Calcium Electrowinning: Production and Principle in Molten Salt Processing, Study Guides, Projects, Research of Process Engineering

Download Calcium Electrowinning: Production and Principle in Molten Salt Processing and more Study Guides, Projects, Research Process Engineering in PDF only on Docsity!

In the name of Allah,

the most beneficent,

the most merciful.

Introduction

 Calcium electrowinning has been performed in a 5 – 10 wt% calcium oxide–calcium chloride molten salt by electrolytic dissociation of calcium oxide.

 This electrolysis requires the use of a porous ceramic sheath around the anode to keep the cathodically deposited calcium and the anodic gases separate.

 Stainless steel cathode and graphite anode have been used in the temperature range of 850–950°C.

Molten Salt Processing

4

 Molten salt processing provides a unique opportunity to process and produce metals where gas-based pyro- reduction, metallothermic reduction, hydrometallurgical methods or aqueous electrochemical techniques are not feasible due to thermodynamic or kinetic constraints.

 High temperature carbothermic or metallothermic smelting reduction methods for metal production are associated with the generation of significant quantity of waste as slags.

 Molten salt processing is intended to have low waste or ideally a ‘zero-waste’ generation.

Electrochemical Principle

 The principles employed here are similar to the Hall cell for aluminum production but the aluminum process is essentially restricted by the solubility of alumina in cryolite.

 Calcium has 4–6 atomic % solubility in the salt depending on the composition and temperature

 A molten flux electrolyte consisting of commercial grade calcium chloride and 10 wt% [20 at.%] calcium oxide, with additions of potassium and sodium chlorides for lowering the melting point, density and viscosity of the salt, has been used in the range of 825– 950 °C.

Electrochemical Principle

The rates of calcium deposition and oxygen evolution are given by the following equations.

Calcium Electrowinning Cell

Cell Parameters

Cell Dimensions

Results

13

 Ionic diffusion through the sheath is the rate- controlling step.

A diffusion coefficient in the range of 10-5-10-6^ cm^2 /sec is obtained for a 30% porous alumina sheath for cell temperatures between 800 and 900°C.