WO1996026159A1

WO1996026159A1 - Treatment of zircon

Info

Publication number: WO1996026159A1
Application number: PCT/US1996/002462
Authority: WO
Inventors: Jacob Adriaan De Beer
Original assignee: Peltek, Inc.
Priority date: 1995-02-21
Filing date: 1996-02-21
Publication date: 1996-08-29
Also published as: AU5172796A

Abstract

A process for treating zircon to produce dissociated zircon comprises providing a high temperature plasma zone by means of a plasma flame generated by at least one non-transfer-arc plasma gun, and feeding particulate zircon, ZrSiO4, into the plasma zone. The zircon is allowed to free-fall through the plasma zone, thereby being converted at least partially to plasma dissociated zircon. The plasma dissociated zircon is quenched in a quench zone below the plasma zone.

Description

TREATMENT OF ZIRCON

THIS INVENTION relates to the treatment of zircon. It relates in particular to a process for treating zircon to produce dissociated zircon, to a reactor for carrying out the process, to dissociated zircon thus produced, and to the use of dissociated zircon.

According to a first aspect of the invention, there is provided a process for treating zircon to produce dissociated zircon, which process comprises providing a high temperature plasma zone by means of a plasma flame generated by at least one non-transfer-arc plasma gun; feeding particulate zircon, ZrSi0₄, into the plasma zone; allowing the zircon to free-fall through the plasma zone, thereby being converted at least partially to plasma dissociated zircon; and quenching the plasma dissociated zircon in a quench zone below the plasma zone.

The feeding of the zircon into the plasma zone may comprise introducing the zircon into a feed zone above the plasma zone, and allowing the zircon to pass from the feed zone into the plasma zone. Instead, or additionally, the feeding of the zircon into the plasma zone may comprise allowing the zircon to pass through the plasma gun into the plasma zone and/or allowing the zircon to pass through at least one conduit located adjacent the plasma gun, into the plasma zone.

The plasma zone may be generated by a plurality of non-transfer-arc plasma guns. In particular, three such guns, arranged in star fashion with operative ends thereof being downwardly inwardly directed, may be provided.

In the plasma zone, the zircon is subjected to high temperatures, preferably above 1800°C, and is dissociated at least partially or substantially into the plasma dissociated zircon, hereinafter also referred to as 'PDZ', according to the following reaction: ZrSi0₄ ^•*■» Zrθ₂-Siθ₂

The above reaction is reversible and, to ensure good yields of PDZ, the heated material should be quenched rapidly to below at least 500°C, to minimize the reverse reaction.

The required residence time of the zircon in the plasma zone is a function of the temperature in the zone and/or the particle size of the zircon. However, at said temperature of at least 1800°C, a residence time of 5 milliseconds or more, will suffice. During the thermal processing of the zircon in the plasma zone or flame, the mineral melts and dissociates into its constituent Zr0₂ and Si0₂ components. Upon quenching, the higher melting Zr0₂ crystallizes as an agglomeration of submicron crystallites, with subsequently solidifying amorphous Si0₂ cementing the

Zr0₂ crystallites together. There is substantially no size reduction, with each plasma dissociated zircon particle having approximately the same particle size as the original zircon feed material. It is, however, important to maintain the correct feed rate and temperature regime inside the plasma zone to achieve dissociation, but at the same time to prevent fusion of the individual dissociated particles together.

The quenching of the PDZ may be effected by means of air or another suitable quenching gas or liquid, which may be at room temperature.

It has been found that the PDZ, consisting of submicron Zr0₂ crystallites, cemented together with the amorphous glassy Si0₂, is more readily comminuted to smaller particle sizes than is undissociated zircon itself. Accordingly, the process may include rendering the resultant PDZ into smaller particulate form, eg even down to sizes as small as submicron sizes, if required. This may be effected by means of milling. The process may then also include leaching contaminants, such as Ti, Fe, Ca and Al, from the milled PDZ.

