Siliceous composition and method for obtaining same

Abstract

A powdery composition based on silica for ceramic welding, in particular by projection, comprising from 10 to 90% of a phase of siliceous particles comprise at least 80% by weight of cristobalite and at most 15% by weight of tridymite, based on the total weight of the composition, from 90 to 10% by weight of conventional additives forming a binding phase, based on the total weight of the composition, said siliceous particles having a d.sub.50 comprised between 350 and 800 μm, preferably between 400 and 500 μm.

Claims

1. A method for preparing a powdery composition based on silica for ceramic welding, comprising the steps: (a) preparing a phase of siliceous particles comprising at least 80% by weight of cristobalite and at most 20% by weight of tridymite, based on the total weight of the phase of siliceous particles; and (b) mixing said phase of siliceous particles with one or more additives forming a binding phase in ceramic welding, said step for preparing said phase of siliceous particles comprising a supply of quarry quartz sand particles having a d.sub.50 comprising between 350 and 800 μm in a rotary furnace attaining 1,400° C. to 1,500° C. in an area called the hottest area and for baking for a predetermined period of time with said phase of siliceous particles being obtained.

2. The preparation method according to claim 1, wherein said quarry quartz sand particles have a d.sub.5 max of 1,100 μm.

3. The preparation method according to claim 1, wherein said quarry quartz sand particles have a d.sub.5 min of 150 μm.

4. The preparation method according to claim 1, wherein said quarry quartz sand particles have before baking an SiO.sub.2 content greater than or equal to 97 by weight, based on the total weight of quarry quartz sand particles.

5. The preparation method according to claim 1, wherein the area called the hottest area has a temperature greater than or equal to 1,430° C.

6. The preparation method according to claim 1, wherein said siliceous particles have a d.sub.3 max of 1,100 μm.

7. The preparation method according to claim 1, wherein said siliceous particles have a d.sub.3 min of 150 μm.

8. The preparation method according to claim 1, wherein said siliceous particles have an SiO.sub.2 content greater than or equal to 97% by weight, based on the total weight of siliceous particles.

9. The preparation method according to claim 1, wherein said phase of siliceous particles comprises at least 80% by weight of cristobalite and at most 15% by weight of tridymite, based on the total weight of siliceous particles.

10. The preparation method according claim 1, wherein, during said mixing step, 20 to 85% by weight of phase of siliceous particles and 80 to 15% by weight of binding phase, based on the total weight of the composition, are brought into a mixing tank.

11. The preparation method according to claim 1, wherein, during said mixing step, 50 to 85% by weight of phase of siliceous particles and 50 to 15% by weight of binding phase, based on the total weight of the composition, are brought into a mixing tank.

12. The preparation method according to claim 1, wherein said binding phase comprises at least one of the following elements or compounds: Al, Si, Mg, Ca, Fe, Cr, Zr, Al.sub.2O.sub.3, SiO.sub.2, MgO, CaO, Fe.sub.2O.sub.3, Cr.sub.2O.sub.3, ZrO.sub.2, CaO.sub.2, MgO.sub.2, BaO.sub.2, and SrO.sub.2.

13. The preparation method according to claim 12, wherein said binding phase comprises at least one of the elements or compounds selected from the group consisting of CaO, MgO, and xCaO.yMgO, wherein the x and y represent mass fractions for which x+y<100.

14. The preparation method according to claim 12, wherein said binding phase comprises at least MgO.

Description

EXAMPLE 1

(1) A quarry quartz sand is selected for its chemical purity (SiO.sub.2>99%) and its grain size suitable for its application in a mixture for ceramic welding (>1,000 μm <5%: d.sub.50 between 400 and 600 μm and <200 μm <5%).

(2) It is introduced as a continuous supply into an industrial rotary furnace reaching 1,500° C. in its hottest area, where it dwells for 2 to 3 hours. After cooling, it is analyzed in terms of mineralogy (XR diffraction) and of grain size (by sifting).

(3) Results (on an Average Sample Resulting from Several Sample Takings)

(4) Mineralogy: cristobalite=89%; tridymite=10%; residual quartz=1%; Grain size: >1,000 μm=1-2%; d.sub.50=400-500 μm; <200 μm=1-3%

(5) The transformation of the quartz is therefore actually complete (residual Q=1%); it leads to a product consisting in a large majority of cristobalite with a minor content of tridymite (=10%); its grain size after transformation is practically similar to that before heat treatment, without significant generation of fines to be discarded.

