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Question
Define pyrometallurgy, hydrometallurgy, electrometallurgy-
State third law of thermodynamics and explain residual entropy of the substance. The solubility of nitrogen gas at 1atm pressure and 25oC is 6.8×10−4mole/dm3. Calculate the solubility of N2(g) at this temp. and pressure is 0.78atm.
State third law of thermodynamics and explain residual entropy of the substance. The solubility of nitrogen gas at 1atm pressure and 25oC is 6.8×10−4mole/dm3. Calculate the solubility of N2(g) at this temp. and pressure is 0.78atm.
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Solution
Pyrometallurgy involves high temperature processes where chemical reactions take place among gases, solids, and molten materials. Solids containing metals are either reacted to form intermediate compounds for further processing, or they are converted into their elemental or metallic state. Pyrometallurgical processes that involve gases and solids are typified by roasting operations. Processes that produce molten products are collectively referred to as smelting operations. The energy required to sustain the high temperature pyrometallurgical processes may come entirely from the exothermic nature of the chemical reactions taking place, usually oxidation reactions. Often, however, energy must be added to the process by combustion of fuel or, in the case of some smelting processes, by the direct application of electrical energy.
Hydrometallurgy is concerned with processes that use aqueous solutions to extract metals from ores such as leaching, which involves dissolution of the metals into the aqueous solution. After the solution is separated from the ore solids, the solution is often subjected to various processes of purification and concentration before the metal is recovered, either in its metallic state or as a chemical compound. The solution purification and concentration processes may include precipitation, distillation, adsorption, and solvent extraction. The final recovery step may involve precipitation, cementation, or an electrometallurgical process. Sometimes, hydrometallurgical processes may be carried out directly on the ore material without any pretreatment steps. More often, the ore must be pretreated by various mineral processing steps and sometimes by pyrometallurgical processes.Electrometallurgy involves metallurgical processes that take place in some form of electrolytic cell. The most common types of electrometallurgical processes are electrowinning and electro-refining. Electrowinning is an electrolysis process used to recover metals in aqueous solution, usually as the result of an ore having undergone one or more hydrometallurgical processes. The metal of interest is plated onto a cathode, while an anode is composed of an inert electrical conductor. Electro-refining is used to dissolve an impure metallic anode (typically from a smelting process) and produce a high purity cathode. Fused salt electrolysis is another electrometallurgical process whereby the valuable metal is dissolved into a molten salt, which acts as the electrolyte, and the valuable metal collects on the cathode of the cell.
The scope of electrometallurgy has significant overlap with the areas of hydrometallurgy and (in the case of fused salt electrolysis) pyrometallurgy. Additionally, electrochemical phenomena play a considerable role in many mineral processing and hydrometallurgical processes.The third law of thermodynamics states that every substance has a positive entropy, but at zero Kelvin the entropy is zero for a perfectly crystalline substance. S(0) = 0 for all perfectly ordered crystalline materialsResidual Entropy: The entropy at zero Kelvin is known as residual entropy. It is the difference in entropy between a non-equilibrium state and crystal state of a substance close to absolute zero temperature.Example: The molecule CO has a very small dipole moment and there is a finite chance that it will crystallize as CO:CO:CO instead of CO:OC:CO. For each molecule there are two possible orientations of the molecule, therefore there are two ways each CO can exist in the lattice. The number of ways/molecule is w=2 for each CO. If we have N CO molecules there are wN ways or 2N ways that all of the CO can be distributed. Therefore, the entropy at zero Kelvin isS=klnW=kln(wN)=Nklnw=nRln2There are number of substances that show similar statistical variations in orientation that lead to a residual entropyUsing Henry's law for solubility of gases in waterc=kHPwhere c = solubility of the gaskH=Henry's constantP=partial pressure of the gas.Given: c=6.8×10−4 mol/dm3, P=1 atm∴kH=6.8×10−4 mol/dm3/atmNow for P=0.78 atmc=6.8×10−4×0.78c=5.3×10−4mol/dm3
Pyrometallurgy involves high temperature processes where chemical reactions take place among gases, solids, and molten materials. Solids containing metals are either reacted to form intermediate compounds for further processing, or they are converted into their elemental or metallic state. Pyrometallurgical processes that involve gases and solids are typified by roasting operations. Processes that produce molten products are collectively referred to as smelting operations. The energy required to sustain the high temperature pyrometallurgical processes may come entirely from the exothermic nature of the chemical reactions taking place, usually oxidation reactions. Often, however, energy must be added to the process by combustion of fuel or, in the case of some smelting processes, by the direct application of electrical energy.
Hydrometallurgy is concerned with processes that use aqueous solutions to extract metals from ores such as leaching, which involves dissolution of the metals into the aqueous solution. After the solution is separated from the ore solids, the solution is often subjected to various processes of purification and concentration before the metal is recovered, either in its metallic state or as a chemical compound. The solution purification and concentration processes may include precipitation, distillation, adsorption, and solvent extraction. The final recovery step may involve precipitation, cementation, or an electrometallurgical process. Sometimes, hydrometallurgical processes may be carried out directly on the ore material without any pretreatment steps. More often, the ore must be pretreated by various mineral processing steps and sometimes by pyrometallurgical processes.
Electrometallurgy involves metallurgical processes that take place in some form of electrolytic cell. The most common types of electrometallurgical processes are electrowinning and electro-refining. Electrowinning is an electrolysis process used to recover metals in aqueous solution, usually as the result of an ore having undergone one or more hydrometallurgical processes. The metal of interest is plated onto a cathode, while an anode is composed of an inert electrical conductor. Electro-refining is used to dissolve an impure metallic anode (typically from a smelting process) and produce a high purity cathode. Fused salt electrolysis is another electrometallurgical process whereby the valuable metal is dissolved into a molten salt, which acts as the electrolyte, and the valuable metal collects on the cathode of the cell.
The scope of electrometallurgy has significant overlap with the areas of hydrometallurgy and (in the case of fused salt electrolysis) pyrometallurgy. Additionally, electrochemical phenomena play a considerable role in many mineral processing and hydrometallurgical processes.
The third law of thermodynamics states that every substance has a positive entropy, but at zero Kelvin the entropy is zero for a perfectly crystalline substance.
S(0) = 0 for all perfectly ordered crystalline materials
Residual Entropy: The entropy at zero Kelvin is known as residual entropy. It is the difference in entropy between a non-equilibrium state and crystal state of a substance close to absolute zero temperature.
Example: The molecule CO has a very small dipole moment and there is a finite chance that it will crystallize as CO:CO:CO instead of CO:OC:CO. For each molecule there are two possible orientations of the molecule, therefore there are two ways each CO can exist in the lattice. The number of ways/molecule is w=2 for each CO. If we have N CO molecules there are wN ways or 2N ways that all of the CO can be distributed. Therefore, the entropy at zero Kelvin is
S=klnW=kln(wN)=Nklnw=nRln2
There are number of substances that show similar statistical variations in orientation that lead to a residual entropy
Using Henry's law for solubility of gases in water
c=kHP
where c = solubility of the gas
kH=Henry's constant
P=partial pressure of the gas.
Given: c=6.8×10−4 mol/dm3, P=1 atm
∴kH=6.8×10−4 mol/dm3/atm
Now for P=0.78 atm
c=6.8×10−4×0.78
c=5.3×10−4mol/dm3
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