AU651847B2

AU651847B2 - Process for refining pitch

Info

Publication number: AU651847B2
Application number: AU86111/91A
Authority: AU
Inventors: John Robert Kershaw; Paul James Smart
Original assignee: Commonwealth Scientific and Industrial Research Organization CSIRO
Current assignee: Commonwealth Scientific and Industrial Research Organization CSIRO
Priority date: 1990-09-13
Filing date: 1991-09-13
Publication date: 1994-08-04
Anticipated expiration: 2011-09-13
Also published as: AU8611191A

Description

PROCESS FOR REFINING PITCH

Technical Field

The present invention relates to a method for refining pitch, especially coal tar pitch. More particularly, the present invention relates to a method for refining pitch, especially coal tar pitch, to produce a refined pitch suitable for use in carbon fibre production. The refined pitch is also suitable as an impregnating pitch for the production of carbon/carbon composites and other high grade carbonaceous products.

Background Art

Carbon/carbon composites have found use in a variety of diverse applications, for example, in rockets and brakes especially those used in modern aircraft. High carbon yields (for example as indicated by high Alcan coking values) are desirable for impregnation pitches used in the manufacture of carbon/carbon composites as this limits the number of impregnation and carbonization steps required in their manufacture.

Carbon fibres which are characterised by their high strength and stiffness together with their low density have found considerable use in reinforcing plastic and metal matrices. It is generally accepted that increased use of carbon fibres depends on reducing the costs associated with producing the fibres. Therefore, the formation of carbon fibres from relatively inexpensive carbonaceous pitches has achieved considerable attention in recent years. Carbon fibres can be produced from pitches by two distinct routes which are: (i) by first converting the pitch into the mesophase state; or

(ii) by forming fibres of intermediate mechanical properties directly from isotropic (non-mesophase) pitch. It is advantageous if the isotropic pitch used has a relatively high softening point to minimise the stabilisation time of the pitch fibre. (Fuller et. al. U.S. Pat. No. 3, 959, 448, 1976).

Carbon fibres produced from optically anisotropic pitch or mesophase pitch have a highly orientated structure in which carbon crystallites are preferentially aligned parallel to the fibre axis and have excellent mechanical properties, that is, high strength and high modulus of elasticity. The production of mesophase pitch from isotropic coal-tar pitch or petroleum pitch requires heat soaking the precursor pitch at temperatures between 350 C and 500 C under an inert atmosphere. In general, temperature of around 400°C have been used to produce mesophase pitch with the required rheological properties for spinning. At this temperature, it takes a long time, typically around 24 hours, to produce mesophase pitch from coal-tar or petroleum pitch (L.S. Singer US 4005183, 1977).

To produce a suitable mesophase pitch for production of carbon fibres it is essential to remove the solids, ash-forming material and coke, which are characterised by their insolubility in quinoline ( I), as the I produce defects in the fibres and lower their tensile strength.

The extraction of coal-tar pitches with supercritical fluids is described in Polish patent No 127934. This patent describes the use of solvents having a critical temperature greater than 30 C and critical pressure of at least 4 MPa. Using toluene at 350°C and lOMPa, the yield of extract was 51% when the pressure and temperature were lowered to 4.5 MPa and 330 C in the separator. Though the anthracene-oil insolubles (equivalent to QI) were low (0.2% compared to 13.2% in the starting pitch) in the extract, the low softening point (55 C compared to 105 C in the starting pitch) and benzene insolubles (4.4% compared 35.4% in the starting pitch) would make the extract unsuitable for mesophase formation and thus unsuitable for use in the production of carbon fibres. A high softening point and a high benzene or toluene insoluble content is desirable in a precursor pitch for mesophase formation or for the direct spinning into pitch fibres and for an impregnating pitch.

The extraction of coal-tar pitches with a supercritical gas in the presence of an entraining agent has been reported in United States patent No. 4,806,228 to Beneke et. al. (1989) (equivalent to AU-A-68593/87) . This patent describes the use of aliphatic or olefinic hydrocarbons containing preferably 2 to 5 carbon atoms, halogen-containing hydrocarbons containing particularly up to 4 carbon atoms or mixtures of these hydrocarbons above their critical temperature and above their critical pressure as the supercritical gases.

According to the patent, mononuculear or polynuclear hydrocarbons which are substituted, when required, by alkyl groups, containing particularly up to 2 carbon atoms or by an amino group and which can be aromatic as well as entirely or partially hydrogenated are suitable as entraining agents. The use of mono- or polynuclear heterocyclic compounds, particularly of nitrogen containing heterocyclic compounds in which one nucleus or the two nuclei are heterocyclic, is possible and so is the use of alkyl esters of aromatic acids containing preferably 1 to 6 carbon atoms in the alcohol component. Suitable entraining agents are, for example, crude benzene, commercial benzene for cleaning, platformates having a boiling range of between 70 and 200 C, preferably between 100 and 150°C, methylnaphthalene and ethylnaphthalene fractions and their mixtures.