(6) After addition to this silica transformed by the method, of other ingredients which complete the mixture for ceramic welding (15% of silicon metal as a powder+3% of quicklime), a ceramic welding test in a pilot furnace is conducted; the obtained ceramic mass is characterized: apparent density=2.2 g/cm.sup.3; open porosity=6 vol %; mechanical compressive strength (crushing of a cylinder)=80 MPa. These characteristics are much greater than those of a conventional silica brick (porosity=18-22 vol %; compressive strength=30-40 MPa).

(7) Further, unexpectedly, the thereby obtained mass by ceramic welding has a clearly lower thermal expansion (ΔL/L=linear 0.6% at 1,000° C.) relatively to that of a conventional silica brick (ΔL/L=linear 1.2% at 1,000° C.). Indeed, the mineralogical analysis revealed a strong proportion of glassy silica (amorphous fraction=60%) and a reduced cristobalite content (cristobalite=15%).

(8) This lowering of the thermal expansion coefficient gives this ceramic mass better resistance to thermal shocks, for example useful in an area close to the door of a coke furnace chamber.

(9) A re-baking step at a high temperature (5 days at 1,200° C.) allowed recrystallization of this amorphous cristobalite fraction, which reinforces the strength at a high temperature and is favorable for a long operating lifetime. Indeed, under the effect of this re-baking, the measured values by collapse under load (refractoriness under load) pass from T0.5=1,490 to T0.5=1,530° C.

EXAMPLE 2

(10) The effect of a reduction in the cristobalite content from the method is illustrated by producing the following mixture: ¾ of silica transformed into cristobalite from the method and ¼ of transformed silica from re-milled bricks, the other ingredients (Si+CaO) being such as in Example 1.

(11) The ceramic mass obtained by ceramic welding has a higher thermal expansion (ΔL/L=linear 1.0% at 1,000° C.), close to that of a conventional silica brick (ΔL/L=linear 1.2% at 1,000° C.). This is due to the amorphous fraction content which only attains 30% instead of 60% (Example 1), the cristobalite content increasing again from 15 to 45%.

(12) Like in Example 1, the re-baking step (5 days at 1,200° C.) allowed complete recrystallization (zero amorphous fraction and cristobalite content increased again to 65%), which reinforces the high temperature strength.

EXAMPLE 3

(13) From the same preparation of thermally transformed silica into cristobalite, a mixture was made for ceramic welding, in which quicklime CaO was replaced with magnesia MgO. The thereby obtained ceramic mass by ceramic welding was characterized: its thermal expansion became extremely low (ΔL/L=linear 0.05% at 1,000° C.) by an increase in its amorphous fraction content (70%), as revealed by mineralogical analysis.

(14) This quasi-zero thermal expansion gives the thereby obtained ceramic mass excellent resistance to thermal shocks.

(15) Like in Examples 1 and 2, a re-baking step for 5 days at 1,200° C. confirmed that this amorphous fraction is able to recrystallize into cristobalite, if the operating temperature exceeds 1,100° C., which is the case in the center of a coke furnace chamber.

EXAMPLE 4

(16) As in Example 2, the silica thermally transformed by the method may also be used as a mixture with silica from a conventional milling-sifting operation on refractory silica bricks (cristobalite+tridymite).

(17) For this purpose, the mixture for ceramic welding this time consists of ¾ of transformed silica from the method and of ¼ of re-milled silica, the other ingredients (Si and MgO) being as in Example 3.

(18) The ceramic mass obtained by ceramic welding has the characteristics mentioned in the table.

(19) TABLE-US-00001 TABLE After After re-baking Characteristics projection (1200° C./5 d) Apparent density (g/cm.sup.3) (measured 2.17 2.22 according to the EN993-1 standard) Open porosity (vol %) (measured 5 6 according to the EN993-1 standard) Cold crushing strength (MPa) (measured 100 150 according to the EN993-5 standard) Abrasion resistance (projection of SiC −11 −9 according to the ASTM C704 standard) - volume loss (cm.sup.3) Expansion at 1,000° C. ΔL/L (lin %) +0.2 +1.2 Collapse under load T0.5 (° C.) (measured 1450 1610 according to the ISO1893 standard) Mineralogy (main phases) Cristobalite 15 55 Tridymite 5 15 Amorphous phase 60 0

(20) Relatively to Example 2 (passing from CaO to MgO), the lowering of the thermal expansion (from linear 1.0 to 0.2% at 1,000° C.), favorable to resistance to thermal shocks, should be emphasized. This is due to the high amorphous phase content (60%).

(21) By recrystallization by re-baking, reinforcement of the thermal and mechanical characteristics is observed, which should be expressed by an extension of the operating lifetime, for example in a coke furnace.

(22) It is quite understood that the present invention is by no means limited to the embodiments described above and that many modifications may be made thereto without departing from the scope of the appended claims.