Production of calcined ceramic pigments

The process for producing zirconium based pigments (also known as stains) comprises the direct use of plasma disassociated zirconium sand. The sand is comminuted to a suitable particle size, mixed with a color determining agent such as ammonium metavanadate and a mineralizer such as sodium fluoride, after which the resulting mixture is calcined to bring about a reaction between the zirconium sand and the color determining agent to produce the required pigments. The resulting calcined pigment is then ground to a suitable particle size ready for application to ceramic tiles.

The use of such stains for ceramics is well established in industry. The 
first zircon stains were zirconium-vanadium blue pigments and examples of 
the preparation of this stain and several variants of it are described in 
the paper by F. T. Booth and G. N. Peel, Trans, Brit. Ceram. Soc. 61, 
359-400 (July 1962). The first yellow pigments included sodium molybdate 
and praseodymium oxide in a zirconia/silica mix, as described by E. Kato, 
Keram, Zeitschrift, 13, (1961) 128-130; but it is more common practice at 
this time for praseodymium oxide or oxalate only to be used, for example 
British Pat. Nos. 965,863 and 895,569. Examples of other zirconium stains 
having an iron colouring agent, usually either ferrous sulphate, hydrated 
ferric chloride, or iron oxide, which produce a coral-pink colour, are 
described in the specifications of British Pat. Nos. 986,751 and 996,033, 
U.S. Pat. No. 3,189,475 and in German Pat. Nos. 1,163,222 and 1,204,996. 
The factor common to all stains of the types indicated is that a mixture of 
zirconia and silica with the colour producing agent is converted by a 
calcination stage to a form of zirconium silicate which is coloured by 
virtue of the fact that the colour determining agent or a transient 
compound or ion derived therefrom is entrapped within, or in some cases 
around, the growing zircon lattice. 
The normally accepted method of producing such a zirconia/silica based 
stain is to calcine a mixture of zirconium oxide and silica in a weight 
ratio of about two to one, i.e. approximately equimolecular proportions, 
together with the colour determining agent, and a catalyst (the 
"mineraliser"). The usual colour determining agents are vanadium (as 
ammonium metavanadate) for the blue; praseodymium (as oxide, carbonate or 
oxalate) for the yellow; and iron (as oxide or sulphate) for the pink. 
Common mineralisers, whose function it is to reduce the temperature 
required for the reaction or to catalyse the reaction itself, include the 
alkali metal halides, especially fluorides. After calcination, the product 
is ground, washed free of soluble salts, dried and pulverised. 
An alternative method as described for instance in British Patent 
specification No. 1,177,676 is based upon the direct reaction or 
"cracking" of the zirconium-bearing ore, zircon sand (zirconium silicate), 
with sufficient alkali to convert the zirconium silicate to a 
non-refractory form (in which the zirconium constituent is in the form of 
sodium zirconate) by heating together at a temperature in excess of 
800.degree. C. The converted product is then mixed with water to decompose 
the sodium zirconate, and a colour determining agent, sulphuric acid, and 
a mineraliser are added. This mixture is dehydrated and the calcined, 
ground, washed, and dried in the usual way. A variation of this process 
using ammonium sulphate instead of sulphuric acid has been reported. 
Further descriptions of processes of these kinds are given in French Pat. 
No. 1,427,877, West German Pat. No. 1,242,500, and Belgian Pat. No. 
695,602. It is a disadvantage of methods such as these that the yields of 
calcined stains are low after extraction of by-products and that the 
colour makers' kiln capacity is unprofitably occupied. In addition, it has 
been found in practice that the stains obtained are highly variable in 
shade and strength. 
SUMMARY OF THE INVENTION 
According to the present invention we provide a process for the production 
of a zircon based stain by reacting a zirconia containing material with a 
colour determining agent, the improvement comprising reacting a zirconia 
containing material which is a comminuted plasma-dissociated zircon sand 
having a zirconia rich phase, a silica rich phase and not more than 30% 
unreacted zircon sand, with a colour determining agent, wherein said 
comminuted plasma dissociated zircon sand contains an amount of silica in 
its original chemical form (SiO.sub.2) which is at least 90% by weight of 
its silica content prior to comminution. 
Preferably the zircon sand is first comminuted and calcined together with 
the colour determining agent and a mineraliser. 