The extraction stage of US 4,806,228 is preferably carried out under temperature and pressure conditions which are above the critical pressure and the critical temperature of the supercritical gas but below the critical temperature of the entraining agent. These pressures are usually between 80 and 300 bars and the temperatures range from 80 to 300 C. The pitch is preferably extracted under a pressure of 150 to 250 bars at a temperature of 120 to 250 C. A uniphase is thus formed. The undissolved solids are deposited as sediments and can thus be separated.

The entrainer was present in the examples quoted in the patent in an amount of 50-70% of the solvent. This is much higher than the concentration of an entrainer in standard supercritical fluid extraction practice where the term is normally used to indicate a minor component (below its critical temperature) present in 5-10% concentration of the major supercritical solvent. The examples given in US 4,806,228 typically used toluene as the entrainer and propane as the supercritical gas at temperatures of 150 C or 190 C, well below the critical temperature of toluene (319 C) . The separation of the pitch into fractions in US 4,806,228 is achieved by passing the extraction mixture through a number of extraction steps by lowering the pressure to separate out distinct fractions. The fraction left in the first separator which is high in toluene insolubles, as required for mesophase formation, also generally contains QI, probably due to the entrainer being well below its critical temperature, making the fraction unsuitable for carbon fibre manufacture. Indeed the patent does not claim that the fractionated pitches would be suited to carbon fibre production.

Further, the pitch residue remaining in the autoclave (Fraction 1 in the example of US 4,806,228) has a very high I content and is unsuitable for carbon fibre production. In both US 4,806,228 and Polish patent 127934, it is the fractions extracted into the supercritical gas and subsequently recovered in a later separation stage that are the fractions of interest.

Disclosure of the Invention

The present inventors have developed an alternative process for refining coal tar pitches.

According to a first aspect, the present invention includes a process for refining pitch comprising substantially removing quinoline insoluble components from the pitch and extracting the pitch with a supercritical fluid whereby a low molecular weight fraction of the pitch dissolves in the supercritical fluid and is thereby removed from the pitch to leave refined pitch as a residue. Preferably, the pitch is a coal tar pitch.

The low molecular weight fraction of the pitch dissolved in the supercritical fluid preferably comprises material with molecular weights predominantly below 450, more preferably below 400. The refined pitch produced by the present invention has been found to be especially suitable for use in the production of carbon fibres.

Accordingly, in a second aspect, the present invention includes a process for refining pitch, preferably coal tar pitch to produce refined pitch suitable for use in the production of carbon fibres comprising substantially removing quinoline insoluble components from the pitch and extracting the pitch with a supercritical fluid whereby a low molecular weight fraction of the pitch dissolves in the supercritical fluid and is thereby removed from the pitch to leave refined pitch as a residue.

According to a third aspect, the present invention includes a process for producing carbon fibres from pitch, preferably coal tar pitch, comprising substantially removing quinoline insoluble components from the pitch, extracting the pitch with a supercritical fluid whereby a low molecular weight fraction of the pitch dissolves in the supercritical fluid and is thereby removed from the pitch to leave refined pitch as a residue, spinning the refined pitch to produce fibres, stabilising the fibres and carbonising the fibres to produce carbon fibres. The refined pitch may be at least partially converted to mesophase prior to the spinning step.

The present invention also encompasses refined pitch produced according to the first and second aspects of the invention, carbon fibres produced from the refined pitch and carbon fibres produced according to the process of the third aspect of the invention and carbon composites and carbon electodes produced from the refined pitch. In a preferred form, the quinoline insoluble components of the pitch are removed by filtering. It is especially preferred that the pitch is filtered at a temperature ranging from 170-230°C through a 0.5-3 micron filter. A suitable filter aid may also be used, especially diatomaceous earth filter aids such as Celite, Hyflo Super-Cel and Dicalite. (Celite, Hyflo Super-Cel and Dicalite are trade names).

Examples of suitable supercritical fluid extracting agents are aliphatic or olefinic hydrocarbons, preferably with 5 to 8 carbon atoms and mixtures thereof above their critical temperature and critical pressure. Suitable supercritical fluid extracting agents are for instance hexane, heptane, cyclohexane, pentane, cyclopentane, methylcyclopentane, trimethylpentane, hexene, heptene and suitable petroleum distillates e.g. petroleum spirit (boiling range 80-110°C) . The extraction may be carried out at temperatures preferably between 200 C and 350°C more preferably between 250°C and 300°C and at pressures preferably between 5MPa and 25MPa, more preferably between 7MPa and 15MPa.

