cylindrical column. 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. Multiple hearth roasters are unpressurized and operate at about 690°C (1300°F). Operating time depends upon the composition of concentrate and the amount of the sulfur removal required. Multiple hearth roasters have the capability of producing a high-purity calcine.
- 9. Flash Roasting Preheated ore particles are made to fall through body of hot air resulting in Instantaneous oxidation or ‘flashing’ of combustible constituents of the ore, mainly sulphur Hence called flash roasting Ore should be of fine size Capacity of flash roaster > hearth roaster Temp. pf combustion zone = 900-950 0 C
- 10. Fluidized bed roasting The ore particles are roasted while suspended in an upward stream of gas Finely ground sulfide concentrates in size over the range 0.005 to 0.05 cm in diameter is used As in the suspension roaster, the reaction rates for desulfurization are more rapid than in the older multiple-hearth processes. Fluidized-bed roasters operate under a pressure slightly lower than atmospheric and at temperatures averaging 1000°C (1800°F). In the fluidized-bed process, no additional fuel is required after ignition has been achieved.
- 11. Fluidized bed Roasting Principle- - Ore particles are roasted while it is suspended in an upward stream of gases - Gas passes through bottom of the bed - Behahaviour of the bed depends on the velocity of gas
- 12. Stage-1 When the gas flow rate is very low, and the ore bed is porous, the gas permeates the bed without disturbing the ore particles Pressure drop across the bed is proportional to flow rate Stage-2 Gas velocity increses, the bed expands upwards due to the effect of the drag forces exerted by gas stream The pressure drop across the bed depends on the gas velocity Stages observed during roasting process
- 13. Stage-3 When gas velocity further increases a stage is reached Pressure drop = wt. of the particle per unit area of the bed Particles remain individually suspended and offer less resistance to gas flow Stage-4 Further increase in gas velocity lead to continued expansion of the bed Results in increase in interparticle distance Pressure drop across bed continues to decrease as the gas velocity increases
- 14. Stage-5 Finally, the expansion of the bed is independent of gas velocity Outcoming gas stream appears in the form of bubbles bursting on the surface of the bed which looks like well stirred boiling liquid In this condition the bed is said to be fluidized. The fluidized bed has an apparent density distinctly different from the density of the solid and is capable of flowing like a liquid.
- 16. Advantages High energy efficiency because it can be autogenously operated Useful in recovery of sulphur because the gas that it produces has high SO 2 content Ideal for roasting of oxide ores because the oxidizing reactions that take place during roasting is highly exothermic. e.g. Pyrite FeS 2 , Millerite NiS, etc.
- 17. Sinter Roasting / Blast Roasting Fine ore & concentrate have to be agglomerated before they can be charged in a blast furnace Treatment of sulphide ore in a sintering machine where roasting and agglomeration take place simultaneously Charge = (fine ore+ moisture) as layer of 15-50 cm thick on revolving belt Combustion is done by burner Speed is adjusted - roasting should be completed before it is discharged Produce porous cinder called sinter Cooled sinter is sized to give uniform product
- 19. Lead Roasting PbS lead ore (Galena) is friable, brittle and fuses easily For blast furnace smelting it should be in the form of hard, strong and porous Hence sinter roasting is carried out in Dwight-Lloyd sintering machine Roasting reacting: PbS + 3/2 O 2 PbO + SO 2 at 800 o C
- 20. Thank You