EP0177981A1 - Process for making high-power graphite electrodes - Google Patents

Abstract

1. A process for making high-power graphite electrodes from two components in the form of anisotropic needle cokes and an electrode binder on the basis of coal tar pitch, which has a proportion of beta-resins of more than 20% by weight and a quinoline-insoluble content (QI) of more than 5% by weight, which process comprises the steps of mixing the two components, moulding the green electrode and burning and graphitizing the electrode, characterized in that 70 to 80 parts by weight of an anisotropic needle coke are mixed homogeneously with 20 to 30 parts by weight of the binder, which has an atomic C/H ratio of the quinoline-insoluble of a maximum of 3 : 1, an ash value of 0.1% by weight or less and a softening point of 80 to 120 degrees C.

Description

Process for the production of high-performance graphite electrodes

The invention relates to a method for producing high-performance graphite electrodes for the production of electrical steel from anisotropic needle coke using an electrode binder by mixing the components, shaping the green electrode, firing and graphitizing the electrode.

The quality of graphite electrodes is determined by physical parameters such as Characterized thermal expansion coefficient, specific electrical and thermal conductivity, ash value, trace elements and density.

Modern high-performance electrodes are characterized, among other things, by a high specific current load. Extreme specific current loads are only achieved, however, if the graphite material with the appropriate Strength has a particularly high electrical conductivity. In order to make the graphite electrode less sensitive to the large radial temperature gradients which are unavoidable in operation, the initial coke quality had to be improved; So-called needle cokes with a very low coefficient of thermal expansion (CTE) of less than 1 · 10 ^-6 K ^{-1 were} developed. Pretreated coal tar pitch has proven to be a particularly suitable raw material for such needle coke with a low coefficient of thermal expansion.

In conjunction with a suitable coking process (DE-OS 30 35 593), needle coke with a previously unattained low CTE can be obtained from this raw material. ₁ ⁶ _K ^-1 by the use of high-quality pitch-needle _coke to _{0 x} 10 ^-6 K - so the CTE of graphite could moldings of previously employed high-grade petroleum needle coke in the temperature range 20-200 ° C to 0.7 x ₁₀ ^{- 1 can be} reduced. However, the large-scale use of needle coke, particularly based on coal tar pitch as a graphite electrode raw material, is impaired because of a negative accompanying property, "puffing".

Technically produced needle coke undergoes the modification change from the burned carbon body to electrographite during a high temperature treatment up to approx. 3000 ° C. In addition to the reversible deformation due to thermal expansion, they also experience an irreversible change in length, which can be up to 4.5%, known as puffing.

The most important physical parameters of a graphite electrode, e.g. the density and, depending on it, the electrical and thermal conductivity as well as the mechanical strength disadvantageously changed.

Puffing with petroleum and pitch needle cokes occurs to different degrees.

For the petro-stemmed needle coke processed up to now to electrographite, it is known that the irreversible volume increase can be correlated with the sulfur content in the calcined coke. By adding e.g. Iron oxide in the production of the molded body paste is largely inhibitable.

However, this connection does not apply to the same extent for the hard coal tar pitch coke that can also be used as a raw material for electrographite. With the same sulfur content, coal tar pitch needle coke electrodes show a stronger puffing, which cannot be significantly inhibited by Fe ₂ 0 ₃ .

In the preprints of the "16th Biennial Conference of Carbon" (pp. 595-596) a method is described to inhibit puffing on petroleum and coal tar pitch coke moldings by adding 1 - 2% er203. This process is hardly economically feasible.

All other known methods for puffing inhibition of graphite electrodes also use inorganic inhibitors, preferably oxides.

It is based on the idea that, in addition to the sulfur compounds that mainly occur in petroleum coke, other previously unknown intercalation compounds spontaneously convert into gaseous components when individual decomposition temperatures are reached and cause an internal overpressure in the coke particles that irreversibly disperses the crystal levels of the carbon. When inhibitors are added, intermediate phases are formed that are stable in the temperature range at risk of puffing.