The plasma dissociated zircon for the purpose of the present invention 
means zircon that has been subjected in particulate form through a zone of 
heat sufficient to raise the temperature of the particles to form them 
into material having a zirconia-rich phase and a silica rich phase, and 
containing not more than 30% of undissociated zircon. 
The plasma dissociated zircon used in the present invention is produced by 
treatment of the zircon sand in a plasma generator which is a device for 
heating gases or solids with an electric arc. It has been observed by 
Charles et al, (Mining & Metall. Trans. 79C 54-59 1970) that zircon if so 
heated to a sufficiently high temperature dissociates into a zirconia rich 
phase and a silica rich phase. It is a characteristic of the equipment now 
being utilised (and described in British Patent specification No. 
1,248,595 Ionarc Smelters Limited and U.S. Pat. Nos. 3,749,763 and 
3,811,907) that the method produces a zirconia rich phase containing 
substantially less than 0.5% silica, and a silica rich phase 
correspondingly low in zirconia. This very efficient dissociation is 
thought to result from the combination of ultra-high temperature and rapid 
quench to which the zircon particles are subjected in the arc. A furnace 
of Ionarc design operates at 300-400 kW and has a throughput of 300-600 
lbs/hour. 
The processed product has a somewhat lower melting point and lower specific 
gravity (3.5 to 4.0) than normal zircon and consists essentially of 
intimate mixtures of zirconia in the form of radially orientated crystals 
(shown diagrammatically as A in FIG. 1) in a matrix of silica (shown as B 
in FIG. 1). Optical examination of the dissociated product shows that 
three main categories of material are present: Type I, the unreacted 
grains of zircon; Type II, the angular grains of partially dissociated 
material; Type III, fully fused, fully dissociated particles usually 
spherical in form.

The proportions of the particles of the Type I, II and III are dependent 
upon the rate of throughput of the zircon sand through the arc and upon 
the power applied. If the throughput is sufficiently slow, then the 
dissociated zircon will be predominantly of Type III. At relatively faster 
throughputs the particles of dissociated zircon will include more 
appreciable proportions of the Type I and II varieties. As a general rule, 
representatives of all three types will be found in any dissociated 
zircon. 
DESCRIPTION OF PREFERRED EMBODIMENTS 
Dissociated zircons of Type II and III are both suitable for the direct 
production of ceramic colours although the yield of the very high strength 
colours now possible will depend on there being a suitable low proportion, 
i.e. no more than 2% by weight of the whole, of Type I (the relatively 
unaffected zircon grains). Whilst the Type III dissociated zircon will 
give best results, the Type II material, characteristic of processing at a 
faster rate, can also be used in the present invention. It should be 
emphasized that Type II contains a proportion as a central kernel of 
undissociated zircon, dependent on the rate of throughput and the 
constitution f the starting zircon sand. However, the greater the 
proportion of fully dissociated material, the stonger the colours which 
can ultimately be produced from the comminuted dissociated zircon, 
hereinafter referred to as DZ. 
A mineraliser is added to the mixture of milled DZ and the colour 
determining agent to reduce the temperature of the calcination reaction in 
the normal way. The plasma dissociated zircon sand may be comminuted to a 
suitable size by dry milling. Preferably the plasma dissociated sand 
suitable for producing zircon based stains has a particle size of 150 to 
350 mesh (British Standard) for the vanadium blue, less than 350 mesh for 
the yellow. However, more advantageously a wet milling process is used in 
which the dissociated zircon may be wet milled in the presence of 
additives which attack the surface of the silica constituent in the 
dissociated zircon. Additives which may be used include: caustic soda, 
caustic potash, hydrofluoric acid, sodium, potassium or ammonium 
bifluorides, alkaline sodium silicates, alkali metal fluorides in the 
presence of hydrofluoric acid. Concentrations of additive of up to 10% 
effective agent relative to the DZ (dissociated zircon) may be used, but 
the optimum, in terms of the final colour which can be produced, is 
usually 1-3%. The amount of water which may be added is between 5 and 25% 
relative to the DZ but best results will normally be obtained between 15 
and 20%. The use of a fluoride as the additive has the added advantage 
that the fluoride also acts as a mineraliser, thus the products of the wet 
milling process may be "self fluxing" and there may be no necessity to add 
another mineraliser prior to calcining as in conventional methods. 
