NZ240674A - Manufacturing a refractory body from ironsand and a pressing agent. - Google Patents

Description

UM7+ i (Stfirir,. |j S, qi' To t— NOV 1992 Patents Form No. 5 Number 240674 PATENTS ACT 1953 Dated November 20, 1991 COMPLETE SPECIFICATION PROCESS FOR MANUFACTURING A REFRACTORY BODY We, HER MAJESTY THE QUEEN in right of New Zealand, having an address at DSIR Chemistry, Gracefield Road, Lower Hutt, New Zealand do hereby declare the invention for which we pray that a Patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement: 240674 The invention relates to a process for manufacturing refractory bodies, particularly heat storage bricks made from bonded ironsand or concentrates derived from ironsand.

Introduction Refractory or heat resistant bodies such as ceramic bricks are characterised by a high heat storage capacity and so are commonly used as a heat storage medium. The material from which they are made must have a high specific heat, and in order to achieve the maximum possible heat storage capacity within given dimensions, the bricks must also have the highest possible density. This is because the heat storage capacity per unit volume of a body is the product of its specific heat and its density.

Heat storage bricks are typically manufactured from iron oxides but at present all such bricks are imported into New Zealand, often in storage heaters. These imported bricks are available in two grades having densities of 2.5 and 3.8 g cm"3 respectively. They commonly contain mixtures of haematite and magnetite, that is Fe203 and Fe304 but are prone to cracking and edge-chipping.

Object of the Invention The object of the present invention is to provide a process for manufacturing a refractory body having a high heat 240674 storage capacity per unit volume which represents an improvement upon, or at least an alternative to, known processes.

Statement of the Invention The present invention comprises a process for i manufacturing a refractory body comprising blending ironsand material and a pressing aid, shaping the mixture, and firing the shaped mixture to form said refractory body.

Preferably the ironsand material is titanomagnetite ironsand or reduced primary concentrate as herein defined. New Zealand North Island titanomagnetite ironsand has been found to be a suitable base material for making refractory bricks with the process of the invention. The ironsand concentrate has a specific heat of greater than 0.20 cal.g_1.K_1. Both titanomagnetite ironsand and "reduced primary concentrate", that is iron-enriched material derived directly from titanomagnetite ironsand by reduction, are suitable.

When a single size fraction of ironsand material is used the refractory body will have a heat storage capacity per unit volume at least substantially equivalent to that of known refractory bodies such as heat storage bricks. Such a refractory body produced by the process of the invention with a single size fraction of from herein be referred to as a "basic brick". Preferably, the average grain size of the ironsand material is less than 140 microns, especially from substantially 110 microns 240674 to substantially 130 microns, and more especially substantially 120 microns.

Superior heat storage capacity per unit volume can be achieved, when two or more size fractions or ironsand material are used and from herein a refractory body made in that way will be referred to as an "optimised brick".

An optimised brick can be made using two size fractions of ironsand material. Preferably the first size fraction of the ironsand material has an average grain size of less than substantially 140 microns, especially from substantially 110 microns to substantially 130 microns, and more especially substantially 120 microns; and preferably the second size fraction of the ironsand material has an average grain size of less than substantially 60 microns, especially from substantially 20 microns to substantially 50 microns, and more especially substantially 34 microns.

Preferably the first size fraction of the ironsand material comprises unmilled magnetically separated titanomagnetite ironsand and the second size fraction comprises milled ironsand in a ratio of substantially 1:2 to substantially 2:1, especially substantially 2:3 to substantially 3:2, and more especially substantially 1:1 respectively. 240674 An optimised brick can be made using three size fractions of ironsand material also. Preferably the first size fraction of the ironsand material has an average grain size of less than substantially 140 microns, especially from substantially 110 microns to substantially 130 microns, and more especially substantially 120 microns; and preferably the second size fraction of the ironsand material has an average grain size of less than substantially 100 microns, especially from substantially 20 microns to substantially 60 microns, and more especially substantially 34 microns; and preferably the third size fraction of the ironsand material has an average grain size of less than substantially 50 microns, especially less than substantially 10 microns, and more especially less than substantially 1 micron.

