EP0174035B1 - Process for preparing needle coke with little irreversible volume expansion from coaltar pitch - Google Patents

Description

The invention relates to a process for the production of needle coke from coal tar pitch with small irreversible volume increases during the graphitization of needle coke moldings, e.g. Graphite electrodes for electrical steel production by smoldering coke from Pechen with a quinoline insoluble content of maximum 1.0% by weight.

The quality of electrographite moldings is determined by several physical properties such as Thermal expansion coefficients, specific electrical and thermal conductivity, ash value, trace elements and density are characterized.

It is known that shaped carbon bodies which are made from petroleum coke or coal tar pitch coke with a binder also show an irreversible increase in volume, the so-called puffing, during the graphitization process in addition to the reversible thermal expansion. Puffing deteriorates the physical properties of the molded body, such as its strength and electrical conductivity.

High-quality electrographites are made from so-called needle cokes with a linear coefficient of thermal expansion of less than 0.7 10-6 K- ¹ (measured in the range 20-200 ° C). In the case of electrographites of electrodes made from petrostatic needle coke, it has been found that the irreversible increase in volume can be correlated with the sulfur content in the calcined coke and can be reduced by adding, for example, iron oxide to the molding paste.

However, this connection does not apply to coal tar pitch needle coke. With the same sulfur content, moldings made from coal tar pitch coke show a stronger puffing, which can be influenced only little by adding Fe ₂ 0 ₃ as a commonly used inhibitor.

In the preprints of the "16th Biennial Conference on Carbon (July 18/22, 1983, San Diego, USA; pp. 595-596) it is described that the irreversible puffing of moldings made from coal tar pitch coke by adding 1-2% Cr ₂ 0 ₃ can be reduced. However, this process is hardly economically feasible on an industrial scale.

Patents EP 0087489 B1 and EP 0085121 B1 describe the targeted introduction of intercalation compounds into the graphite and subsequent heating to higher temperatures at which the intercalation compounds spontaneously decompose into gaseous phases in the production of expanded graphite from petroleum coke. This creates an internal overpressure in the graphite. In order to break it down, the gas must diffuse through the crystal layers of the carbon, the crystal layers being expanded irreversibly. At extreme diffusion current densities, expansion factors of 200: 1 to 400: 1 occur.

Electrographite from high-quality coal tar pitch coke may be puffed using an analogous mechanism and is normally up to 0.9% relative change in length. Only macroscopically isotropic cokes can be produced from coal tar pitch with a high content of quinoline-insoluble substances (Q1), from which graphite bodies can be produced without irreversible linear expansion. However, these have a low conductivity and a high coefficient of thermal expansion.

This results in the need to free the coal tar pitch as a precursor for the coal tar pitch needle coke from quinoline-insoluble constituents. This also reduces the concentration of harmful intercalation compounds in the carbon body and thus the expected diffusion current density, but small amounts of harmful compounds remain in the precursor. A targeted reduction in sulfur and nitrogen in the raw material, however, is difficult.

JP 5898,385 claims the production of a low-puffing needle coke from coal tar pitch, the content of which is 0.03% in quinoline-insoluble constituents. With a Fe ₂ 0 ₃ content of 0.5%, an electrographite is obtained after heat treatment up to 2800 ° C, which only shows an irreversible change in length of 0.5% in the axial direction of the electrode. An electrographite made from a coal tar pitch that contains no insoluble components has a change in length of 2.1%. Even on the basis of low-ash coal tar pitch as a raw material for needle coke and thus for electrographite, the addition of Fe ₂ 0 ₃ as an inhibitor cannot be dispensed with. However, the addition of Fe ₂ 0 ₃ increases the ash value of the electrographite to an undesirable extent.

The object of the invention is therefore to develop a method for producing such needle coke from coal tar pitch, which can be processed into low-puffing electrographite without additional inhibiting agents.

The object is achieved according to the invention in that the ash-forming constituents are removed from the coal tar pitch down to at least 0.01% by weight and the crystalline order of the coke is disturbed by suitable raw material selection and / or by the coking parameters so that the true density of the at 1,300 ° C calcined coke is reduced to an amount of 2.08 to 2.14 g / cm ³ .

The ash-forming constituents in the feed product can be reduced by known mechanical or thermal processes, as are customary for the separation of solid particles. These processes include, for example, centrifuging and separating, filtering, extracting and promoter-accelerated settling. With this ash removal, however, the puffing can only Ash removal, however, can only partially reduce puffing.

The crystal structure of coke can be influenced by changing the coking tendency of the feed, e.g. by using selected hard coal tars and pitches, or also by adding up to 50% by weight, based on the pitch, of a high-boiling carbo- and petro-derived hydrocarbon fraction. Another option is to vary the coking conditions such as pressure, heating rate, coking time and coking temperature. A combination of both measures is possible, but not necessary in every case.

In the procedure claimed according to the process, carbonization and subsequent calcination up to 1,300 ° C., for example coke, can be obtained, which can be characterized by the following data:

On the basis of some selected test data in Table 1, it can be seen that by varying measures (1) and (2), needle coke is obtained which is characterized by different densities and ash values. As a simple measurement, the true density _ß is a measure of the degree of crystalline orientation in the coke.

If these cokes obtained are then processed under known, constant conditions (Journal of Materials Science 18, 1983, pages 3161 EZ 176) into shaped bodies and graphitized up to 2500 ° C., different irreversible changes in length are found.

