GB2131789A - Binder for calcined dolomite refractory compositions - Google Patents

Abstract

Calcined dolomite refractory compositions comprise a major proportion of calcined dolomite and, as binder therefore, a phenolformaldehyde resin which has a phenol; formaldehyde molar ratio of from 1:0.7 to 1:4.0, and alkali metal hydroxide:phenol molar ratio of from 0.2:1 to 1.5:1 and a water content of less than 20%, preferably not greater than 5%, by weight. Refractory articles are made by forming the composition into the desired shape and then curing at an elevated temperature.

Description

SPECIFICATION Calcined dolomite refractory compositions Coal tar pitch has been traditionally used to bond dolomite and magnesite refractory articles.

However, because of the potential health hazards in the handling of pitch and the evolution hazards of pyrolysis products, there is a tendency to use polymers to replace pitch. Phenolic resins, both novolaks and resoles, are favoured because they are thermosetting, have a high carbon yield and, under some conditions, can be pyrolysed during service.

Calcined magnesite (MgO) can be easily bonded with both:- (1) phenol-formaldehyde novolaks, either in solution or as powders, with hexamine added to provide thermosetting characteristics; and (2) resoles, as high solids solutions in water or in alcohol/water mixtures.

However, when bonding calcined dolomite the mix exotherms rapidly and the resin tends to gel.

The mix quickly dried up and is difficult to form into the bricks after only a few minutes. If the mix has sufficient life to enable bricks to be made, then the are very weak and tend to crack up on standing and even further on attempting to cure at 1 500 C.

There are two problems in using calcined dolomite as a refractory binder with phenolic resins.

(1) reaction of the CaO fraction of the calcined dolomite with the phenolic hydroxy group. This reaction is exothermic so that the mix heats up, the molecular weight of the phenolic novolak or resole increases as a result of polymerisation and water tends to evaporate. The bivalent calcium ion, CA++, crosslinks the polymer chains. The overall effect is that the mix appears to dry out and can no longer be formed into the required article.

(2) the presence of water in the resin causes rapid hydration to calcium hydroxide. This reaction is also exothermic and expansion occurs for two reasons; (a) because of the temperature rise and (b) because the volume of the calcium hydroxide molecule is larger than that of the oxide.

The expansion of the inside of the compressed calcined dolomite article breaks resin bonds and causes surface cracking.

Heating of calcined dolomite in air accelerates hydration due to absorption of water from the air.

(It is important to carry out bonding of calcined dolomite in as dry an atmosphere as possible).

We have now found that the above problems may be circumvented by employing, as binder for the calcined dolomite, a phenolic resin in which at least some of the phenolicOH groups are blocked by forming the alkali metal salt and which has a low water content.

The present invention provides a composition comprising a major proportion of calcined dolomite and, as binder therefore, a phenol-formaldehyde resin having a phenol:formaldehyde molar ratio of from 1:0.7 to 1:4.0, an alkali metal hydroxide:phenol molar ratio of from 0.2:1 to 1.5:1 and a water content of less than 20% by weight.

The present invention also provides a process for producing refractory articles which process comprises mixing calcined dolomite with, as binder, a phenol-formaldehyde resin having a phenol:formaldehyde molar ratio of from 1:0.7, to 1:4.0, an alkali metal hydroxide:phenol molar ratio of from 0.2:1 to 1.5:1 and a water content of less than 20% by weight, forming the mixture and curing the formed mixture at elevated temperature.

It is known to employ phenolic novolak resins to bond refractory granules, for example, sand, in the manufacture of foundry moulds and cores. It would, in theory, be possible to employ phenolic novolak resins, in which the phenolicOH groups have been converted to the salt form using an alkali metal hydroxide, as binders for calcined dolomite. However, in practice, the hexamethylenetetramine, commonly employed to make up the deficit of formaldehyde in order to render the novolak resin thermosetting, is destroyed by the alkali, with the liberation of ammonia. It is, nevertheless, possible to employ a phenolic novolak resin in conjunction with a phenolic resole resin as a curing agent therefore, both the phenolic novolak and the phenolic resole having a proportion of their phenolicOH groups blocked by alkali.Alternatively, the novolak could be a normal novolak containing no alkali.