Preferably the first size fraction of the ironsand material comprises unmilled magnetically separated titanomagnetite ironsand, the second size fraction comprises coarsely milled ironsand, and the third size fraction comprises finely milled ironsand in a ratio of 15 to 25 : 15 to 25 : 1 respectively.

Preferably the pressing aid is a sodium silicate solution or a wet clay, such as a mixture of halloysite clay and water. 2.40674 The use of a sintering aid is not essential but it is preferred especially if optimised rather than basic bricks are to be produced.

Sodium silicate solution, or a mixture of halloysite clay and water are preferred sintering aids.

If the blend of ironsand material includes an alkaline metal silicate solution the "green" or unfired strength of the mixture may then be increased prior to firing by exposing the mixture to gaseous carbon dioxide which in turn maximises the strength of the final product. This is disclosed in our New Zealand patent specification 212330 which is incorporated herein by reference.

Preferably the blended fractions are shaped by being pressed into a mould at a pressure greater than substantially 50 MPa, especially greater than substantially 60 MPa, and more especially substantially 7 0 MPa, and fired at a temperature not exceeding substantially 1300°C.

Detailed Description The invention will now be particularly described with reference to the accompanying drawings: Fig. 1 illustrates the effect of temperature on the density of 25 mm diameter ceramic pellets, 240674 Fig. 2 illustrates the effect of rod-milled ironsand on the density of 25 mm diameter ceramic pellets fired at different temperatures, and Fig. 3 illustrates the effect of pressure on the density of 25 mm diameter ceramic pellets fired at different temperatures.

We have found ironsand material such as titanomagnetite ironsand and reduced primary concentrate to be suitable material from which to manufacture refractory bodies such as bricks for storage heaters. Such ironsand material will from herein be described as simply "ironsand".

While, a "basic" brick can be produced using a single size fraction of ironsand, the density and thus the heat storage capacity per unit volume of a refractory body can be maximised by manufacturing the body from material of a variety of particle sizes such that the spaces between large grains can be filled by smaller grains. We describe such an improved brick as an "optimised" brick.

We have found that a combination of a first size fraction of separated ironsand having an average grain size of less than substantially 140 microns, especially from substantially 110 to substantially 130 microns, and more especially substantially 120 microns, and a second size fraction of ironsand having an average grain size of less than substantially 60 microns, especially from substantially 20 microns to substantially 50 microns, and more especially substantially 34 microns, gives optimal densities after firing when the fractions are blended in a proportion of substantially 1:2 to substantially 2:1, especially substantially 2:3 to substantially 3:2, and more especially substantially 1:1 respectively. Preferably the first fraction is unmilled ironsand and the second fraction is milled ironsand.

We have also found that a combination of a first size fraction of magnetically separated ironsand having an average grain size of less than substantially 140 microns, especially from substantially 110 microns to substantially 130 microns, and more especially substantially 120 microns; a second size fraction of ironsand having an average grain size of less than substantially 100 microns, especially from substantially 20 microns to substantially 60 microns, and more especially substantially 34 microns; and a third size fraction of ironsand having an average grain size of less than substantially 50 microns, especially less than substantially 10 microns, and more especially less than substantially 1 micron, gives further optimised densities after firing when the fractions are blended in a proportion of 15 to 25 : 15 to 25 : 1 respectively. Preferably the first fraction is unmilled ironsand, the second fraction is coarsely milled ironsand, and the third fraction is finely milled ironsand. 240674 The third size fraction can be particularly useful to optimise the density when the average grain size is less than substantially 10 microns, and especially less than substantially 1 micron. In most instances small quantities, such as less than 10% by weight, will be sufficient. i The effectiveness of the second fraction is shown in Fig. 2.

Using two or three fractions not only provides better packing in the green state but the finely milled ironsand fragments sinter more rapidly at temperatures below 1300°C. Very high densities can be achieved by using large proportions of finely milled ironsand but those mixtures often have low green densities and shrink greatly during firing, resulting in warping and cracking. It is therefore important to choose an appropriate ironsand composition and balance the density against the degree of shrinkage.