According to a second aspect of the invention, there is provided a reactor for treating zircon to produce dissociated zircon, which reactor comprises reaction chamber defining means defining a reaction chamber; - A - at least one non-transfer arc plasma gun protruding into a plasma zone within the reaction chamber; feed means for feeding zircon into the plasma zone such that the zircon can free fall through the plasma zone; and quenching agent feed means for feeding a quenching agent into a quench zone below the plasma zone.

The feed means may comprise a zircon feed conduit terminating in a feed zone above the plasma zone. Instead, or additionally, the feed means may comprise a zircon feed conduit or passageway through the plasma gun and/or at least one zircon feed conduit adjacent the plasma gun and adapted to discharge zircon in the plasma zone.

A plurality, eg three, of the plasma guns, arranged in star fashion with operative ends thereof being downwardly inwardly directed, as hereinbefore described, may be provided. The reaction chamber defining means may comprise a lined reactor shell.

Opacifiers used in glazes or enamels (hereinafter referred to simply as 'glazes' for brevity, with derived words such as 'glazing' having a corresponding meaning) are sufficiently low in light transparency effectively to hide from view the underlying substrate or ceramic body to which such a glaze is applied. Such glazes are usually white, but their colour can vary if desired. The more opaque the glaze, the greater the hiding power per unit of glaze thickness. Opacity in glazes is the result of reflection and refraction of light by phases and particles suspended in a clear glaze matrix. To be opaque, the glaze layer must contain materials in a fine state of subdivision and that have refractive indices different from that of the glaze matrix. Thus, an opacifier must have a sufficient degree of difference in refractive index as compared to the transparent matrix, and the size of the dispersed opacifier particles should be sufficiently small since, generally, the smaller the particle, the more effective is its opacifying ability, up to certain limits. Furthermore, the material must be resistant to high firing temperatures which are characteristic of the ceramic industry, and must be relatively cheap.

Zircon possesses a favourable refractive index of 2,05, is sufficiently resistant to high temperatures, and is used as an opacifier in the ceramic industry. However, mined zircon has a relatively large particle size, and thus requires milling in order to impart satisfactory opacifying properties thereto. Thus, opacifier grade zircon is milled down from an as-mined size of approximately lOOμ to 200μm (microns) , to a more favourable size of approximately lμm to 5μm. However, due to its inherent hardness, zircon is notoriously difficult to mill, so that long milling times, are normally required. As a result, milling costs are inevitably high. Furthermore, there exists appreciable potential for contamination of the milled zircon with milling media during prolonged milling.

In addition, particle sizes of lμm to 5μm are not optimum with respect to opacifying efficiency. Studies have shown that maximum opacity is achieved with particle sizes of about 0,25μm. Furthermore, according to Rayleigh's law, the scattering coefficient for extremely small particles varies with the fourth power of the particle diameter. Thus it is highly beneficial in ceramic glazes to have particles less than lμm in diameter. However, with zircon, the high milling costs associated with producing such small particles have precluded this from industrial application.

It is therefore an aim of the invention to provide a zircon-based or zircon-derived opacifying agent or opacifier whereby these drawbacks are at least reduced and which complies with the requirements hereinbefore set out.

Thus, according to a third aspect of the invention, there is provided a ceramic glaze opacifier, which comprises plasma dissociated zircon in comminuted particulate form.

This aspect of the invention is based thereon that PDZ can be comminuted, eg milled, down to smaller, even submicron sizes, much more readily than can be undissociated zircon itself. Submicron PDZ therefore constitutes an affordable high quality opacifying agent or opacifier.

More particularly, the inherent submicron nature (approximately 0,25μm) of the Zrθ₂ crystallite content of PDZ is used to obtain a high quality opacifying capability or property. Thus, equivalent or even better opacifying properties may be obtained by utilizing PDZ of much larger particle size than would be the case with undissociated zircon itself. The only limitation would be to mill the PDZ down to suitable sizes for efficient fluxing and good distribution/coverage. Thus, the PDZ may be milled down to particle sizes with a d^, value of less than 40μm, preferably a ά_κ value of less than 10μm. Most preferred, however, would be to mill the PDZ down to particle sizes with a dr_ø of less then lμm. During the glazing process the submicron Zr0₂ constituent of the PDZ particles will react with the silica of the glaze to form submicron zircon particles in the glaze, thereby imparting high quality opacifying properties thereto. By 'd^' is meant that 90% of the particles are smaller than the value indicated.