It may be necessary, particularly when using novolak resole combinations, to employ an accelerator to speed up the reaction. Examples of such accelerators include dihydrophenols, for example, resorcinol, amines and esters such as monoacetin or triacetin.

Where, in the above, reference is made to a phenol:formaldehyde molar ratio and to an alkalimetal hydroxide:phenol molar ratio, it is to be understood that these ratios relate the the overall molar ratios, so that, where a mixture of phenolic resins is employed, for example, a phenolic novolak resin together with a phenolic resole resin as curing agent therefore, it is the ratio of the combined weights of phenol and formaldehyde and of phenol and alkali-metal hydroxide used which is of relevance.

Typically, the resin will be used in an amount of from 2 to 10%, preferably from 3 to 8%, by weight based on the weight of the mixture with calcined dolomite.

In order to create a mixture which can be formed to the desired shape it is desirable that the binder should be in the liquid state or contain sufficient liquid to enable it to adhere to the calcined dolomite particles and to flow under heat and pressure. Consequently, because it is necessary to minimise the water content of the resin, it is desirable to incorporate, as pick up agent, an amount of a non-aqueous liquid to permit the composition to be formed to shape. To prevent drying out during use, the non-aqueous liquid should preferably be of low volatility, although it is within the scope of the invention to employ a mixture of non-aqueous liquids for this purpose and such a mixture may include some volatile liquid.

It is not essential that any or all of the non-aqueous liquids employed to render the compositions formable should be solvents for the resin. Suitable non-aqueous liquids for this purpose include glycols, such as ethylene glycol or diethylene glycol. Examples of volatile liquid which may be used in combination with non-volatile non-aqueous liquids include lower alcohols, such as methanol or ethanol.

The pick up agent, when used, will typically be present in an amount of up to 10% by weight of the composition. Preferably, however it is used in an amount of from 1.5 to 5% by weight of the composition.

As indicated above, however, the presence of water is generally undesirable, since it reacts with the calcined dolomite to produce heat, as well as resulting in expansion of the composition. It is, nevertheless, not essential to eliminate all of the water and satisfactory results may be achieved with a phenolic resole containing water up to 20%.

To reduce to a minimum the heating and expansion effect of the binder, however, it is preferred that the phenolic resole should have a water content no greater than 5.0% by weight.

In this connection, it should be noted that commercially available spray-dried phenolic resole resins may typically exhibit water contents of from 10 to 14% by weight based on the weight of the resin.

In one embodiment of the invention, the binder is a phenolic resole prepared by condensing a phenol and formaldehyde in the presence of sodium hydroxide, distilling under vacuum to reduce the water content to below 5.0% by weight and adding a high-boiling liquid, such as a glycol, and, optionally, a volatile solvent, such as methanol to reduce viscosity. This approach is best achieved with low molecular weight resoles or novolaks since the amount of high boiling solvent is kept to a minimum and solubility is increased.

In a further embodiment of the invention, a phenol and formaldehyde are condensed in the presence of sodium hydroxide and spray-dried prior to mixing with a pick-up agent, such as ethylene glycol, and employment as a binder for the calcined dolomite particles.

This technique is only applicable to resins which form powders on dehydration (i.e. these which are sufficiently high in molecular weight). However, we have found that such resins do act as adequate bonds for calcined dolomite when used with a high boiling solvent as a pick up agent as described in the examples.

The phenol employed in preparing the resins of the present invention is preferably phenol itself, but all or part of the phenol may be replaced by any one (or more) of the many phenols well known for the manufacture of phenolic resins, such as, for examle, the various cresols, xylenols and chlorophenols.

For reasons of convenience and availability, the formaldehyde employed will normaily be in the form of an aqueous solution, such as, for example, a 50% w/w aqueous solution. Since much of the water present in the mixture of reactants must be removed prior to use, however, it may prove to be advantageous to replace all or part of the aqueous formaldehyde solution by paraform, or by one of the commercially available high strength flake or block forms of formaldehyde.