The material can be milled by conventional processing such as ball milling or rod milling, wet or dry, but the exact ratio of unmilled: coarse milled: fine milled ironsand will depend on the specific particle size distributions achieved during milling which will depend on the facilities used. Optimal brick density can be achieved by carefully choosing the particle sizes and proportions.

The chemical compositions of the magnetically separated New Zealand North Island titanomagnetite ironsand and the reduced primary concentrate used for the purposes of the present invention are summarized in Table 1 below.

Table 1: Chemical composition of magnetically separated ironsand and reduced primary concentrate. Values represent the percentage of total weight.

Reduced Ironsand Primary Cone Fe (total) 58.0 71.6 Fe (metallic) 63.0 Ti02 8.0 9.9 A1203 4.0 .0 Si02 3.4 CM • MgO CO • o 3.6 CaO 0.8 1.1 MnO 0.6 0.7 v2o5 0.6 0.7 c 0.6 Other 0.1 0.1 The mineral composition of the reduced primary concentrate is summarised in Table 2 below. 240674 Table 2: Mineral composition of reduced primary-concentrate (NT Evans, Iron and Steel Society of the AIME, 37th Conf: Chicago (1978) 122). 63% iron metal (Fe) 12% ilmenite (FeTi03) 9% pyroxene (Ca(FeMg) Si206) 8% spinel ((FeMg)Al204) 6% magnetite-ulvospinel (Fe304-Fe2Ti04) 3% armalcolite ( (FeMg)Ti2Os) When an optimised brick is required the first target in processing is to achieve a high "green" or unfired density as this will assist in obtaining a high fired density. This is achieved by using an optimum amount of additional pressing aids (but no excess). Clay powders, for example wet halloysite clay, and alkali metal silicate solutions, for example sodium silicate solution, have been found to be good in this respect.

Optionally, if sodium silicate solution is used, the shaped green body can be exposed to carbon dioxide gas to increase the green strength. This is discussed in our aforementioned New Zealand patent specification No 212330 and may be advantageous if the unfired bodies need to be handled excessively. 240674 The term "alkali metal silicate" used herein includes silicates of sodium or potassium, particularly sodium, and chemical equivalents thereof. The alkali metal silicate is preferably added to the raw material mixture as a commercial aqueous solution which generally contains about 40-50% solid sodium silicate. However, if desired, the viscosity can be adjusted by adding a small amount (eg up to substantially 5%) of water to obtain the required consistency. Alternatively, but less preferably, the alkali metal silicate may be added in powdered form and water subsequently blended in.

Optionally as pressing agents oleic acid or engine oil may be used. Water can be used either alone or in association with clay or sodium silicate to improve the plasticity of the body and homogeneity of the distribution of the pressing aids.

A sintering aid can be used to assist the high temperature bonding process. It is not required to produce a basic brick but it is preferred in the production of optimised bricks. Suitable sintering aids include saturated aqueous sodium silicate solution, sodium silicate powder, ground waste glass, lead bisilicate, magnetite powder (Fe304), hematite powder (Fe203) , colemanite (Ca2B60u.5H20), borax (Na2B407) , boric oxide (B203), feldspar, nepheline syenite, halloysitic clay, bentonite clay, allophane-rich clay, kaolinitic ball clay, milled quartz sand, and amorphous silica. 240674 Both sodium silicate solution and clay/water have dual roles as sintering aids and pressing aids. Excellent results have been obtained using pyroxene materials such as diopside, which occurs naturally in ironsand. t The ironsand fractions, pressing aids and/or sintering aids are blended by art-known means to ensure homogeneity in their distribution, packed and then pressed into an appropriately shaped mould. The greater the pressure used, the higher the green and fired densities will be. This is shown in Fig 3. The use of higher pressures is also accompanied with less shrinkage and distortion on firing.

