2013

Aashirwad jindal

Chemistry project.

12/11/2013

General and Processes of Isolation of Elements…

I would like to express my special thanks of gratitude to my teacher vishal sir as well as our principal chakrapani sir

who gave me the golden opportunity to do this wonderful project on the topic general principles and isolation of

elements which also helped me in doing a lot of Research and i came to know about so many new things I am really

thankful to them.Secondly i would also like to thank my parents and friends

who helped me a lot in finalizing this project within the limited time frame.

Aashirwad jindal..

Magnetic separation takes advantage of differences in the magnetic properties of minerals. Minerals fall into one of three magnetic properties: ferromagnetic, paramagnetic and diamagnetic.

Ferromagnetic minerals are themselves magnetic (i.e., magnetite and pyrrhotite) and can be easily separated from other minerals with a magnet since they will stick to the poles of the magnet. These minerals can be separated by wrapping the poles of a magnet in paper, passing the magnet over the

mineral mixture. The ferromagnetic minerals will stick to the magnet and may be easily separated by removing the paper covering the magnet. Paramagnetic and diamagnetic minerals are not magnetic, but they differ in how they interact with a magnetic field. Paramagnetic minerals are weakly attracted into a magnetic field and diamagnetic minerals are weakly repelled by a magnetic field. Thus, if a mixture of paramagnetic and diamagnetic minerals is passed through a magnetic field, they will be pulled into the field (paramagnetic) or repelled from the field (diamagnetic) and may be separated. Furthermore, paramagnetic minerals with different degrees of paramagnetism can be separated from one another in the same way. The device used to separate minerals based on their magnetic properties is called a Frantz Isodynamic Magnetic

Separator. The magnetic separator consists of a large electromagnet through which mineral mixtures can be passed on a metal trough which is divided near its exit end. Varying the strength of the magnetic field and/or slope of the separation trough is used to separate minerals.

All forms of mineral separation suffer from one difficulty. It is impossible to completely eliminate impurities. Depending on what the impurities are, that may or may not be a major problem. For example, if you were separating hornblende from a granite for Zr analysis, potential contamination by zircon inclusions in the hornblende might be a major problem. A typical hornblende crystal might have a Zr content of 50 ppm. A zircon crystal (ZrSiO4) has approximately 500,000 ppm Zr. Thus, if the hornblende separate contained only 0.01% Zr, the hornblende would contribute 4999.5 units of Zr and the zircon impurity would contribute 5000 units of Zr. The resulting concentration you would measure would be 100 ppm, which is twice the correct result. This is a major problem that cannot be eliminated when mineral separations are involved in the analysis.

Leaching is the process by which constituents of a solid material are released into a contacting water phase. Although some species may be more of an environmental concern than others, the leaching process is indiscriminant such that all constituents (e.g., major or minor matrix components as well as inorganic, organic and radionuclide contaminants) are released under a common set of chemical phenomena which may include mineral dissolution, desorption and complexation, and mass transport processes. In turn, these phenomena are affected by certain factors that can alter the rate or extent of leaching. Among these factors are:

internal chemical and physical reactions external stresses from the surrounding

environment physical degradation of the solid matrix due to

erosion or cracking, and loss of matrix constituents due to the leaching

process itself.

Physical and Chemical Factors Influencing Leaching

The process of leaching includes the partitioning of contaminants between a solid and liquid phase (e.g., assuming local equilibrium) coupled with the mass transport of aqueous or dissolved constituents. Mass transport is the summation of diffusion, hindered diffusion, tortuosity effects, and effective surface area effects through the pore structure of the material to the environment. Important chemical factors, those that influence the liquid-solid partitioning (LSP) of a constituent, include solution pH, redox, the presence of dissolved organic matter, and biological activity. Physical factors, such as relative hydraulic conductivity, porosity and fill geometry, play an important role in determining the rate at which

constituents transport through a solid into a passing liquid phase.

The process itself is universal, as any material exposed to contact with water will leach components from its surface or its interior depending on the porosity of the material considered.

Froth flotation is considered to be the most widely used method for ore beneficiation. In ore beneficiation, flotation is a process in which valuable

minerals are separated from worthless material or other valuable minerals by inducing them to gather in and on the surface of a froth layer. Sulfide and non-sulfide minerals as well as native metals are recovered by froth flotation. This process is based on the ability of certain chemicals to modify the surface properties of the mineral(s). Other chemicals are used to generate the froth and still others are used to adjust the pH. Certain chemicals are even capable of depressing the flotation of minerals that are either to be recovered at a later time or are not to be recovered.

