Silico-aluminate containing aggregates for production of monolithic refractory compositions, their method of production and their use
The present invention relates to a porous aggregate, comprising Al 2 O 3 , SiO 2 and optionally Fe 2 O 3 , having a d 50 of equivalent pore diameter between 1 μm or more and 50 μm or below and a total porosity between 20% and 60%, for use in the formation of monolithic refractories. Also part of the invention is a method of formation for said aggregates, their use in the formation of monolithic refractories and monolithic refractories comprising such aggregates.
1. A porous aggregate comprising from 70 to 95 wt.-% Al 2 O 3 , from 3 to 10 wt.-% SiO 2 and optionally up to 5 wt.-% Fe 2 O 3 , having a d 50 of equivalent pore diameter between 1 μm or more and 50 μm or below, a total porosity between 20% and 60%, and an apparent density of 2.5 g·cm −3 or less.
2. A porous aggregate according to claim 1 having a d 10 equivalent pore diameter of 30 μm or below.
3. A porous aggregate according to claim 1 having an open porosity of 20% or more.
4. A porous aggregate according to claim 1 , which is additionally coated with a polymers and which has an open porosity of 20% or less.
5. A porous aggregate according to claim 4 , wherein said polymer is a petroleum by-product.
6. A porous aggregate according to claim 1 having corundum and mullite as primary phases.
7. A porous aggregate according to claim 1 , which maintains a porosity of 20% or above and/or a d 50 of equivalent pore diameter of 50 μm or below through subsequent firings at a temperature of 1300° C. or below.
8. A method of preparation of a porous aggregate, comprising the steps of
(a) providing a particulate mixture of
(i) a pulverised alumina containing base material;
(ii) a pore former; and
(iii) metal hydroxide;
(b) adding a binder to said particulate mixture in a pelletiser in order to obtain a pelletised mixture; and
(c) firing the pelletised mixture obtained at the end of step (b) in order to obtain a porous aggregate;
wherein the porous aggregate comprises from 70 to 95 wt.-% Al 2 O 3 , from 3 to 10 wt.-% SiO 2 and optionally up to 5 wt.-% Fe 2 O 3 , having a d 50 of equivalent pore diameter between 1 μm or more and 50 μm or below, a total porosity between 20% and 60%, and an apparent density of 2.5 g·cm −3 or less.
9. A method according to claim 8 , further comprising a step of (d) coating the fired pellets obtained at the end of step (c) with a hydrophobic polymer.
10. A method according to claim 9 , wherein said polymer is a polymer selected from the group of thermosetting binding agents, thermo-hardening binding agents and multi-component reacting binding agents.
11. A method according to claim 8 , wherein the pulverised alumina containing base material is selected from silico-aluminates, pure alumina, and mixtures thereof.
12. A method according to claim 8 , wherein the pulverised alumina containing base material comprises a calcined bauxite having a d 90 of 200 μm and a d 50 of 45 μm or lower, and/or a raw bauxite having a d 90 of 1000 μm and a d 50 of 500 μm or lower and/or wherein the pulverised bauxite comprises greater than 80 wt.-% Al 2 O 3 , less than 10 wt.-% SiO 2 and less than 5 wt.-% Fe 2 O 3 after calcination.
13. A method according to claim 8 , wherein the said pore former is selected from the group of carbohydrates, carbon black, polymer particles, cereal flour and mixtures thereof.
14. A method according to claim 8 , wherein the pore former is an ultrafine monosaccharide or polysaccharide having a d 90 of 150 μm or less and a d 50 of 45 μm or less.
15. A method according to claim 8 , wherein the metal hydroxide is selected from the group aluminium hydroxide, aluminium oxide hydroxide and hydrated alumina.
16. A method according to claim 8 , wherein the metal hydroxide has a d 90 of 60 μm and a loss on ignition at 1000° C. of 40% or less.
17. A method according to claim 8 , wherein the mixture of step (a) consists of from 65 to 90 wt.-% pulverised alumina containing base material, from 5 to 20 wt.-% pore former and from 5 to 20 wt.-% metal hydroxide, based on the total weight of the mixture.
18. A method according to claim 8 , wherein in step (b), the amount of binder added is no more than 15 wt.-%, on the basis of the total weight of the said particulate mixture, such as for example from 2 wt.-% to 10 wt.-%.
19. A method according to claim 8 , wherein in step (c), the firing is carried out at a temperature of 1200° C. or more, for a duration of 2 hours or more.
20. A method of using the porous aggregate according to claim 1 , in the preparation of a monolithic refractory composition.
21. A method according to claim 20 , wherein the said porous aggregate forms 60 wt.-% or less of the said monolithic refractory composition.
22. A method according to claim 20 , wherein the said monolithic refractory composition comprises 70 wt.-% or more Al 2 O 3 , 20 wt.-% or less SiO 2 and 5 wt.-% or less CaO.
23. A method according to claim 20 , wherein the said monolithic refractory composition has a density of less than 2.8 t·m −3 at 800° C.
24. A method according to claim 20 , wherein the said monolithic refractory composition has a thermal conductivity of 3 W·m −1 ·K −1 or less.
25. A monolithic refractory composition comprising the porous aggregate of claim 1 .
26. A porous aggregate according to claim 4 , wherein the polymer is selected from thermosetting binding agents, thermo-hardening binding agents, and multi-component reacting binding agents.
27. A porous aggregate according to claim 5 , wherein the polymer is a petroleum by-product polymer that is a hydrocarbon containing from 20 to 40 carbon atoms per molecule.