According to one embodiment of the present invention, when wet milling the 
colour determining agent is ammonium metavanadate, with a phosphorous 
containing compound included in the mineraliser composition. 
The phosphorous containing compound may be a phosphate such as sodium 
triphosphate, a monofluorophosphate such as sodium monofluorophosphate, a 
phosphorous ester such as trixylyl phosphate or a polyphosphate such as 
sodium tripolyphosphate. 
In all cases, the fully inorganic phosphates are the most effective, and 
features common to all the reactions are: 
i. a change in shade from the greenish blue of the basic formulations 
without phosphate to a more reddish tone at phosphate radical levels of 
about 0.1%, 
ii. a change in colour from blue to a blue/violet or grey/violet at 
phosphate radical levels of about 1% in the case of dry milled dissociated 
zircon but not wet milled, 
iii. extensive bleaching of the colour at phosphate radical levels about 
and in excess of 2% particularly in the case of the wet milled DZ. 
DESCRIPTION OF SPECIFIC EXAMPLES 
The wet milling process may be carried out as described in the following 
Examples 1 and 2: 
EXAMPLE 1 
900 Pounds of 11/2-2 inch high density alumina balls are charged to a 3 
foot 6 inch ball mill with 450 pounds of dissociated zircon, 9 pounds of 
caustic soda flake and 75 pounds of water. After milling for 20-30 hours 
or until 99% is less than 200 mesh, the charge is neutralised with 
hydrochloric acid (27 lbs) or sulphuric acid. The slurry is dumped from 
the mill, spray dried or dried by any other convenient means, and the 
particle size of the powder product determined (say 3-12 microns on the 
Fisher Sub-Sieve Sizer). 
EXAMPLE 2 
This alternative method would use with 450 pounds of DZ 1% of sodium 
fluoride (41/2 pounds) and 1/2% hydrofluoric acid (4 pounds of 60% HF 
acid) by weight relative to the DZ. After milling for 15-30 hours or until 
99% is less than 200 mesh, the charge is neutralised with caustic soda. 
The particles size will be 3-12 microns (Fisher). The slurry is then 
dumped and dried. Alternatively, the charge is neutralised after dumping 
but before drying. 
It is further feasible and occasionally advantageous to neutralise the 
charge at any time during the course of the milling process. 
In both Examples 1 and 2, the dried product contains all of the original 
silicon content and at least 90% of it is still in its original chemical 
condition (SiO.sub.2), only a very small amount being converted to sodium 
silicate or other compounds. Some or all of the silicate or other 
compounds could be removed if desired before drying. 
For wet milling the additive or total of the additives for attacking the 
surface of the silica may be 0.5 to 10% by weight of the zircon sand. 
Thus, only the surface of the silica is attacked and a maximum quantity of 
the silica converted to silicate or other compounds will be less than 10% 
by weight of the initial weight of the silica. By this means the zircon 
grains can then be comminuted. 
It is also possible to wash out all water soluble materials before drying 
-- but in the case of the "fluoride attack" (Example 2 above) some loss of 
zirconium values will result. In the case of the "caustic attack" (Example 
1 above) the strength of the colours which can be produced with the 
comminuted product will then be somewhat impaired. 
The following Examples 3 to 8 describe colour formulations which can be 
produced in this manner. 
In the Examples which follow, the milled product is referred to as DIZIRC 
which is a trade mark of Keeling & Walker Limited (No. 1,027,872). Unless 
otherwise stated all percentages and parts referred to herein are by 
weight. 
EXAMPLE 3 
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DIZIRC from 1 above at 7 microns (Fisher) 
90 parts 
silica (quartz) 10 parts 
ammonium metavanadate 
5-8 e.g. 6 parts 
sodium fluoride 3-7 e.g. 6 parts 
______________________________________ 
Calcine at 650.degree.-900.degree. C, grind as required, wash, dry, 
pulverise the resulting blue pigment. 