The process of froth flotation entails crushing and grinding the ore to a fine size. This fine grinding separates the individual mineral particles from the waste rock and other mineral particles. The grinding is normally done in water with the resultant slurry called the pulp. The pulp is processed in the flotation cells, which agitate the mixture and introduce air as small bubbles.

The ability of a mineral to float depends upon its surface properties. Chemical modification of these properties enables the mineral particles to attach to an air bubble in the flotation cell. The air bubble and mineral particle rise through the pulp to the surface of the froth or foam that is present on the flotation cell. Even though the air bubbles often break at this point, the mineral remains on the surface of the froth. The mineral is physically separated from the remaining pulp material and is removed for further processing.

Frothers -

Frothers are liquids that produce the froth or foam on which the flotation process depends. The froth resembles soap suds and provides the physical

separation between the mineral(s) floated and the pulp containing the waste. The froth must be strong enough to support the weight of the mineral floated and yet not be tenacious and non-flowing. It should have the tendency to break down when removed from the flotation cell. The frother should not enhance the flotation of unwanted material. Many other characteristics are required for a foaming agent to be a good flotation frother. Typical frothers include:

pine oil certain alcohols low molecular weight polypropylene glycols

Collectors -

A collector is a chemical that attaches to the mineral surface and produces a hydrophobic (water-fearing) surface. While certain minerals are naturally hydrophobic and do not require a collector, recovery is often improved when a collector is used. This water-repellent film facilitates the attachment of the mineral particle to the air bubble. Many different chemicals are used as collectors, such as:

oils xanthates dithiophosphates petroleum sulfonates fatty amines

Depressants - O

 Depressants are chemicals that inhibit the flotation of minerals.  They are used to improve the selectivity of a flotation process.  They often make it feasible to separate minerals that were initially floated together. 

The response of many minerals to the flotation process is often dramatically affected by pH.  Flotation circuits are often operated at a pH range of 7.5 to 11.5.  The exact range at any given plant is optimized for the ore at that site. 

Lime is often used to raise the pH of the pulp and also reduce the flotation of iron pyrite. 

Hall-Héroult process is used to produce aluminum in an electrolytic cell are shown.

Alumina, an aluminum oxide (Al203), is dissolved in molten cryolite (Na3AlF6). It is

decomposed electrolytic between carbon and aluminum electrodes at about 950° C to

aluminum and oxygen. The carbon anode is continuously consumed by reacting with the

oxygen to give carbon dioxide (CO2). The typical features of the electrolysis cells in an

aluminum plant are described. Technical cells are operated with a current intensity of 100

to 500 kA. They are equipped with devices to operate the cells in a highly automatic i.e.

computer controlled way. The waste gases of the process are collected and thoroughly

cleaned (scrubbed) before being exhausted to the atmosphere. 

The discussion of alternative methods to produce aluminum in technical quantities

mentions the Direct Carbothermic Reduction Process and the Alcoa Smelting Process

which were developed to replace the classical Hall-Héroult process. Technical problems

which could not be solved stopped however these projects.

Inert anodes must sustain the chemical attack of the anodic oxygen and of the electrolyte

at electrolysis temperature. Several candidate materials were investigated however

without con-vincing success so far. Using materials which are wetted by aluminum as

cathode would avoid several problems essentially the magnetic effects and the chemical

reactions of the carbon lining. Several patents and publications propose materials and

arrangements to replace the aluminum metal pad as cathode.

In 1886 Hall of the USA and Héroult of France invented simultaneously and independently of each other the process to produce aluminum by

electrolysis.

Alumina, an oxide of aluminum (Al2O3), is dissolved in molten cryolite (Na3AlF6) and decomposed electrolytically to give liquid aluminum. The anode of the electrolytic cell is made of carbon and the pool of already produced aluminum acts as cathode. The oxygen of the alumina is discharged at the anode where it reacts with the carbon anode to produce carbon dioxide (CO2).

1.3 Schema of a Hall-Héroult Electrolytic Cell.

Figure 1.3 shows schematically such a Hall-Héroult electrolysis. A steel shell which is lined with carbon blocks and thermal insulation material contains the liquid cryolite electrolyte and liquid aluminum. The process uses electrical energy to reduce electrolytically aluminum oxide and to keep the electrolyte at a temperature of about 950° C. During aluminum production the chemical reactions consume continuously alumina and anodes which must be added respectively replaced to the electrolytic cell. Aluminum and anode gases (in essence carbon dioxide and carbon monoxide) are produced and removed from the cell.