What is surprising about these investigations is the fact that cokes with the same ash value at lower density, i.e. show a significantly improved puffing behavior with a lower crystalline order, without the needle coke properties being impaired in an impermissible manner. It must be assumed that in addition to the reduction of the ash components that are responsible for the puffing, the disturbed crystal structures of the coke facilitate the diffusion of the gases produced by the remaining intercalation compounds during the graphitization, so that the graphite structure is expanded.

With the measures described - reducing the ash builders in the pitch and influencing the crystallinity of the pitch coke - for influencing irreversible linear expansion in the case of pitch needle cokes, a way has been found to minimize the undesired molding puffing without contaminating additives.

The individual measures are effective on their own, but only in combination do they achieve an optimal result. The invention is explained in more detail with reference to the following examples, without being restricted thereto.

Examples

Pyrolyserückstandsöl aus der Wasserdampfspaltung:

Anthracenöl aus der Steinkohlenteeraufarbeitung:
Siedebereich 355 bis 470°C.10 cokes are compared, which are made from pitches, partly in a mixture with highly aromatic oils, with a low content of quinoline-insoluble substances (QI). The input products are characterized by the following analysis data:

Pyrolysis residue oil from steam splitting:

Anthracene oil from coal tar processing:
Boiling range 355 to 470 ° C.

These feedstocks are pretreated according to the specified procedures in order to ash the pitches. The separation takes place in a plate separator at 200 ° C, without the addition of solvents. The feed products are filtered through a gap filter with a gap width of 100 to 200 µm at 240 to 280 ° C and a maximum pressure of 8 - 10 ⁵ Pa is. This method is particularly suitable for ash removal from pitches. Ashing by promoter-accelerated settling is also possible, as comparative example 1 shows, and equivalent to separation. The feed is mixed homogeneously with 50% by weight of methylnaphthalene and 50% by weight of kerosene, based on the feed, at 175 ° C. and left at 170 ° C. A heavy phase settles on the bottom of the container. The remaining light phase is removed and the low-boiling solvents are removed by flash distillation. Coking of the precursor obtained in this way takes place either according to the delayed-coking process under 5 bar at a tube furnace outlet temperature of 480 ° C or in pressure-resistant retorts under a pressure of 1-5 bar and a heating rate of 5 or 10 K / h to 500 ° C instead.

Different pretreatments and coking conditions produce cokes with different ash contents and true densities, measured on coke calcined at 1300 ° C.

The coke is crushed, ground and sieved so that there is a uniform granulometry of 100% <1 mm 0, 75% <0.5 mm 0 and 15% dust (<63 µm 0).

The ground coke is made using a commercially available electrode binder that meets the specifications

aufweist, zu Pasten mit einem Binderanteil von 24 Gew.-%, bezogen auf das Gemisch, vermengt.

has, to pastes with a binder content of 24 wt .-%, based on the mixture.

The pastes are then extruded into shaped bodies in a suitable device and fired to solid carbon bodies in a conventional manner.

The burned carbon bodies are graphitized under identical conditions and the irreversible change in shape that occurs is measured.

The results are shown in Table 2.

As a comparison of Examples 1 and 2 with the corresponding comparative examples shows, puffing is influenced not only by ash removal, but to a considerable extent by the choice of coking conditions. For example, the coking process most regarded as the most suitable for coking in retorts with slow heating (5 K / h) and low shear forces results in a coke with relatively high irreversible linear expansion. The delayed coking process with rapid heating and then almost constant temperature at high shear forces, which prevents the growth of large crystallites, leads to a coke with less puffing. Although the linear expansion coefficient increases as a result of this procedure, it is still within the range favorable for needle coke. In addition, the electrical conductivity of the graphitized moldings also increases, which is a further positive feature of the method according to the invention.

Claims (7)

1. A process for manufacturing needle coke from coaltar pitch with low irreversible volume increase during the graphitisation of the moulded bodies manufactured therefrom by low-temperature coking of pitches having a content of quinoline-insoluble matter of 1.0%-weight maximum, characterised in that the ash-forming components are removed from the raw material to at least 0.01%-weight and the crystalline structure of the coke is disturbed by a suitable choice of raw material and/or by the coking parameters in such a manner that the true density of the coke calcined at 1300°C is reduced to a level of between 2.08 and 2.14 g/cm³.

2. A process according to Claim 1, characterised in that the raw material is de-ashed by filtering at 240 to 280°C and at a maximum pressure of 8 · 10⁵ Pa with the addition of double the quantity of filtering agent in relation to quinoline-insoluble matter in the raw material via a gap filter with 100 to 200 µm gap width.

3. A process according to Claim 1, characterised in that up to 50%-weight, in relation to the pitch, of a high-boiling-point aromatic petroleum or carbon-derived hydrocarbon fraction is added to the coaltar pitch.

4. A process according to Claim 3, characterised in that the hydrocarbon fraction is a residue oil from water vapour pyrolysis of crude oil fractions.

5. A process according to Claim 3, characterised in that the hydrocarbon fraction is a brown coaltar pitch.

6. A process according to Claim 1, characterised in that, during coking, a heating gradient of at least 5 K/h is maintained under a pressure of 1-5 bar.

7. A process according to Claim 1, characterised in that the precursor is coked in a delayed coker at a pressure of 5 bar maximum and at a tube furnace exit temperature of approximately 480°C.