However, the ash value of electrographite is undesirably increased by the known type of chemical puffing inhibition.

In contrast, this patent application describes a method to minimize the puffing of graphite electrodes from pitch needle coke without the additional use of inhibiting agents.

In contrast to the known methods, this application places particular emphasis on an extremely low-ash electrode coke, which also has a specifically set degree of crystalline orientation. The crystalline degree of orientation of the coke is achieved through the selection of suitable raw materials and a coking process adapted to the raw material. The irreversible deformation (Δ1) of standard pitch needle With optimized needle cokes, koksen is reduced from 0.9% to less than 0.2% (measured in the longitudinal direction).

However, the development of high-quality coke types alone does not fully utilize the quality potential in the raw materials.

The object was therefore to develop a process for producing high-performance graphite electrodes from anisotropic needle cokes using an electrode binder by mixing the two components, shaping the green electrode, firing and graphitizing the electrode, in which the properties of the needle coke are fully utilized so that a graphite electrode with optimal properties is obtained.

This object is achieved in that 70 to 80 parts by weight of a highly anisotropic needle coke with 20 to 30 parts by weight of a binder, predominantly based on coal tar pitch, with a proportion of ß-resins (toluene-insoluble - quinoline-insoluble) of more than 20% by weight, a quinoline-insoluble content of more than 5% by weight with an atomic C / H ratio of at most 3: 1, an ash value of 0.1% by weight or less and a softening point (Kraemer-Sarnow) of 80 to 120 ° C are mixed homogeneously and the mixture is formed into green electrodes, fired and graphitized according to methods known per se. High-performance graphite electrodes are usually made from anisotropic filler coke and a largely isotropically coking binder. Therefore, a fired electrode body consists morphologically of two phases with different crystallinity. This results in divergent physical properties that lead to opposite effects at the phase interfaces.

Already during the firing process there are different thermal expansions between filler and binder coke, which lead to relative movements, which result in cracking and peeling. Because of the normally higher level of trace components, electrode binders are foreign atom emitters, which diffuse into the anisotropic filler grain preferably before and during the firing phase. In the subsequent thermal conversion of the carbon body consisting of two different crystal phases into graphite, the recrystallization in the more pre-oriented filler coke is completed earlier than in the isotropic binder coke. Filler coke areas that have already been converted are exposed to foreign atom diffusion at higher temperatures and show irreversible puffing. If, instead of the previously used petroleum needle coke, a better pre-oriented coal tar pitch needle coke is used, the effects described are enhanced, above all, by the graphitization of the filler coke, which was completed earlier.

Improved morphological properties of the polycrystalline carbon body are more surprising Wisely achieved when anisotropic filler cokes, preferably coal tar pitch needle coke, are processed with largely ash-free, also anisotropically coking binders to form electrode moldings. In addition to other measurable parameters, the puffing behavior is also effectively improved. The more the puffing tendency of an anisotropic coke, the more pronounced is the puffing-damping effect of an anisotropic ash-free binder coke in the electrode body.

Correlating the mean true density S of the filler ^coke calcined at 1300 ° C with the proportion of irreversible deformation ^Δl _lo when graphitizing the electrode ^reveals two characteristic curves depending on the crystal ^{modification of the binder coke} , as shown in FIG. 1.

An anisotropic binder coke structure in the electrode body improves the electrical conductivity and the modulus of elasticity with approximately the same breaking strength.

The anisotropically coking binder, preferably with 0.01 to 0.1% by weight of ash, is preferably produced in the usual way from coal tar pitch alone. The ash removal is carried out by methods known per se, such as centrifuging, separating, filtering or promoter-accelerated settling (settling) at elevated temperature. Aromatic petro- or carbo-derived hydrocarbon fractions can be added to the coal tar pitch to facilitate the separation of the ash formers.

Another important requirement for an anisotropically coking binder is the atomic C / H ratio of the quinoline-insoluble (QI). It should be in the range of 2.3: 1 to 3.0: 1. Higher C / H ratios lead to isotropic binder cokes.

