FIELD OF THE INVENTION

The present invention relates to an apparatus and process for purifying graphite containing one or more contaminants.

BACKGROUND OF THE INVENTION

Graphite, obtained either from mining operations or through synthetic processes, generally contains impurities which prevent its use altogether or severely reduce its performances. Those impurities vary widely pending on the sources of graphite. But generally, silica, calcium oxide, magnesium oxide, alumina and iron oxide will predominate with many minor components such as derivatives of 'sodium, potassium or lithium, titanium oxide, with sulphates, chlorides, phosphates and carbonates as typical anions. Pending on intended uses, some impurities are more detrimental than others. For example, in applications where friction is important, silica which is abrasive must be taken out. In nuclear applications, the presence of strong neutron adsorbers, such as boron or cadmium, must be avoided at concentrations lower than parts per million. Electrical applications, such as in batteries, call for extremely high purity since it has been found that some contaminants will reduce markedly the useful life of the products by internal discharge.

SUBSTITUTESHEET

For these reasons, substantial efforts have been directed towards the purification of graphite.

These purification systems can be broadly divided into two categories, namely chemical purification and high temperature treatment.

Chemical purification involves the action of specific reagents in order to render soluble the impurities which are then leached out of the graphite by washing. For example, in order to remove silica, hydrofluoric acid is used. With calcium oxide or magnesium oxide, washing with an acid, such as hydrochloric acid, will dissolve these basic impurities. Such chemical treatments are currently practised. However, they lead to a graphite which has a purity seldom greater than 99.5% if the large flakes of the material are to be preserved. The use of a wet technology with potent reagents, such as hydrofluoric acid, creates an environmental situation which is complex and costly to handle.

The high temperature purification is based on the unique ability of graphite to withstand extremely high temperature without damage. Whereas all refractory oxides, such as magnesia, will sinter well below 2000°C, graphite can be treated at temperatures above 2500°C, if the atmosphere is neutral or reductive. This has led to the preparation of highly pure graphite in the course of

the production of silicon carbide. This side production results from the circulation of high intensity electrical current through a mass of silica and coke. The core temperature of the reacting mass reaches values so high as to induce the volatilization of all elements but graphite carbon. Similar high temperature treatment on a purely graphite mass is said to be practised in some instances.

OBJECTS AND STATEMENT OF THE INVENTION It is an object of the present invention to combine the benefits of chemical treatment with thermal treatment, thus avoiding the wet chemistry which is less than perfect as a purification system and the extreme conditions induced by a purely thermal process operating at 2500°C.

This is achieved by combining the chemical and thermal approach in the form of a treatment with chlorine and distillation of the resulting chloride.

It is well known that all oxides, in the presence of carbon, combines with chlorine to give chlorides that are more volatile than the corresponding oxides. For example, titanium oxide (melting point: 1640°C) combines with chlorine at 1200°C, in the presence of carbon, to give titanium tetrachloride (boiling point: 136°C) . Chlorine having the ability to diffuse readily

through the lattice of graphite is ideal to achieve a total purification at a temperature which would be in the range of 1700-1800°C since the least volatile chlorides boils below these temperatures.

Attempts have been made in the past for example when ultra pure graphite was needed for nuclear reactors, to provide a moderately high temperature treatment with chlorine. However, it was observed that such operation was destructive for the equipment and therefore this approach did not receive an industrial application. This was due mainly to the great permeability of graphite toward chlorine and it was noted that for lack of material that can contain chlorine at an operating temperature between 1600 and 1800°C, the gas would not discriminate between the impurities contained in graphite and the heating system, thus resulting in the dissolution and volatilization of both. Therefore, extreme corrosion prevented the use of a technique which would otherwise have allowed the high purification of graphite without the extreme temperatures required by a purely thermal approach.

Although chlorine is a well proven agent for the purification of graphite, the permeability of massive graphite, which is the only possible containment material with which the reactor for purification can be made of.

has prevented the use of this halogen in industrial upgrading of graphitic materials.

Therefore, the present invention is concerned with a method whereby the permeability of graphite is put to use in order to avoid leakage of aggressive chlorine outside of the graphite reactor used for the treatment.

The purification of graphite with the assistance of chlorine calls for temperatures in the range of 1700°C in order to ensure complete volatilization of chlorides. At these temperatures, a graphite reactor must be protected from atmospheric oxidation. Therefore, the present invention uses an atmosphere of an inert gas which does not react with carbon.

It has been found that if a blanket of inert gas is circulated within the walls of the reactor, an if the pressure inside the reactor is kept slightly below the pressure of the inert gas, there is an inward sweep to the inner wall of the vessel which contains the atmosphere of chlorine completely inside said reaction vessel.

