Leaching and its kinetics What is Leaching?  ➢ Also known as liquid-solid extraction.  ➢ Two phases are in intimate contact and the solute or solutes can diffuse from the solid to the liquid phase.  ➢ The process of extracting a substance from a solid material that has come into contact with liquid.  ➢ The liquid that enters will separate the components originally in the solid.  Examples of Leaching: 1) Extracting tea from tea leaves. 2) Production of vegetable oil using hexane. 3) Copper salt leaching from copper ores. 4) Production of nickel from ore.  Kinetics of Leaching:   The above figure shows the concentration profile when a mineral surface dissolves in a leaching medium. Subscripts  S and B refer to interface and bulk respectively. The situation is simple for complete dissolution.   However when a mineral decomposes, and dissolves only partially, then a new solid residue appears. Leaching would continue if this new phase is porous. This situation is shown in figure 4.13. Sub

  BORON FIBER   Introduction Normally boron is a brittle material. Weintraub was the one who obtained the first Boron fiber in 1911 by the reduction of boron halide with hydrogen   on a hot wire (substrate). Boron fibers were developed after 1960s to increase the strength and stiffness of composite by reinforcement. Fabrication Ø Boron fibers are produced by CVD on a substrate by following two methods.                       1.     Thermal reduction of boron halide o   Low temperature. o   Carbon coated glass fiber used as a substrate. o   Weak and less dense boron fiber   is produced by this method because of gas entrapment. 2.     Reduction of Boron halide by hydrogen o   High temperature. o   Tungsten wire (12micron) used as a substrate which has a high density and melting point. o   Uniform quality of Boron fiber is the result of this method. Reaction : 2BX 3 + 2H 2 ------> 2B + 6HX Where X =   Cl, Br, I, mostly Cl. In the above reaction

  RETAINED AUSTENITE   ·      As we know, for ferrous alloys the austenite to martensite transformation is never practically completed and a certain amount of austenite always remains even after the completion of the process. It is the untransformed austenite. It is usually more found in Hardened and High carbon steels. ·      The amount of retained austenite depends largely on the M s and M f temperature which are the martensitic start and martensitic finish temperature respectively. Both M s and M f temperatures depend upon the carbon content  and both are inversely proportional to the amount of carbon. As the carbon content increases the M s and M f temperature  lowers down and the amount of retained austenite increases. It  is shown in the figure below.                             Figure: Effect of carbon on retained austenite. ·      Except for cobalt and Aluminium, all other alloying elements lower the M s and M f temperatures and so the alloy steels are found to