The CjH ratio in the binder is preferably 1.85: 1 to 2.04: 1 with a QI content of preferably 10 to 20% by weight.

The invention is explained in more detail with reference to the following examples, without being restricted thereto.

Examples

Three needle cokes with different degrees of pre-orientation and for comparison an isotropic pitch coke are processed according to the invention with an anisotropically coking electrode binder and for comparison with a conventional isotropically coking electrode binder in a known manner to shaped body pastes and converted into fired electrodes. The analysis data of the electrodes graphitized under the same conditions clearly show the advantages of electrode production in the combination of puffing coke with anisotropically coking binder.

Ein Vergleich der Beispiele 1 und 2, 3 und 4 sowie 5 und 6 zeigt deutlich den Einfluß des anisotrop verkokenden Bindemittels auf das Puffing-Verhalten der Elektrode beim Graphitieren. In allen Fällen verringert sich das Puffing vor allem in Längsrichtung um mehr als 70%. Bei den Nadelkoksen aus Steinkohlenteerpech nimmt das Puffing senkrecht (quer) zur Längsachse der Elektrode um etwa 30 % ab. Bei der Verwendung von Petrolkoks ist dieser Effekt geringer. Gleichzeitig werden die übrigen Kenndaten der Elektrode verbessert. Bei der Verwendung eines isotropen Pechkokses für die Elektrodenherstellung hingegen wirkt sich ein anisotrop verkokendes Bindemittel negativ auf die mechanischen Eigenschaften der Elektrode aus.The characteristics of the coke are shown in Table 1. The binders used are characterized in Table 2. The pastes produced in accordance with the recipe given in Table 3 are extruded into green shaped bodies by known technology and fired under identical conditions at 1000 ° C and then graphitized to 2800 ° C. The important characteristics of the fired electrodes are shown in the upper part of Table 4, the characteristics of the graphitized electrodes are in the lower part of Table 4.

A comparison of Examples 1 and 2, 3 and 4 and 5 and 6 clearly shows the influence of the anisotropically coking binder on the puffing behavior of the electrode when graphitizing. In all cases, puffing is reduced by more than 70%, especially in the longitudinal direction. With needle coke made from coal tar pitch, the puffing decreases perpendicularly (transversely) to the longitudinal axis of the electrode by about 30%. This effect is less when using petroleum coke. At the same time, the remaining characteristics of the electrode are improved. When using an isotropic pitch coke for the manufacture of electrodes, however, an anisotropically coking binder has a negative effect on the mechanical properties of the electrode.

It follows from this that, surprisingly, the use of an anisotropically coking binder, in particular in connection with coal tar pitch needle cokes for the production of high-performance graphite electrodes, leads to an improvement in the electrode properties and to a reduction in puffing.

Claims (4)

1. Process for the production of high-performance graphite electrodes from anisotropic needle coke using an electrode binder by mixing the two components, shaping the green electrode, firing and graphitizing the electrode, characterized in that 70 to 80 parts by weight of an anisotropic needle coke are used 20 to 30 parts by weight of a binder, predominantly based on coal tar pitch, with a proportion of β-resins (toluene-insoluble - quinoline-insoluble) of more than 20% by weight, a quinoline-insoluble content (QI) of more than 5 parts by weight % with an atomic C / H ratio of maximum 3: 1, an ash value of 0.1% by weight or less and a softening point (Kraemer-Sarnow) of 80 to 120 ° C and the mixture itself known methods is shaped into green electrodes, fired and graphitized. 2. The method according to claim 1, characterized in that the anisotropic needle coke is made of coal tar pitch. 3. The method according to claim 1, characterized in that the electrode binder is made of filtered coal tar pitch and has an ash value of 0.01 to 0.1%. 4. The method according to claim 1, characterized in that the electrode binder has a quinoline-insoluble content of 10 to 20 wt.% With an atomic C / H ratio in the range from 2.3: 1 to 3.0: 1.

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