The PDZ may be that obtained from the process as hereinbefore described, or the reactor as hereinbefore described.

According to a fourth aspect of the invention, there is provided a method of producing a glazed ceramic artefact, which method comprises applying a glazing layer containing plasma dissociated zircon in comminuted particulate form, to a ceramic or enamel substrate.

The glazing layer may comprise a matrix or slip with which is admixed the PDZ particles of a suitable size, as hereinbefore described.

According to a fifth aspect of the invention, there is provided a glazed ceramic artefact, which comprises a ceramic substrate and a glazing layer on the substrate, the glazing layer comprising an admixture of a matrix and PDZ in comminuted particulate form. The invention will now be described in more detail with reference to the accompanying diagrammatic drawings and the non-limiting examples set out hereunder.

In the drawings FIGURE 1 shows, schematically, a side view of a reactor according to the invention; and

FIGURE 2 shows, schematically, a plan view of the reactor of Figure 1.

In the drawings, reference numeral 10 generally indicates a reactor for producing dissociated zircon.

The reactor 10 comprises a cylindrical upright reactor shell 12. A zircon inlet conduit 14 leads axially into the upper end of the shell 12, while a PDZ withdrawal conduit 16 leads from the lower end thereof. Three non-transfer-arc plasma guns 18 protrude into the shell 12 near its upper end. The guns 18 are spaced circumferentially apart, and are inclined downwardly inwardly. A quench air conduit 20 leads into the shell 12 below the guns 18. As previously indicated, the quench medium may be a liquid instead of air.

In use, particulate zircon, ZrSi0₄, is fed into the top of the shell 12 through the conduit 14. The zircon thus enters a feed zone 22 located above the plasma guns 18. The zircon free-falls from the feed zone 22 through a plasma zone 24 containing plasma flames generated by the guns and in which a temperature, under plasma conditions, of at least 1800°C is attained and maintained. The zircon is converted to PDZ as hereinbefore described, and is rapidly quenched by contact with air entering, at room temperature, along the conduit 20. It is quenched rapidly to a temperature of 500°C or less. The cooled particulate PDZ is withdrawn through the conduit 16. This stream can be subjected to gas-solid separation, filtration, etc in a stage 28, to recover the PDZ.

Instead, or additionally, the zircon can be fed into the plasma zone 24 along a passageway or conduit (not shown) extending through one or more of the plasma guns 18, and/or along a plurality of conduits (not shown) located alongside at least one of the burners and adapted to discharge zircon passing therethrough into the plasma zone 24.

The PDZ comprises submicron Zr0₂ crystallites, cemented together with amorphous glassy Si0₂. It can readily be milled down to desired particle sizes, by means of wet milling in a stage 30. Thereafter, contaminants such as Ti, Fe, Ca and Al can substantially be removed by leaching in a stage 32 so that there can be withdrawn, along a flow line 34, opacifier grade zircon which is substantially free of, in particular, Ti and Fe.

EXAMPLE 1

PDZ which was obtained by plasma dissociating zircon to a degree of 80%, was subsequently milled to a d^ of 40μm. This material, as well as a commercial zircon opacifier having particle sizes with a d₅₀ of 3,4μm, was used in comparative opacifier tests. To the trained eye it was clear that equivalent opacifying efficiency was obtained with both materials.

EXAMPLE 2

The above experiment was repeated with PDZ (90% dissociation and milled down to a d^ of 40μm) and two commercial zircon opacifiers, the one having particle sizes with a d_so of l,5μm and the other having particle sizes with a d₅₀ of 3,4μm. To the trained eye it was clear that better opacifying efficiency was obtained with the PDZ opacifier in comparison to the opacifier with the d_J0 of 3,4μm. In fact, the opacifying efficiency of the PDZ approached that obtained with the much finer commercial zircon opacifier having the particle sizes with the d₅₀ of l,5μm.

