GB2231562A - Process for the production of non-puffing shaped carbon bodies - Google Patents

Description

PROCESS FOR THE PRODUCTION OF NON-PUFFING SHAPED CARBON BODIES The

invention relates to a process for the production of graphitized shaped carbon bodies from a coal tar pitch coke-containing dry material and at least one cokable binder by mixing of starting materials, shaping, coking and graphitizing.

Graphitized shaped carbon bodies, termed in the following graphite bodies are used in large measure in electrothermal and electrochemical processes as well as in process technology on account of their good electrical and thermal conductivity, their thermal shock resistance, corrosion resistance, mechanical strength and the outstanding temperature resistance. The chief field of application is in the processing of electrosteel in which an arc burns between graphite electrodes with diameters up to 700 mm and lengths up to 2700 mm to produce the melting heat.

The production of graphite bodies, which requires a few weeks, extends over several expensive process steps. The required raw materials are expensive. Graphite bodies have as a consequence of this a comparatively high price. One of the most important aims of the graphite producer is therefore to minimise production rejects and to produce products with high economic value. Graphite bodies are produced from coke, a carbonisable binder and optionally additions of auxiliaries. From the coke fractions obtained after the grinding and sieving, there are produced dry starting materials according to the compositions provided. Then the dry material is mixed with a binder generally while hot and is shaped into bodies under compression, e.g. by means of extrusion presses. The shaped bodies are fired to form coke bodies at up to temperatures of 700 to 10000C, with there being conversion of the binder material into a coke matrix, and the coke bodies are converted to graphite bodies in electric furnaces by heating to 2500 to 30000C.

The most important raw materials for coke are, these days, petroleum cokes, especially the anisotropic premium petroleum cokes which, on account of their frequently to be observed structure, are also known as needle cokes. The needle cokes have comparatively excellent properties such as a low thermal coefficient of expansion, low electric resistance, good mechanical strength and a high thermal conductivity. They are therefore used for the production of graphite bodies which are to withstand the highest stresses, like electrodes for Ultra High Powerelectric melting furnaces.

In the last few years, there have also become available high value cokes produced from coal tar pitch, so called coal tar pitch needle cokes. The production of large, loadable graphite bodies from such cokes is however uneconomic because, on graphitization, high rejection occurs as a result of formation of cracks.

The formation of cracks is a general problem with graphitization of shaped bodies formed of almost all commercially used cokes. The source of this is a rapidly occurring irreversible volume expansion in the temperature range of 1400 to 20000C which is termed "puffing" by the man skilled in the art. In this procedure, there occur in the bodies mechanical stresses which, besides leading to the formation of micro and macro cracks in the texture, also lead to rejection as a result of breaking open of the graphite bodies. This puffing effects moreover an impairment of important properties of the graphite bodies such as for example the mechanical strength, the electrical resistance and the thermal conductivity. The puffing can be reduced by slower heating. This is however uneconomical and leads to losses in quality.

A source of the puffing with petroleum cokes is the sulphur content which, with commercially available types, lies between 0. 3 and 1.5%. When the carbon shaped bodies pass through the temperature region of 1400 to 20000C, e.g. on graphitization, the sulphur is suddenly liberated in the form of gas and as a consequence of the formation of a significant gas pressure in the bodies connected therewith, mechanical stresses build up which can lead to cracks. With petroleum cokes, there has been success in strongly reducing or suppressing the puffing by the addition of suitable inhibitors. The number of proposed puffing inhibitors is large and always it is a question of their being used in a fine distribution in the bodies to be graphitized. The amount of the inhibitor substances added in practice is 1 to 2% related to the amount of coke in the starting mixture. An essential disadvantage with the use of puffing inhibitors is that the thermal coefficient of expansion of the graphite is increased thereby. This harms its resistance to temperature change and leads to a higher consumption of graphite with electrodes in steel works. It must therefore be the aim to use as little as possible of a substance which is as effective as possible. This object is not easily to be solved and there have been for this purpose a large number of proposals.

In DE-A-1 073 368, the use of salts of the alkali metals, like sodium or potassium carbonate, as puffing inhibitors is described. The electrode blanks cooled after the firing are impregnated with a sodium or potassium carbonate solution and then graphitized.

The addition of chromium oxide to a coke-pitch mixture is disclosed by French Patent No. 1 491 497. In addition to the inhibition of puffing, the addition acts as graphitization catalyst.

To be noted from British Patent No.733 073 is a process in which oxides of chromium, iron, copper or nickel are added on grinding of the coke and, in this way, are finely distributed over the surface of the coke in the subsequent mixing with pitch. On graphitization of the shaped and fired bodies, they then act as puffing inhibitors.

U.S. Patent No. 3 563 705 teaches the addition of mixtures of iron or calcium compounds with small amounts of titanium and zirconium compounds to the mixture of coke and binder in order to prevent the puffing.

In U.S. Patent No. 3 338 993, there is described, for the same purpose, the addition of calcium, magnesium, strontium and barium fluorides to the mixture of green or calcined coke and the binder.