Figure 1.5 shows the slide show of the schematic cross section for such a technical cell.Electrolyte, Metal Pad, TopCrust and Side Ledge

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1.4 The AP 30 Electrolytic Cell (Rio Tinto Alcan former Péchiney).

This figure shows the AP 30 electrolytic cell of Péchiney (now Rio Tinto Alcan). This cell is operatated at 300 kA and is equipped with 20 double anodes and five side raisers that carry the electric current to the anode beam (not shown).

zone melting,  any of a group of techniques used to purify an element or a compound or control its composition by melting a short region (i.e., zone) and causing this liquid zone to travel slowly through a relatively long ingot, or charge, of the solid. As the zone travels, it redistributes impurities along the charge. The final distribution of the impurity depends on its distribution in the starting charge of material; its distribution between the liquid and solid phase of the material (called its distribution coefficient, k, which is a characteristic of the particular impurity); and on the size, number, and travel direction of the zones.

Zone melting is a means of using the freezing process to manipulate impurities. It combines the fact that a freezing crystal differs in composition from the liquid from which it crystallizes with the idea of passing a short liquid zone along a lengthy solid.

Zone refining is the most important of the zone-melting techniques. In zone refining, a solid is refined by passing a number of molten zones through it in one direction. Each zone carries a fraction of the impurities to the end of the solid charge, thereby purifying the remainder. Zone refining was first described by the U.S. scientist W.G. Pfann and was first used in the early 1950s to purify germanium for transistors. The purity achieved was hitherto unheard of—less than one part of detectable impurity in 10,000,000,000 parts of germanium. The method was adopted in transistor manufacture around the world.

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The principles of zone refining are quite general, and so the method has been applied to many substances. More than one-third of the elements and hundreds of inorganic and organic compounds have been raised to their highest purity by zone refining. Many of these were, for the first time, made pure enough for their intrinsic properties to be determined.

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The mobile and stationary phases of chromatographic systems are arranged in such a way that migration is along a coordinate much longer than its width. There are two basic geometries: columnar and planar. In column chromatography the stationary phase is contained in a tube called the column. Apacked column contains particles that either constitute or support the stationary phase, and the mobile phase flows through the channels of the interstitial spaces. Theory has shown that performance is enhanced if very small particles are used, which simultaneously ensures the additional desired feature that these channels be very narrow. The effect of mobile-phase mass transfer on band (peak) broadening will then be reduced (see discussions of mass transfer and peak broadening in Efficiency and resolution and Theoretical considerations below). Constructing the stationary phase as a thin layer or film will reduce band broadening due to stationary-phase mass transfer. Porous particles, either as adsorbents or as supports for liquids, may have deep pores, with some extending through the entire particle. This contributes to band broadening. Use of microparticles alleviates this because the channels are shortened. An alternate packing method is to coat impermeable macroparticles, such as glass beads, with a thin layer of microparticles. These are the porous-layer, superficially porous, or pellicular packings. As the particle size is reduced, however, the diameter of the column must also be decreased. As a result, the amount of stationary phase is less and the sample size must be reduced. Detection methods must therefore respond to very small amounts of solutes, and large pressures are required to force the mobile phase through the column. The extreme cases are known as

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microbore columns; an example is a column 35 centimetres (14 inches) long of 320-micrometre (1 micrometre = 10−4 centimetre) inside diameter packed with particles of 2-micrometre diameter.

A second column geometry involves coating the stationary phase onto the inside wall of a small-diameter stainless steel or fused silica tube. These are open tubular columns. The coating may be a liquid or a solid. For gaseous mobile phases, the superior performance is due to the length and the thin film of the stationary phase. The columns are highly permeable to gases and do not require excessive driving pressures. Columns in which a liquid mobile phase is used are much shorter and require large driving pressures.

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In this geometry the stationary phase is configured as a thin two-dimensional sheet. In paper chromatography a sheet or a narrow strip of paper serves as the stationary phase. In thin-layer chromatography a thin film of a stationary phase of solid particles bound together for mechanical strength with a binder, such as calcium sulfate, is coated on a glass plate or plastic sheet. One edge of the sheet is dipped in a reservoir of the mobile phase, which, driven by capillary action, moves through the bed perpendicular to the surface of the mobile phase. This capillary motion is rapid compared to solute diffusion in the mobile phase at right angles to the migration path, and so the solute is confined to a narrow path.