The compositions of the invention may be prepared in any convenient manner which will ensure substantially uniform and intimate mixing of the binder with the particles of calcined dolomite. After the binder and calcined dolomite have been mixed, the resultant composition is formed to the desired shape under pressure and finally cured at elevated temperature.

The following examples illustrate the invention: Example 1 1) Preparation of powdered resin 30 g of caustic soda flake was dissolved in 30 g of water and the solution allowed to cool to 200C.

94 g of phenol was charged to a flask and the caustic soda solution added slowly. The solution produced was cooled to 400C and 86 g of 52% formalin added, raised to reflux and reacted for 1 hour 30 minutes, then cooled to 250 C.

The resin solution was spray dried to produce a powdered resin having the following characteristics: Approximate water content 1 1.6% NaOH:phenol 0.75:1 P:F 1:1.5 2) Bonding of calcined dolomite with powdered resin 750 g of fresh coarse ground calcined dolomite was mixed with 250 g fresh fine ground material powdered resin of Example 1 and ethylene or diethylene glycol pickup agents added. The temperature of the mix before and after mixing was measured. 2x2 cyclinder specimens were made at 1 tonne/in2 pressure and baked in an oven at 1 600C for 1 hour. The attached table describes the mixed made. All specimens were extremely strong and showed no signs of surface cracking, even after baking for 1 hour at 3000C.

Resin content 5% 4% 3% 3% Pickup agent EG EG EG DEG % Pickup agent 3.75% 3% 2.25% 2.25% Temperature after mixing* 300C 280C 280C 280C Cure 1 hr1600C 1 hr1600C 1 hr1600C 1 hr1600C +1 hr 3000C +1 hr 3000C *Initial aggregate temperature 1 70C.

EG=Ethylene glycol.

DEG=Diethylene glycol.

All mixes remained sticky for above 24 hours although they had stiffened up after about 1 hour.

All specimens were so strong it was impossible to break them using a laboratory Houndsfield tensometer (above 20 KN).

This example shows that adequate bonding can be obtained with a resin which does not have complete salt-like character but contains some free hydroxy groups. However, the mix does exotherm slightly which tends to advance the resin and could result in loss of bench life in bulk production mixers.

Example 2 1) Preparation of liquid resin 40 g of caustic soda flake was dissolved in 85 g of water and the solution cooled to 200 C.

94 g of phenol was charged to a flask and the caustic soda solution added slowly.

The solution produced was cooled to 200C and 69.2 g of 52% formalin added slowly over 1 hour maintaining cooling to keep the temperature down to 4045 C. After a further 2 hours at 40-450C the free formaldehyde fell to 0.2%. The resin was vacuum distilled under full vacuum until the temperature rose to 650C under full vacuum (28 mmHg). 25 g of ethylene glycol were added and vacuum distillation continued until the temperature rose to 800 C, whereupon 10 g of ethylene glycol were added and vacuum distillation continued until the temperature rose to 800 C. 20 g of ethylene glycol and 35 g methanol were then added and the resin, which had a viscosity of 10.4 poises at 250C, was cooled to 250C.

The product, which was opaque in appearance and tended to be thixotropic, had the following characteristics: Approximate water content 3.2% Mo ratio NaOH:Phenol 1:1 P:F 1:1.2 2) Bonding of calcined dolomite with liquid phenolic resin A similar procedure was adopted as for Example 1 except that 8% of the above-described liquid resin was used. No wetting agent was required in this case. The mix showed no temperature rise and remained in a formable state for at least 3 days at room temperature. Again the specimens exhibited strengths in excess of 20 KN after curing for 1 hour at 1600 C.