EXAMPLE 4 
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DIZIRC from 2 above at 7 microns (Fisher) 
90 parts 
silica (quartz) 10 parts 
ammonium metavanadate 
5 - 8 e.g. 6 parts 
sodium fluoride 3 - 7 e.g. 6 parts 
sodium chloride 2 - 7 e.g. 5 parts 
______________________________________ 
Calcine at 650.degree.-900.degree. C, grind as required, wash, dry, 
pulverise the resulting blue pigment. 
EXAMPLE 5 
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DIZIRC from 1 above at 7 microns (Fisher) 
90 parts 
silica (quartz) 10 parts 
praseodymium oxide 
2 - 5 e.g. 4 parts 
sodium fluoride 1 - 7 e.g. 6 parts 
______________________________________ 
Calcine at 800.degree.-1,000.degree. C, grind as required, wash, dry, 
pulverise the resulting yellow pigment. 
EXAMPLE 6 
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DIZIRC from 2 above at 7 microns (Fisher) 
90 parts 
silica (quartz) 10 parts 
praseodymium oxide 
2 - 5 e.g. 4 parts 
sodium fluoride 1 - 7 e.g. 5 parts 
sodium chloride 1 - 7 e.g. 5 parts 
______________________________________ 
Calcine at 800.degree.-1,000.degree. C, grind as required, wash, dry 
pulverise the resulting yellow pigment. 
EXAMPLE 7 
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DIZIRC 90 parts 
silica (quartz) 0-10 e.g. 5 parts 
iron sulphate 25-70 e.g. 40 parts 
sodium fluoride 10-20 e.g. 15 parts 
sodium chloride 5-15 e.g. 10 parts 
sodium potassium nitrate 
5-15 e.g. 10 parts 
______________________________________ 
Calcine at 850.degree.-1,000.degree. C, grind as required, wash, dry, 
pulverise the resulting peach pigment. 
EXAMPLE 8 
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DIZIRC 90 parts 
silica (quartz) 10 parts 
red iron oxide 5-23 e.g. 15 parts 
sodium fluoride 5-15 e.g. 10 parts 
______________________________________ 
Calcine at 850.degree.-1,000.degree. C, grind as required, wash, dry, 
pulverise the resulting peach pigment. 
The procedure for preparing a DIZIRC product on a pilot plant scale is 
described above; but alternatively the dissociated zircon can be 
comminuted in situ with or without the other components of the colour 
formulation. However, in these latter cases, care must be taken to ensure 
that there is no loss of water-soluble constituents and allowance made for 
the fact that, with the high proportion of other materials, the particle 
size of the dissociated zircon cannot be so conveniently controlled. 
The following Example 9 described the method of dry milling with subsequent 
fractionation. 
EXAMPLE 9 
The process may be carried as follows in the laboratory: 
6 kilos of 1/2 inch cylinder zircon grinding media are charged to a 1 
gallon porcelain ball mill and 1,000 gm. of dissociated zircon added. 
After dry milling for several hours until 98% is less than 150 mesh (BS) 
the extracted material is separated first through a 150 mesh sieve then 
through a 325-350 mesh sieve to give two fractions. The finer of the two 
is used to produce the yellow pigment, the coarser the blue. 
The following Examples 10 to 17 describe colour formulations which can be 
produced using DIZIRC produced by the dry milling process of Example 9. 
EXAMPLE 10 
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DIZIRC 100 parts by weight 
of material passing 
a mesh size of 200 
(British Standard) 
but standing on 325 
mesh 
ammonium metavanadate 
5-10 parts e.g. 8 
sodium fluoride 3-7 parts e.g. 6 
sodium chloride 3-7 parts e.g. 6 
______________________________________ 
Calcine at 700.degree.-900.degree. C, grind as required, wash, dry, 
pulverise the resulting blue pigment. The pigment produced is comparable 
in intensity with the earlier commercial zirconium vanadium blues. 
EXAMPLE 11 
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DIZIRC 100 parts by weight of 
material passing a 
mesh size of 200 
(British Standard) but 
standing on 325 mesh 
ammonium metavanadate 
5-10 parts e.g. 8 
silica (quartz) 0-10 parts e.g. 8 
sodium or potassium silicofluoride 
1-7 parts e.g. 5 
sodium chloride 5-10 parts e.g. 6 
potassium nitrate 1-7 parts e.g. 5 
______________________________________ 
Calcine at 680.degree.-900.degree. C, grind as required, wash, dry, 
pulverise the resulting blue pigment. Formulations of this type produce 
higher strength stains than can be obtained with Example 9. 