The addition of 1 to 3 calcium cyanamide or calcium carbide as sulphur binding and puffing inhibiting agents to green coke before the calcining is disclosed in US Patent Specification No.3 642 962.

According to US Patent No 4 308 177, additions of chlorinated naphthalenes with 4 to 8 chlorine atoms on the naphthalene nucleus also have a puffing inhibiting effect besides their effect as pressing auxiliary and condensation material for pitch. Especially strong effects inhibiting the puffing are produced with simultaneous addition of chloronaphthalenes and inhibiting metal compounds like iron, chromium, copper, cobalt or manganese oxide, as well as alkaline earth metal fluorides to the mixture of production components before the shaping, apparently as a result of synergistic effect.

U.S. Patents Nos. 4 312 745 and 4 334 980 teach the production of cokes which have no puffing. For this purpose, chromium compounds, preferably chromium oxide (US Patent 4 312 745) or iron compounds, preferably iron oxide or calcium fluoride (US Patent 4 334 980) are added to a sulphur containing starting composition and then coke is produced by the delayed coking process. All previously described literature references concern the addition of inhibitors in the production or working of petroleum cokes. 35 A particular problem is obtained with use of cokes which have been produced from coal tar pitch. Investigations (K.W.Tucker et al, 13th Biennial Conference on Carbon in Irvine, California, Extended Abstracts, pages 191, 192 and I. Letizia, M.H.Wagner, 16th Biennial Conference on Carbon in San Diego, California, Extended Abstracts, pages 593, 594 as well as E.G.Morris et al, ibidem, pages 595, 596) and experiences in the technical processing have shown that the correlation existing for petroleum cokes between the level of sulphur content and the puffing is not valid for coal tar pitch cokes and that in particular the puffing of the coal tar pitch cokes and the coal tar pitch needle cokes as a result of addition of the usual inhibiting materials for petroleum cokes, such as e.g. iron oxide or chromium oxide cannot be reduced or cannot be reduced in sufficient measure. Coal tar pitch cokes and coal tar pitch needle cokes with sulphur contents, which no longer give rise in practice to any puffing with petroleum cokes show a marked puffing. The puffing of petroleum coke is therefore not comparable with the puffing of cokes produced from coal tar pitches. The technical world therefore assumes that with cokes produced from coal tar pitches, besides sulphur, above all other influencing factors like, for example nitrogen content are causes and speak of an "anomalous puffing" of coal tar pitch cokes. This property of anomalous puffing has, in spite of the availability of a large number of puffing inhibitors for petroleum cokes, hitherto prevented absolutely the advantageous utilisation of coal tar pitch cokes and of coal tar pitch needle cokes otherwise of equal value to premium petroleum cokes for the economical production of large size graphite shaped bodies such as e.g. electrodes for steel production for reasons of availability and possession of raw material as well as for economic reasons.

According to the present invention, there is provided a process for the production of graphitized shaped carbon bodies from a coal tar pitch cokecontaining dry material and a cokable binder by mixing of starting materials, shaping, coking and graphitization, in which a substance reducing the irreversible expansion of the shaped bodies on heating to temperatures above 14000C is added to said starting materials prior to shaping, which substance is a compound of one of the metals of the group magnesium, calcium, strontium and barium which is not soluble in the binder.

Preferably alkaline earth metal compounds, i.e. Mg, Ca, Sr, Ba compounds in the context of this invention, from the group of carbonates, oxides, carbides, fluorides and sulphates are used as additives according to the invention to control the puffing. In order to be effective, the inhibiting additives must be as finely particulate as possible and be distributed uniformly in the mixture specified for the shaping. A degree of fineness of 80% lets than 50 micrometer particulation or finer is to be used. The inhibiting agent is advantageously added in such amount that its metal content corresponds to 0.1 to 3.0% by weight related to the coke components. The procedure in respect of addition of the inhibitors can be different. They can firstly be dispersed in the pitch binder and then, by means of this, can be mixed in the dry material. Alternatively, they can be supplied to one or several dry material components, e.g. coke fractions and mixed into these. They can be added to the binder-free composition components directly before the mixing process or during it. Finally, they can even be added during the mixing of the mixture containing all the other components of the mixture. Mixtures of different inhibiting substances can also be added and one can furthermore make use of combinations of individual processing steps in the addition process so that the addition of partial amounts of the inhibitors can take place according to one or more of the foregoing procedures. The mixing is to be carried out so that the inhibitor particles are distributed uniformly over the entire mixing of the dry material W constituents and binder. This happens advantageously by use of suitable mixers, such as of intensive or kneader mixers wherein the mixing times are matched to the set mixing objectives. The further processing is as follows:

after the mixing, the so-called "green mixture" obtained is processed in shaping arrangements known per se, such as e.g. extrusion process, die presses, isostatic or vibrational compactors. These shaped bodies are then fired in known manner in firing ovens with use of special temperature conditions with exclusion of oxygen at up to temperatures of about 700 to 10000C, whereby a conversion to coke bodies takes place. These coke bodies are then converted to graphite bodies in electrically heated furnaces likewise with exclusion of oxygen, at temperatures of about 2500 to 30000C.