The shaped unfired bodies are heated by art-known methods to temperatures in the range of 1100°C to 1350°C (most preferably 1200-1300°C). Within limits the greater the pressure used, the higher the fired density will be. Excessive temperature causes the bodies to warp and can cause bloating depending on which pressing/sintering aids are chosen. Instead of using excessive temperature it is better to increase the firing time rather than further increase the temperature. The effect of temperature on density is illustrated in Fig. 1.

The invention will now be described with reference to particular examples but which are not intended to limit the scope of the invention. 240674 Example 1 Unmilled ironsand having an estimated average grain size of 120 microns and New Zealand China Clays halloysite clay were blended in the ratio of 96:4 by weight with 6% water added. The mixture was transferred to a die, spread uniformly and pressed to a pressure of at least 70 MPa. The shaped body was allowed to stand and dry to develop green strength. The body was then heated gradually by 5°C per minute to 1250°C, allowed to soak for 2 hours, and then cooled in a similarly controlled manner to avoid cracking due to thermal stresses.

Example 2 Unmilled ironsand having an estimated average grain size of 120 microns, partially milled ironsand having an estimated average grain size of 34 microns and New Zealand China Clays Halloysite clay were blended in the ratio of 48:48:4 by weight with 6% added water. The mixture was transferred to a die, spread uniformly and pressed to a pressure of at least 70 MPa. The shaped body was allowed to stand and dry to develop green strength. The body was then heated gradually by 5°C per minute to 1250°C, allowed to soak for 2 hours, and then cooled in a similarly controlled manner to avoid cracking due to thermal stresses.

Example 3 Unmilled ironsand having an estimated average grain size of 120 microns, partially milled ironsand having an 2.40674 estimated average grain size of 34 microns and New Zealand China Clays Halloysite clay were blended in the ratio of 48:48:4 by weight with 6% added water. The mixture was transferred to a die, spread uniformly and pressed to a pressure of at least 70 MPa. The shaped body was prefired at 100-120°C for about 1 hour, heated gradually by 5°C per minute to 1250°C, allowed to soak for 2 hours, and then cooled in a similarly controlled manner to avoid cracking due to thermal stresses.

Example 4 Unmilled ironsand having an estimated average grain size of 120 microns, milled reduced primary concentrate having an estimated grain size of 20 microns and sodium silicate solution were blended in the ratio 60:34:6 by weight transferred to a die, spread uniformly, and pressed to a pressure of at least 70 Mpa. The shaped body was then exposed to gaseous carbon dioxide for 2 minutes to harden the body, heated gradually by 5°C per minute to 1200*C allowed to soak for 2 hours and then cooled in a similarly controlled manner to avoid cracking due to thermal stresses.

The foregoing describes the invention and preferred forms thereof. Alterations and modifications as will be obvious to those skilled in the art are intended to be incorporated within the scope of the invention as defined in the following claims.

Claims (44)

WHAT WE CLAIM IS:

1. A process for manufacturing a refractory body of density greater than 3.0 g/cm3, comprising blending ironsand material and a pressing aid, shaping and pressing the mixture, and firing said mixture to form said refractory body.

2. A process according to claim 1, wherein the ironsand material is titanomagnetite ironsand or a reduced primary concentrate thereof as herein defined.

3. A process according to claim 1 or 2 wherein the average grain size of the ironsand-material is less than substantially 140 microns.

4. A process according to claim 3 wherein the average grain size of the ironsand material is from substantially 110 microns to substantially 130 microns.

5. A process according to claim 4 wherein the average grain size of the ironsand material is substantially 120 microns.

6. A process according to claim 1 or 2, wherein there are two size fractions of ironsand material. c 16 240674

7. A process according to claim 6, wherein the average grain size of the first fraction is less than substantially 140 microns.

8. A process according to claim 7, wherein the average grain J size of the first fraction is from substantially 110 microns to substantially 130 microns.

9. A process according to claim 8, wherein the average grain size of the first fraction is substantially 120 microns.

10. A process according to claim 6, wherein the average grain size of the second fraction is less than substantially 60 microns.