EXAMPLE 12 
______________________________________ 
DIZIRC 100 parts by weight of 
material passing a mesh 
size of 325 (British 
Standard) 
praseodymium oxide 2-5 parts e.g. 4 
sodium fluoride 3-6 parts e.g. 4 
sodium chloride 3-6 parts e.g. 4 
______________________________________ 
Calcine at 850.degree.-1,000.degree. C, grind as required, wash, dry, 
pulverise the resulting yellow pigment. 
EXAMPLE 13 
______________________________________ 
DIZIRC 100 parts by weight 
of material passing 
a mesh size of 325 
(British Standard) 
praseodymium oxide 2-5 parts e.g. 4 
silica (quartz) 0-10 parts e.g. 6 
barium silicofluoride 
1-5 parts e.g. 4 
sodium chloride 3-20 parts e.g. 10 
______________________________________ 
Calcine at 850.degree.-1,000.degree. C, grind as required, wash, dry, 
pulverise the resulting yellow pigment. Formulations of this type produce 
higher strength stains than can be obtained with Example 11. 
EXAMPLE 14 
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DIZIRC 100 parts by weight 
of material passing 
a mesh size of 325 
(British Standard) 
iron sulphate 25-70 parts e.g. 40 
sodium fluoride 10-20 parts e.g. 15 
sodium chloride 5-15 parts e.g. 10 
sodium or potassium nitrate 
5-15 parts e.g. 10 
______________________________________ 
Calcine at 800.degree.-1,000.degree. C, grind as required, wash, dry, 
pulverise the resulting pink pigment. 
The procedure for preparing, in the laboratory, the appropriate DIZIRC 
fractions is described above; but alternatively the dissociated zircon can 
be comminuted in situ with or without the other components. However, these 
latter methods result in a reduction in the final colour strength of the 
stain produced since the particle size of the dissociated zircon can not 
be so conveniently controlled. 
Examples 15 and 16 illustrate the use of a phosphorous containing compound 
as a component of the mineraliser for the purpose of modifying the shade 
of the basic blue stain. 
Example 15 
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DIZIRC 100 parts by weight 
of material passing 
a mesh size of 200 
(British Standard) but 
standing on 325 mesh 
ammonium metavanadate 
5-10 parts e.g. 6 
sodium fluoride 3-7 parts e.g. 5 
sodium chloride 3-7 parts e.g. 5 
sodium monofluorophosphate 
0.1-4 parts e.g. 2 
______________________________________ 
Calcine at 700.degree.-900.degree. C, wash, dry, pulverise the resulting 
blue to blue/violet pigment, grind as necessary. 
Example 16 
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DIZIRC 100 parts by weight 
of material passing a 
mesh size of 200 
(British Standard) 
standing on 325 mesh 
ammonium metavanadate 
5-10 parts e.g. 8 
sodium or potassium silicofluoride 
1-7 parts e.g. 5 
sodium chloride 5-10 parts e.g. 6 
trisodium phoshpate 
1-7 parts e.g. 5 
______________________________________ 
Calcine at 700.degree.-900.degree. C, wash, dry, pulverise the resulting 
blue to blue/violet pigment, grind as necessary. 
The following Example 17 illustrates the use of a lead compound as a 
component of the mineraliser. 
Example 17 
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DIZIRC 100 parts 
praseodymium oxide 2-5 parts e.g. 4 
sodium fluoride 3-7 parts e.g. 4 
sodium chloride 3-7 parts e.g. 4 
and red lead 1-10 parts e.g. 6 
or lead bisilicate 1-10 parts e.g. 6 
______________________________________ 
Calcine the mixture at 850.degree.-1,000.degree. C, wash, dry, and 
pulverise the resulting yellow pigment. The pigment is thereafter milled 
to the required particle size. 
The process of fractionation subsequent to milling may be omitted, but the 
colours then produced will be weaker. In all cases the use of a 
silicofluoride in the mineraliser formulation is advantageous. 
We have also discovered that it is possible to induce a degree of chemical 
milling in what are, in practical terms, damp conditions by milling with 
an appropriate additive in the presence of between 2 and 5% of moisture 
(relative to the DZ in the mill). 