Example 3 1) Preparation of a higher phenol-formaldehyde molar ratio liquid resole resin The procedure of Example 2 was foliowed except that the molar ratio of phenol to formaldehyde' used was 1:2.5 by increasing the amount of 52% formalin used to 144.2 g. The product had the following characteristics: Approximate water content 2.0% Molar ratio of NaOH to phenol 1:1 Phenol:formaldehyde 1:2.5 2) Bonding of calcined dolomite with powdered resin 750 g of coarse ground Doloma and 250 g of fine ground Doloma were mixed with 6% of powdered resin as prepared in step 1) above and 3% of a powdered novolak resin made by condensing phenol and formaldehyde in a molar ratio of 1:0.8 in the presence of oxalic acid, as catalyst, and then dehydrated by heating up to 1 500 C. This novolak did not contain caustic soda or hexamine. The mix remained formable for at least one day at room temperature. Specimens compressed at 1 tonne/in2 and cured for 1 hour at 1 600C again showed strengths in excess of 20 KN and no signs of surface cracking.

Claims (23)

Claims

1. A composition comprising a major proportion of calcined dolomite and, as binder therefore, a phenol-formaldehyde resin having a phenol:formaldehyde molar ratio in the range of from 1:0.7 to 1 :4.0, an alkali metal hydroxide:phenol molar ratio in the range of from 0.2:1 to 1.5:1 and a water content of less than 20% by weight.

2. A composition according to claim 1 , wherein the phenol-formaldehyde resin has a water content not greater than 5.0% by weight.

3. A composition according to either claim 1 or claim 2, wherein the molar ratio of the phenol to formaldehyde in the resin is in the range of from 1:1 to 1 :2.5.

4. A composition according to any one of claims 1 to 3, wherein the molar ratio of alkali metal hydroxide to the phenol in the resin is in the range of from 0.75:1 to 1:1.

5. A composition according to any one of claims 1 to 4, wherein the phenol component of the resin is phenol.

6. A composition according to any one of claims 1 to 5, wherein the alkali metal hydroxide is sodium hydroxide.

7. A composition according to any one of claims 1 to 6, which additionally comprises a nonaqueous liquid.

8. A composition according to claim 7, wherein the non-aqueous liquid is selected from ethylene glycol, diethylene glycol and mixtures thereof.

9. A composition according to either claim 7 or claim 8, wherein the non-aqueous liquid contains a minor amount of a volatile liquid.

10. A composition according to claim 9, wherein the volatile liquid is selected from methanol, ethanol and mixtures thereof.

11. A process for the production of refractory articles which process comprises mixing calcined dolomite with, as binder therefor, a phenol-formaldehyde resin having a phenol:formaldehyde molar ratio in the range of from 1:0.7 to 1 :4.0, an alkali metal hydroxide:phenol molar ratio in the range of from 0.2:1 to 1.5:1 and a water content of less than 20% by weight, forming the mixture and curing the formed mixture at elevated temperature.

12. A process according to claim 11, wherein the phenol-formaldehyde resin has a water content not greater than 5.0% by weight.

1 3. A process according to either claim 11 or claim 12, wherein the molar ratio of the phenol to formaldehyde in the resin is in the range of from 1:1 to 1 :2.5.

14. A process according to any one of claims 11 to 13, wherein the molar ratio of alkali metal hydroxide to the phenol in the resin is in the range of from 0.75 to 1:1.

15. A process according to any one of claims 11 to 14, wherein the phenol component of the resin is phenol.

1 6. A process according to any one of claims 11 to 1 5, wherein the alkali metal hydroxide is sodium hydroxide.

17. A process according to any one of claims 11 to 16, wherein the calcined dolomite and the binder are mixed with a non-aqueous liquid.

1 8. Process according to claim 17, wherein the non-aqueous liquid is selected from ethylene glycol, diethylene glycol and mixtures thereof.

1 9. A process according to either claim 1 7 or claim 18, wherein the non-aqueous liquid contains a minor amount of a volatile liquid.

20. A process according to claim 19, wherein the volatile liquid is selected from methanol, ethanol and mixtures thereof.

21. A process according to any one of claims 11 to 20, wherein the mixture, after being formed to the desired shape, is cured by heating at a temperature of from 100 to 3500C for from 15 minutes to 48 hours.

22. A process for producing a refractory article substantially as hereinbefore described in the Examples.

23. Refractory articles made by the process according to any one of claims 11 to 22.