11. A process according to claim 10, wherein the average grain size of the second fraction is from substantially 20 microns to substantially 50 microns.

12. A process according to claim 11, wherein the average grain size of the second fraction is substantially 34 microns.

13. A process according to any one of claims 6 to 12, wherein the ratio of the first and second fractions is substantially 1:2 to substantially 2:1 respectively. - 17 - / 240674

14. A process according to claim 13, wherein the ratio of the first and second fractions is from substantially 2:3 to substantially 3:2 respectively.

15. A process according to claim 14, wherein the ratio of the first and second fractions is substantially 1:1.

16. A process according to claim 1 or 2, wherein there are at least three size fractions of ironsand material.

17. A process according to claim 16, wherein the average grain size of the first fraction is less than substantially 140 microns.

18. A process according to claim 17, wherein the average grain size of the first fraction is from substantially 110 microns to substantially 130 microns.

19. A process according to claim 18, wherein the average grain size of the first fraction is substantially 120 microns.

20. A process according to claim 16, wherein the average grain size of the second fraction is less than substantially 100 microns. - 18 - 240674

21. A process according to claim 20, wherein the average grain size of the second fraction is from substantially 2 0 microns to substantially 60 microns.

22. A process according to claim 21, wherein the average grain size of the second fraction is substantially 34 microns.

23. A process according to claim 16, wherein the average grain size of the third fraction is less than substantially 50 microns.

24. A process according to claim 23, wherein the average grain size of the third fraction is less than substantially 10 microns.

25. A process according to claim 24, wherein the average grain size of the third fraction is less than substantially 1 micron.

26. A process according to any one of claims 16 to 25, wherein the ratio of the first, second and third fractions is substantially 10 to 25:10 to 25:1 respectively.

27. A process according to any preceding claim, wherein the pressing aid is selected from one or more of clay, an alkali metal silicate solution, oleic acid, oil and water.

28. A process according to claim 27, wherein the clay is halloysite clay. - 19 - • 240674

29. A process according to claim 27, wherein the pressing aid is an alkali metal silicate solution and the mixture is exposed to gaseous carbon dioxide prior to firing to increase its green strength.

30. A process according to claim 27 or 29, wherein the alkali metal silicate solution is sodium silicate solution.

31. A process according to any preceding claim wherein the ironsand material and the pressing aid are blended with a sintering aid prior to the shaping and firing.

32. A process according to claim 31, wherein the sintering aid is selected from sodium silicate solution and a mixture of halloysite clay and water.

33. A process as claimed in any preceding claim, wherein the mixture is shaped by pressing the mixture into a mould.

34. A process according to claim 33, wherein the mixture is exposed to a pressure greater than substantially 50 MPa.

35. A process according to claim 33, wherein the mixture is exposed to a pressure greater than substantially 60 MPa.

36. A process according to claim 33, wherein the mixture is exposed to a pressure of substantially 70 MPa. - 20 -

37. A process according to any preceding claim, wherein the mixture is fired at a temperature from substantially 1100°C to substantially 1350°C.

38. A process according to claim 37, wherein the mixture is fired at a temperature from substantially 1200°C to substantially 1300°C.

39. A process for manufacturing a basic brick as herein defined substantially as herein described with reference to Example 1.

40. A basic brick as herein defined when prepared by the process of any one of claims 1 to 5 and 27 to 38 when dependent on any one of claims 1 to 5.

41. A basic brick as herein defined according to any one of claims 1 to 5 in the form of a brick suitable for a heat storage medium.

42. A process for manufacturing an optimised brick as herein defined substantially as herein described with reference to any one of Examples 2 to 4.

43. An optimised brick as herein defined when prepared by the process of any one of claims 1, 2, 6 to 26, and 27 to 38 when dependent on any claim other than any one of claims 3 to 5. - 21 - 240674

44. An optimised brick as herein defined according to any one of claims 1 and 2, and 6 to 26 in the form of a brick suitable for a heat storage medium. WEST-WALKER McCABE Per: ATTORNEYS FOR THE APPLICANT