From these examples, it is thus evident that the opacifying power of PDZ increases with the degree of plasma dissociation.

EXAMPLE 3

In comparative milling tests between zircon and 75% dissociated PDZ and with both materials starting at 150μm, the following advantages were demonstrated for PDZ.

Milled down to a d^ of 75μm, the specific energy consumption was 15,4kWh/ton of zircon, and 5,9k h/ton for the PDZ. Therefore a decrease in specific energy consumption of 2,6x ('times') was realised. - Milled down to a d^ of 25μm, a decrease in specific energy consumption of 2,lx was obtained. - Milled down to a d^ of 4,7μm, a decrease in specific energy consumption of l,5x was obtained. The above results were obtained under non-optimal conditions, due to the presence of 25% harder-to-mill residual zircon. In the case of >90% dissociation, more marked ease of millability or comminution should be obtained.

Claims

C A I M S

1. A process for treating zircon to produce dissociated zircon, which process comprises providing a high temperature plasma zone by means of a plasma flame generated by at least one non-transfer-arc plasma gun; feeding particulate zircon, ZrSi0₄, into the plasma zone; allowing the zircon to free-fall through the plasma zone, thereby being converted at least partially to plasma dissociated zircon; and quenching the plasma dissociated zircon in a quench zone below the plasma zone.

2. A process according to Claim 1, wherein the feeding of the zircon into the plasma zone comprises introducing the zircon into a feed zone above the plasma zone, and allowing the zircon to pass from the feed zone into the plasma zone.

3. A process according to Claim 1, wherein the feeding of the zircon into the plasma zone comprises allowing the zircon to pass through the plasma gun into the plasma zone.

. A process according to Claim 1, wherein the feeding of the zircon into the plasma zone comprises allowing the zircon to pass through at least one conduit located adjacent the plasma gun, into the plasma zone.

5. A process according to Claim 1, wherein the plasma zone is generated by three non-transfer-arc plasma guns arranged in star fashion with operative ends thereof being downwardly inwardly directed.

6. A process according to Claim 1, wherein the quenching of the plasma dissociated zircon is effected rapidly to a temperature below 500°C.

7. A process according to Claim 1, wherein the quenching of the plasma dissociated zircon is effected by means of air which is at room temperature.

8. A process according to Claim 1, which includes comminuting the resultant plasma dissociated zircon into smaller particulate form, and leaching contaminants from the milled plasma dissociated zircon.

9. A reactor for treating zircon to produce dissociated zircon, which reactor comprises reaction chamber defining means defining a reaction chamber; at least one non-transfer arc plasma gun protruding into a plasma zone within the reaction chamber; feed means for feeding zircon into the plasma zone such that the zircon can free fall through the plasma zone; and quenching agent feed means for feeding a quenching agent into a quench zone below the plasma zone.

10. A reactor according to Claim 9, wherein the feed means comprises a zircon feed conduit terminating in a feed zone above the plasma zone.

11. A reactor according to Claim 9, wherein the feed means comprises a zircon feed conduit or passageway through the plasma gun.

12. A reactor according to Claim 9, wherein the feed means comprises at least one zircon feed conduit adjacent the plasma gun and adapted to discharge zircon in the plasma zone.

13. A reactor according to Claim 9, wherein three of the plasma guns, arranged in star fashion with operative ends thereof being downwardly inwardly directed, are provided. „_m

WO 96/26159

- 14 -

14. A ceramic glaze opacifier, which comprises plasma dissociated zircon in comminuted particulate form.

15. An opacifier according to Claim 14, wherein the plasma dissociated zircon is that obtained from the process as claimed

5 in Claim 1.

16. A method of producing a glazed ceramic artefact, which method comprises applying a glazing layer containing plasma dissociated zircon in comminuted particulate form, to a ceramic or enamel substrate.

10 17. A method according to Claim 16, wherein the glazing layer comprises a matrix or slip with which is admixed the PDZ particles.

18. A glazed ceramic artefact, which comprises a ceramic substrate and a glazing layer on the substrate, the glazing layer " comprising an admixture of a matrix and plasma dissociated zircon in comminuted particulate form.