GB2171684A - Rod suitable for use as an electrode and method of preparing same - Google Patents

Abstract

A carbon rod, suitable for use as an electrode is formed by hardening an extruded mixture of carbon and an inorganic binder at a temperature below 250 DEG C. Suitable binders include sodium or potassium silicate, ethyl silicate aluminium orthophosphate and magnesium oxychloride.

Description

SPECIFICATION Rod suitable for use as an electrode and method of preparing same Description of invention From one aspect, the present invention relates to a method of preparing a carbon rod, by which we mean a rod which is predominently of carbon. The invention has been devised primarily for the manufacture of carbon rods intended to be used as electrodes.

Carbon electrodes for use in dry Leclanche cells are presently manufactured by mixing particles of carbon with pitch at an elevated temperature, forming a block of the mixture in a press, transferring the block to an extrusion press, extruding the mixture in the form of "green" rods, cutting up the rods and then subjecting the rods to heating for a period of many hours, typically thirty hours. During the heating, a temperature in excess of 10000C is achieved and the pitch is completely carbonised.

The carbon which is formed during the heating of the rods binds the original particles together into a mass which has good electrical conductivity, since both the original particles and the material binding those particles in the product consist substantially entirely of carbon.

According to the first aspect of the invention, there is provided a method of preparing a carbon rod wherein particles of carbon are mixed with an inorganic binder, the mixture is extruded, the extruded mixture is hardened at a temperature not exceeding 250 C and the extrusion is cut up either before, during or after the hardening step.

Hardening of the extruded mixture may be achieved by chemical change, a physical change or a combination of these.

During the hardening step, the extruded mixture may be warmed above the ambient temperature to promote curing or evaporation. However, it is envisaged that the extruded mixture would be maintained at an elevated temperature for a period which is short, as compared with the thirty hours for which the known "green" rods are heated and the high temperatures used in the known process would be avoided. Thus, the energy required for hardening of the extruded mixture in a method in accordance with the present invention is relatively low.

In order to bring about extrusion of the mixture, the mixture must be subjected to a pressure which exceeds the pressure to which the newly extruded mixture is subjected. However, the mixture is relieved of this larger pressure before hardening is completed and, preferably, before hardening occurs to any significant degree.

According to a second aspect of the invention, there is provided a method of preparing a carbon rod wherein particles of carbon are mixed with an inorganic binder, the mixture is extruded, the extruded mixture is hardened whilst subjected to a pressure not exceeding 100 kilograms per cm2 and without the temperature of the extruded mixture being raised to a level in the region of 1,000 C, and the extrusion is cut up either before, during or after the hardening step.

In a method in accordance with the second aspect of the invention, the extruded mixture is preferably hardened whilst being subjected to a pressure not substantially greater than atmospheric pressure.

The temperature at which hardening takes place is not sufficiently high to bring about sintering of the carbon. Preferably, no carbon is formed during hardening.

The binder used may have an electrical conductivity which is low, as compared with the conductivity of carbon.

The binder may be initially in an aqueous condition. It is believed that an aqueous liquid does not penetrate to an excessive degree pores which are present in particles of carbon and that a high proportion of an aqueous binder will remain between adjacent particles, rather than being absorbed inside.

The binder may be a water-soluble silicate, magnesium oxychloride or aluminium orthophosphate.

Silicates which may be used include sodium silicate, potassium silicate and ethylsilicate.

During the hardening step, the extruded material may be treated by a further agent. For example, in a case where the binder includes a water soluble silicate, the extruded material may be treated by carbon dioxide, a calcium salt or an aluminium salt during the hardening step. The further agent may be applied after extrusion or may be incorporated in the mixture prior to extrusion. Incorporation prior to extrusion would be appropriate in a case where the further agent is a metal salt or is an acid.

In a first example of the preparation of a carbon electrode by a method in accordance with the invention, four parts by weight of carbon particles is mixed at 60 C with one part by weight of a 38% solids aqueous solution of sodium silicate. The carbon particles include particles derived from metallurgical coke, needle coke, graphite particles and carbon black. The mixture is extruded to form a rod. Prior to extrusion, the mixture may be formed into a block having a suitable size and shape for insertion into an hyraulic extrusion press.

The extruded rod is warmed from ambient temperature to a temperature in the range 500C to 600C and maintained at that temperature until a major part of the water present during extrusion has evaporated. The extrusion is then heated to a temperature within the range 90 C to 100 C to dry the extrusion. The extrusion is cut up, either before, during or after drying. Drying may be completed under vacuum.

In a second example, the procedure of the first example is repeated but one part of the solution of sodium silicate is used with eight parts of carbon.

In a third example, there was mixed in a Z-blade mixer a first quantity of Durrans coke, a second quantity of graphite and a third quantity of carbon black. The weight of graphite was somewhat more than ten times the weight of coke and somewhat more than twice the weight of carbon black. To the mixture of carbon particles, there was added an aqueous solution of sodium silicate in sufficient quantity for the resulting, dried product to contain 16% sodium silicate.

The mixture was subjected to a force of twelve tonnes in the extruder, with the extrusion orifice closed, to compact the mixture, was then relieved of this larger force and was extruded under a pressure established by the application of a one tonne force to the extrusion ram. The extruded rod was allowed to dry for sixteen hours at 230C and then heated in a vacuum oven to a temperature of 140 C, at which temperature it was maintained for two hours. The resulting rod was found to have a resistivity of 7.8 m.ohm.cm and a breaking strength of 199 kg.cm-2.

We prefer that the extruded rod, after drying, should be treated at a temperature within the range 100-200 C.

We have also found that an acceptable rod could be produced by mixing with the carbon particles and sodium silicate solution a quantity of an acetate of glycerol, for example glycerol triacetate and heating the extruded rod formed from this composition to a temperature of 600C for drying and hardening. In this case, heating to a temperature above 1000C was avoided. The amount of glycerol triacetate used was 10% of the extruded composition.

The features disclosed in the foregoing description, or the accompanying drawings, expressed in their specific forms or in terms of a means for performing the disclosed function, or a method or process for attaining the disclosed result, or a class or group of substances or compositions, as appropriate, may, separately or any combination of such features, be utilised for realising the invention in diverse forms thereof.

Claims (10)

1. A method of preparing a carbon rod wherein particles of carbon are mixed with an inorganic binder, the mixture is extruded, the extruded mixture is hardened at a temperature not exceeding 2500C and the extrusion is cut-up either before, during or after the hardening step.

2. A method according to Claim 1 wherein the extruded mixture is heated to a temperature within the range 500 to 2500.

3. A method according to Claim 1 or Claim 2 wherein the mixture is subjected to atmospheric pressure immediately upon emerging from an orifice through which the mixture is extruded.

4. A method of preparing a carbon rod wherein particles of carbon are mixed with an inorganic binder, the mixture is extruded, the extruded mixture is hardened whilst subjected to a pressure not exceeding 100 kg./cm2 and without the temperature of the extruded mixture being raised to a level in the region of 1000 C and the extrusion is cut-up either before, during or after the hardening step.

5. A method according to any preceding Claim wherein the binder which is mixed with the carbon particles is in an aqueous condition.

6. A method according to Claim 5 wherein the binder comprises sodium silicate.

7. A method according to any one of Claims 1 to 5 wherein the binder includes a water-soluble silicate and the binder is treated by a further agent during the hardening step.

8. An extruded rod comprising particles of carbon and an inorganic binder.

9. A method substantially as herein described with reference to any one of the examples of producing a carbon rod.

9. A method substantially as herein described with reference to any one of the examples of producing a carbon rod.

10. Any novel feature or novel combination of features disclosed herein.