Example 18 
This process may be carried out in the laboratory as follows: 6 Kilos of 
3/4 inch alumina milling media are charged to a 1 gallon porcelain ball 
mill and 600 gms of DZ added, together with 12 gm caustic soda flake and 
12 gms of water. After milling for several hours until 95% is less than 
200 mesh, the extracted material is used directly in formulations such as 
given in Examples 3, 10, 12 and 13. 
It is to be noted that we have discovered that comminuted dissociated 
zircon is suitable for the direct preparation of pigments so obviating the 
necessity as in the prior art, to prepare an intimate mixture of (fully 
separated) zirconia and silica in a 2:1 ratio since this proportion of a 
substantially similar or suitable ratio already exists in the dissociated 
zircon. Whereas the Ionarc process is used for dissociating zircon from 
which zirconium oxide is produced by first leaching out substantially all 
of the silica (ref: Ravinder & Wilks, "The commercial production of 
submicron ZrO.sub.2 via plasma," A. I. Ch. E. Dec. 2, 1971), it is now not 
necessary to take this latter step. 
Furthermore, we have discovered that for the present purposes the zircon 
can be dissociated at higher rates of throughput than those necessary to 
produce the fully leached oxide provided that the product does not contain 
more than 15% of the unreacted grains of zircon (Type I) and between 2 and 
35% of Type II i.e. not more than say 30% of unreacted zircon in total. 
The economic advantage of the present invention results from the 
possibility of using milled plasma dissociated zircon for the production 
of ceramic colouring materials comparable to the usual commercial yellow, 
blue and pink shades without incurring the costs of producing the 
zirconium oxide values by the normal methods of leaching or extraction and 
precipitation. 
By normal leaching we mean removal of silica as far as practicable from the 
Zircon sand -- at least 90% of the silica, and generally much more is 
converted into a different compound which is removed from the product. 
For the purpose of this invention we take care to remove as little as 
possible of the silica. The small addition of silica attacking agent 
during wet or damp grinding is only sufficient to attack the surface of 
the silica. The temperature of the wet grinding will usually be normal 
grinding process temperature, and in any event below 150.degree. F. The 
proportion of silica attacking agent added whether in wet or damp milling 
will be less than 0.5% of the stoichiometric amount required to convert 
all the silica to corresponding alkali metal silicate. We retain at least 
90% of the silica originally present in the dissociated zircon sand in its 
original chemical form (SiO.sub.2) for inclusion in the final stain. The 
comminuted DZ added to the ingredients for calcining into the stain 
contains at least 25% silica by weight of the zircon. The quantity of 
silica attacking agent used during milling is in any case less than 10% by 
weight of the zircon. 
The amount of silica added with the colouring agent for calcination is 
always less than 15% by weight of the DZ. We have found that the addition 
of silica (quartz) improves the colour strength of the stain. 
When calcining the DZ with the colouring agent, we may add one or more 
alkali metal salts, e.g. sodium fluoride, sodium chloride and sodium 
potassium nitrate preferably totalling 1 to 35 parts to 90 parts DZ. 
The DZ mixed with the colour agent for calcining is in fused amorphous 
condition whereas it is to be noted that heretofore manufacturers of 
stains would not be use amorphous silica in making stains. 
To summarise, the following main advantages are achieved by our process: 
a. Facilitation of pigment manufacture in good yields by using a ready-made 
mixture of zirconia and silica in highly reactive forms, 
b. cost reduction arising from a most economical use of expensive raw 
materials, 
c. higher yields and elimination of a calcination in a colour makers' kiln 
when compared with "chemical crack" processes, 
d. colour strength pigments comparable to that which may be produced from 
high quality chemically precipitated oxides, 
e. good stability in colour shade from batch to batch provided the DIZIRC 
material is produced from fully dissociated zircon sand and provided also 
the characteristics of the comminution process are maintained constant, 
f. the production of "self-fluxing" DIZIRC products by wet milling 
dissociated zircon in the presence of additives which attack the silica 
constituent and which are mineralisers in the subsequent colour forming 
reactions, 
g. the production of hitherto unavailable grey violet stains -- at 
reasonable cost -- from dry milled DIZIRC.