As stated above, the refined pitch produced according to the present invention is especially suitable for use in the production of carbon fibres. Without wishing to be bound by theory this is believed to be mainly due to the decreased heating time of the refined pitch required to produce mesophase pitch when compared to the heating time required to produce mesophase pitch from isotropic coal-tar pitch. It is believed that the low molecular weight components of the pitch inhibit the production of mesophase pitch. Unless removed prior to the heat-soaking stage, the lower molecular weight components of the pitch are removed from the pitch as volatiles, or are polymerised, during mesophase pitch formation. The heat soaking to produce mesophase takes place at temperatures below the boiling points of most of the lower molecular weight components. Therefore, the slow evolution of these components, together with their stability which limits their rate of polymerisation, contributes to the slow rate of mesophase formation from isotropic or unrefined pitch. These problems are not present when refined pitch according to the present invention is used for mesophase formation because the low molecular weight fraction of the material has been removed prior to the heat-soaking, and this contributes to the much lower time required for mesophase formation.

EXAMPLE 1

lOOOg of a coal-tar pitch having 8.8% by weight of quinoline insolubles and a softening point of 111°C (Mettler) was mixed with 70g of Hyflo Super-Cel and filtered through a lμm filter under a pressure of IMPa at 200 C. The resultant pitch (pitch A) had a QI content of less than 0.1% by weight, a softening point of 108 C and a toluene insoluble (TI) content (ISO 6376) of 21% by weight, an Alcan coking value of 54.5% and an average molecular weight of 471. lOOg of pitch A and 600ml of hexane were placed in a 1 litre autoclave and heated to 250 C. Hexane (1 litre/hour) was passed through the autoclave while the pressure was maintained at 10 MPa. The extraction was carried out for 1 hour. 29% by weight of the pitch was extracted during this time. The pitch residue (pitch B) left in the autoclave after the extraction accounted for 71% of the starting pitch (pitch A) had a softening point of 171 C, a QI content < 0.1wt% and a TI content of 30.6 wt%, an average molecular weight of 567 and contained no mesophase as shown by optical examination. This pitch (pitch B) was heat-treated in a nitrogen atmosphere at 400°C (method A) and formed mesophase in a substantially shorter period than did pitch A (see Figure 1). After 6 hours 50% mesophase was formed from pitch B compared to 24% from pitch A. Using a different reactor (stainless steel autoclave) and a different pitch quantity (method B), pitch B formed 36% mesophase compared to 12% for pitch A after 6 hours at 400°C.

EXAMPLE 2

The extraction of lOOg of pitch A in example 1 was repeated using cyclohexane as the extractant at a temperature of 300 C. The pressure, flowrate and extraction time were the same as in example 1. 49% by weight of pitch remained after the extraction. This pitch residue (pitch C) had a softening point of 190 C, QI content of < 0.2% by weight, a TI content of 49.7% by weight and an average molecular weight of 790. This residual pitch formed mesophase much more readily than did pitch A as shown in Figure 1. After 6 hours pitch C had formed 70% mesophase (method A) and 51% mesophase (method B) compared to 24% and 12% respectively for pitch A.

EXAMPLE 3

The extraction of lOOg of pitch A in example 1 was repeated using n-heptane as the extractant at a temperature of 290°C and a pressure of 15MPa. The flow rate and extraction time were the same as in example 1. 55% by weight of pitch remained after the extraction. This pitch residue had a QI content of < 0.2% by weight and a TI content of 44.7% by weight. This residual pitch (pitch D) formed mesophase much more readily than pitch A (see Figure 1). After 6 hours, pitch D formed 65% mesophase (method A) and 54% mesophase (method B) compared to 24% and 12% respectively for pitch A.

EXAMPLE 4

Pitch A was extracted as in example 1 except that the pressure was 15 MPa. The pitch residue (pitch E) left in the autoclave after the extraction accounted for 64% of the pitch and had a softening point of 186 C, a QI content of < 0.1wt% and a TI content of 34.2% by weight. 200g of pitch E was heat-treated in an autoclave at 400 C with stirring (330 rpm) and nitrogen sparging (0.51/min) to form 85% mesophase in 12.5 hours compared to 24.5 hours for pitch A to form the same percentage of mesophase.

EXAMPLE 5

The extraction of lOOg of pitch A in example 1 was repeated using petroleum spirit (boiling range 80 110 C) at a temperature of 300 C and a pressure of 15 MPa. The flow rate and extraction time were the same as in example 1. 55% by weight of the pitch remained after extraction. This pitch residue had a QI content of < 0.2% by weight, a TI content of 46.4% by weight, an Alcan coking value of 74.8% and an average molecular weight of 721. This pitch formed 47% mesophase compared to 12% for pitch A after 6 hours at 400°C.