The present invention therefore relates to a process for purifying graphite containing one or more contaminants, the graphite being contained in the core of

a reactor defined by enclosing walls made of graphite, the walls including a passageway therein; the process comprising the steps of: circulating an inert gas under pressure in the passageway; circulating chlorine under pressure in the core to react with contaminants in the graphite; maintaining the pressure of the inert gas at a value higher than the pressure of the chlorine; and - maintaining a temperature in the core at a value sufficient to eliminate the one or more contaminants

The present invention also relates to an apparatus for purifying graphite containing one or more contaminants which comprises: a reactor having a core to receive therein graphite to be purified and walls around the core, the walls being made of graphite and defining a passageway therein; first inlet means to the core for allowing ingress of chlorine to react with the graphite in said core; - second inlet means to the passageway for allowing ingress of an inert gas under pressure in the passageway;

heating means surrounding the walls to maintain a temperature at a value sufficient to eliminate one or more contaminants; and outlet means from the core for allowing egress of said chlorine inert gas and eliminated contaminants.

In one preferred form of the invention, the inert gas is taken from the group comprising nitrogen, argon and helium, preferably nitrogen.

In another form of the invention, the ration of the pressure of the inert gas to the pressure of the chlorine is from 1.2 to 5, preferably 3.

Other objects and further scope of applicability of the present invention will become apparent from the detailed description given hereinafter. It should be understood, however, that this detailed description, while indicating preferred embodiments of the invention, is given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art.

IN THE DRAWINGS

Figure 1 is a cross-sectional view schematically representing an apparatus for carrying out the present invention.

DESCRIPTION OF PREFERRED EMBODIMENTS

The annexed figure 1 illustrates an apparatus, generally designated 10, having a main body 12 provided with a central chamber 14 in which may be received graphite 16 to be purified. The body 12 has an outlet port 18 with a passage 19 in communication with the upper part of the chamber 14 and an inlet port 20 having a central passage 22 in communication with the lower part of the chamber 14. Surrounding the inlet port is a second body 24 having a pair of inlets 26 and 28 to receive an inert gas, such as nitrogen. Body 24 is enclosed within a third body 30 having an inlet port 32 and an outlet port 34 for the circulation of water inside the body. The water serves as a refrigerant for the walls of the body 24.

A coil 50 is schematically represented to indicate that heat is required and provided in order to purify the content 16 in the reactor.

Body 24 has an inlet 36 for chlorine to be received within the chamber via passage 22. The inert gas introduced at inlets 26 and 28 circulates in the

space between port 20 and body 24 and then through the surrounding area 38 defined between the inner wall 40 and outer wall 42 of the body 12.

Based on the above description of the process of the present invention, it can be seen that the chlorine introduced within the central chamber and passing outwardly through the wall 40 is returned inwardly as it reaches the passageway 38 filled with inert gas which may also permeate through the walls 40 and 42.

The implementation of the present invention calls for the adjustment of three variables: namely, the relative pressure within the reactor, the pressure ratio existing between chlorine and nitrogen and the ratio of the thickness of the inner wall 40 and the outer wall 42 of the reactor.

The relative pressure within the reactor as related to atmosphere must be kept negative so as to insure that the excess chlorine and other products are evacuated through the intended port 18 and do not tend to diffuse through the top cover. A negative pressure from 100 to 1000 Pascal proved to be appropriate.

The ratio of pressure of nitrogen to chlorine is a critical parameter. Chlorine, as it flows through

the layer 16 of graphitic particles to be purified, has to have a positive pressure to insure an appropriate flow through the reacting bed. This positive pressure existing in the chlorine inlet and reactor bottom is the main site of potential leaks. To correct this situation, the absolute pressure of nitrogen within the wall must be kept at values from two to five times the absolute pressure of chlorine. Under those conditions, no leakage of chlorine is observed.

The ratio of the thickness of the inner and outer walls determines the distribution of nitrogen leakage. This ratio may also be influenced by the density of the graphite walls, such density being related to the porosity. Mechanical constraints may also enter into consideration. An inner chamber with high density material and with a 1:1 to 1:2 inner-to-outer wall width ratio gave excellent performances.

Examples

1. Determination of chlorine leakage. A demonstration reactor has been built, having an inner diameter of 7.62 cm, the thickness of the inner wall being 2.54 cm and the thickness of the outer wall being 3.55 cm (ratio 1.4) . A layer of 25 cm of graphite having an average particle size of 80-100 mesh (Tyler) or 0.177 to 0.149 mm was loaded in the reactor. A vacuum of 1100 Pa was applied at the outlet of the reactor.

A positive pressure of chlorine was admitted at the bottom of the reactor (1400 Pa) while the nitrogen pressure in the wall was kept at 4200 Pa. The system operated for eight hours and the outside surface probed for chlorine leak using a silver nitrate detector. No detectable chlorine could be measured either by chemical test or by olfaction.

2. Purification of graphite.

Using a reactor as described in Example 1, a sample of natural graphite was treated with chlorine in excess at 1750°C for 30 minutes. The starting material, Stratmin grade +5094, contained 94% of elemental carbon. After the operation, the carbon content was 99.99% ± 0.01%.

Although the invention has been described above with respect with one specific form, it will be evident to a person skilled in the art that it may be modified and refined in various ways. It is therefore wished to have it understood that the present invention should not be limited in scope, except by the terms of the following claims.