The advantage of the invention lies in the possibility now of also controlling the puffing of coal tar pitch cokes and coal tar pitch needle cokes and accordingly to utilise these cokes to greater extent than hitherto done industrially. This is of effect particularly in the production of shaped bodies of large format from coal tar pitch cokes. There the formation of defects giving rise to rejection, as for example cracks, drops to an acceptable extent.

The following Example illustrates this invention:

EXAMPLE

Four productions of graphitized shaped bodies with addition of different inhibitors, as well as a comparative production, were carried out. With each of the four test productions, one of the metal compounds Fe203, M90, CaO and Bao was added. Since the course of the productions differed only by the type of respective inhibitor substance added, the description of the test bodies is summarised. 100 parts by weight of ground coal tar pitch needle coke (sulphur content DIN 51724 Tl:

0.33, hydrogen content DIN 51912: 0.115t, density DIN 51901: 2.085 g/cm3) with a maximum particle size of 1 mm were treated with 30% by weight of coal tar pitch (softening point DIN 52025: 890C, coking residue DIN 51905: 59). To each of the starting compositions with the exception of the comparative composition was added, as inhibitor substance, one of the compounds Fe203, M90, CaO and BaO, degree of fineness 80 less than 50 micrometer and 50% less than 20 micrometer particulation, in an amount which corresponded to 1% by weight of the metals iron, magnesium, calcium and barium related to the amount of coal tar pitch used. The components of each starting composition were then mixed in a heatable Z-arm kneader at 1650C for 45 minutes. Then the mixtures were shaped in a piston rod press to rods of 70 mm diameter and 1000 mm length and the rods were subsequently fired in an annular chamber furnace up to a temperature of 8000C. For carrying out the measurement of the puffing, specimens sized 8 x 8 x 100 mm, were cut out from these rods. The puffing measurements were carried out in the temperature range of 1400 to 24000C with a high temperature torque rod dilatometer as described by M.H.Wagner et al in High Temperatures High Pressures 13, 153 (1981). The volume expansion totalled over the measuring range is indicated as a measure of puffing. These values were obtained from the linear dilation values of the test bodies measured parallel and at right angles in relation to the extrusion direction of the rods, according to A volume = A length parallel + 2 A length at right angles. The results are indicated in the Table Added Content of inhibitor A volume [] inhibitor metal in the coal tar in the range pitch of 1400 to to 24000C No addition 0 6.3 Fe203 1.0 5.9 -g- CaO 1.0 3.3 Mgo 1.0 2.8 BaO 1.0 1.6 The values show the good effect of the metals of the alkaline earth group, especially of barium, as puffing inhibitors with coal tar pitch cokes.

Claims (14)

Claims:

1. A process for the production of graphitized shaped carbon bodies from a coal tar pitch coke- containing dry material and a cokable binder by mixing of starting materials, shaping, coking Lnd graphitization, in which a substance reducing the irreversible expansion of the shaped bodies on heating to temperatures above 14000C is added to said starting materials prior to shaping, which substance is a compound of one of the metals of the group magnesium, calcium, strontium and barium which is not soluble in the binder.

2. A compound

3. A compound

4. A compound

5. A 1, wherein said carbonate. 1, wherein said oxide. 1, wherein said carbide. 1, wherein said fluoride. 1, wherein said sulphate.

process according to claim is an alkaline earth metal process according to claim is an alkaline earth metal process according to claim is an alkaline earth metal process according to claim compound is an alkaline earth metal

6. A process according to claim compound is an alkaline earth metal

7. A process according to claim 1, wherein there is added to said starting materials at least two of the compounds indicated in one or more of claims 1 to 6.

8. A process according to any preceding claim, wherein the compound(s) is/are added in such an amount that the metal content provided thereby corresponds to 0.1 to 3.0% by weight related to the components of the coke.

9. A process according to any preceding claim, wherein the compound(s) is/are added in a fineness of at least 80% less than 50 micrometer.

10. A process according to claims 1 to 9, wherein the compound(s) is/are added to one or to several of the components to be mixed before the mixing procedure.

11. A process according to any one of claims 1 to 9, wherein the compound(s) is/are added to the components to be mixed during the mixing procedure.

12. A process according to any one of claims 1 to 9, wherein the or each compound is first mixed with the binder and this mixture is added to particulate 5 components of the mixture.

13. A process for the production of graphitized shaped carbon bodies as claimed in claim 1, substantially as hereinbefore described with reference to the foregoing Example.

14. A graphitized shaped carbon body, whenever produced by the process claimed in any preceding claim.

F,ib'ished '990 a' The Pal ent Of:,,ce. S-a-.c HousF 66 71 High Holborn. Lcndon WC 1R 4TP- Flarther copies maybe obtained from The Patent OfficeSaleú Branch, St Mary Cray, Orpington. Kent BR5 3RD. Prjl,^.ad by Multiplex tecliniques ltd, St Mary Cray. Kent, Con. 1.87