This pitch was melt spun through a multi-hole spinnerette (hole diameter 0.008 inch) at 222 C and the filaments taken up by a reel at the rate of 600-950m/minute to give filaments of 10 microns diameter. Long spin times could be achieved without filament breakage. EXAMPLE 6

Pitch A was extracted as in example 1 except that the pressure was 15 MPa and the temperature 300 C. The pitch residue left in the autoclave after the extraction accounted for 50% of the pitch and had a QI content of < 0.2 wt%, a TI content of 43.3% and a softening point of 178°C.

The pitch formed 36% mesophase compared to 12% for pitch A after 6 hours of 400 C. 200g of this pitch was heat-treated in an autoclave with stirring (330 rpm) and nitrogen sparging (0.5L/min) to form a pitch (softening point 364°C) with 80% mesophase after 10.5 hours.

EXAMPLE 7

Pitch A was extracted as in example 1 except that the pressure was 15 MPa, temperature 300 C and extraction time 20 minutes. The pitch residue left in the autoclave after extraction accounted for 77% of the pitch and had a softening point of 147°C, a QI content of < 0.1% by weight and a TI content of 26% by weight.

Those skilled in the art will appreciate that the invention described herein is susceptible to variations and modifications other than those specifically described. It is therefore to be understood that the invention includes all such variations and modifications which fall within its spirit and scope.

Claims

CLAIMSThe claims defining the invention are as follows:

1. A process for refining pitch comprising substantially removing quinoline insoluble components from the pitch and extracting the pitch with a supercritical fluid whereby a low molecular weight fraction of the pitch dissolves in the supercritical fluid and is thereby removed from the pitch to leave refined pitch as a residue.

2. A process according to claim 1 in which the pitch is a coal tar pitch.

3. A process according to claim 1 or claim 2 wherein the low molecular weight fraction comprises material with molecular weights predominantly below 450.

4. A process according to claim 3 wherein the low molecular weight fraction comprises material with molecular weights predominantly below 400.

5. A process according to claim 2 wherein the quinoloine insoluble components are removed from the pitch by filtering the pitch.

6. A process according to claim 5 wherein the pitch is filtered at a temperature in the range of 170-230°C.

7. A process according to claim 5 wherein the pitch is filtered through a filter having a pore p ze in the range of 0.5-3 micrometres.

8. A process according to claim 1 or claim 2 wherein the supercritical fluid is an aliphatic or olefinic hydrocarbon or a mixture thereof.

9. A process according to claim 8 wherein the supercritical fluid is an aliphatic or olefinic hydrocarbon having 5-8 carbon atoms.

10. A process according to claim 8 or claim 9 wherein the supercritical fluid is selected from the group comprising hexane, heptane, cyclohexane, petroleum spirit having a boiling range of 80-110 C and mixtures thereof.

11. A process according to any one of the preceding claims wherein the pitch is contacted with the supercritical fluid at a temperature of from 200-350 C and at a pressure in the range of 5-25 MPa.

12. A process according to claim 11 wherein the pitch is contacted with the supercritical fluid at a tteemmppeerraattuurree ooff ffrroomm 225500-300 and at a pressure in the range of 7-15 MPa.

13. A process for producing carbon fibres from pitch comprising substantially removing quinoloine insoluble components from the pitch, extracting the pitch with a supercritical fluid whereby a low molecular weight fraction of the pitch dissolves in the supercritical fluid and is thereby removed from the pitch to leave refined pitch as a residue, spinning the refined pitch to produce fibres, stabilising the fibres and carbonising the fibres to produce carbon fibres.

14. A process according to claim 13 wherein the pitch is a coal tar pitch.

15. A process according to claim 13 or claim 14 wherein the low molecular weight fraction comprises material with molecular weights predominantly below 450.

16. A process according to any one of claims 13-15 wherein the refined pitch is at least partially converted to mesophase prior to spinning.

17. A process according to claim 16 wherein the refined pitch is at least partially converted to mesophase by heating the refined pitch to a temperature of about 350°-500°C for a period of 6-12.5 hours.

18. Refined pitch produced by the process as claimed in any one of claims 1-12.

19. Carbon fibres produced from the refined pitch as claimed in claim 18.

20. Carbon fibres produced by the process as claimed in any one of claims 13-17.

21. A carbon composite produced from the refined pitch as claimed in claim 18.

22. A carbon electrode produced from the refined pitch as claimed in claim 18.