PROCESS FOR PREPARING SYNTHETIC WOLLASTONITE

Abstract: Process for preparing synthetic wollastonite which does not contain any detectable phases other than crystalline cyclowollastonite and which upon crushing and/or grinding yields particles having an irregular, round-edged shape without any detectable fibrousness, wherein the purest possible CaO- and SiO2-containing raw materials are finely ground, preferably to a grain size less than 10 mm, mixed in the proportions stated below, gradually added to a melting furnace, melted to a homogeneous melt in the melting furnace by heating to above 1550 DEG C, and cooled to a solidified product which is crushed/ground to the desired particle size, the amount of starting material being adjusted so that the molar ratio in the melt between SiO2 and divalent metal oxides, including CaO, is above 1 - 0.014 . (% Al2O3), where ''% Al2O3'' represents percent by weight of Al2O3.

Description: PROCESS FOR PREPARING SYNTHETIC WOLLASTONITE The present invention relates to a process for pre¬ paring a synthetic raw material of cyclowollastonite (previously designated pseudowollastonite and represented by α-CaO-SiO ) which after crushing and grinding is an attractive additive in glazes and ceramics, in powder mixtures used as lining/barrier layers in electrolysis furnaces for the production of aluminium, in different powder mixtures used in welding and in continuous casting of steel, and in plastics, paint and paper where it may be applied as filler and/or as pigment, as per se known. Wollastonite is a naturally occuring mineral formed by contact metamorphosis in limestone. The mineral has triclinic crystal structure and occurs normally in ray¬ like, usually fibrous aggregates. In pure form the compo¬ sition on weight basis is 48.3% CaO and 51.7% Si0 2 , but calcium may to some extent be replaced by contaminations, such as divalent iron, manganese and magnesium, as the case usually is if the wollastonite has been formed from impure limestone. Products which today are marketed with the designation natural wollastonite, are concentrates which by beneficiation methods have been prepared on basis of deposits of this type. In addition to those contaminations which by nature are present in the wolla¬ stonite mineral, these concentrates will also contain about 10% rock forming minerals such as free quartz (SiO ) , calcite (CaC0_) and various silicates containing Al O , Fe ? 0 and alkali oxides. The purity requirements stipulated by the market with respect to the mineral composition of the concentrates and the composition and properties of the wollastonite mineral limit the selection of commercially exploitable deposits to a small number. Thus, there is only one producer of natural wollastonite in Europe, viz. Partek OY in Finland, and on world wide basis there is only one producer which is dominating, viz. Nyco Inc., USA. Upon crushing and grinding natural wollastonite will yield particles with a fibrous shape. The fibrous character is characterized by the socalled "aspect-ratio", which represents the mean ratio between fibre length and diameter. For certain applications such as in plastics and rubber industry where the reinforcing effect of the additive is of importance, wollastonite powder with an aspect-ratio in the range 1/15 - 1/20 is demanded and well paid for. However, for other applications the fibrous character of the powder may for health reasons be directly undesired. Examples are ceramic and metallurgic industry where there are strict restrictions with respect to the handling of dry powder materials with a fibrous grain shape. The alternative for these consumers is to choose powder products prepared from synthetic wollastonite which do not yield fibrous grains upon grinding. As far as we know, today there are such products on the market from two producers, viz. Onodo Cement Co. Ltd. in Japan and Rheinische Kalksteinwerke GmbH in Germany. Both of these are using rotary kiln technology where socalled wolla¬ stonite clinker is prepared by starting with relatively pure CaO- and Si0„-containing raw materials. The rotary kiln technology is in no way simple, cf. British patents Nos. 1174919 and 1465635. Among other things reactive raw materials are required, preferably also addition of mineralisation reagents, careful charge preparation in the form of grinding, mixing and com- pacting, and control of residence time and temperature at a level which is close to the limit of what may be tolerated without the occurence of local melting and resulting ring formation in the kiln. The process must be operated optimally in order for the resulting clinker to contain more than 90% pseudo-wollastonite. The rest may be unreacted SiO- and silica rich glass phase, in the worst case also unreacted CaO and Ca_Si0.. The presence of the latter two phases is particularly unfortunate, primarily due to their pronounced tendency to form hydrates. In addition there is the polymorph inversion of the disilicate between the modifications β/τ which is accompanied by change in volume as per se known. The intention with our work was to develop an industrially interesting process for preparing a synthetic raw material which does not contain any other detectable phases than cyclowollastonite and which upon crushing and grinding will yield particles with irregular shape with rounded edges without any detectable fibrous character. We found it natural first to try what initially appeared to be simplest and under Norwegian conditions also appeared to be cheapest, viz. to melt readily avail¬ able raw materials of CaO and SiO_ in properly adjusted proportions and then make a casting, whereupon the melt is allowed to solidify and cool to room temperature. Approximately the same process is described in German patent No. 568832 from 1933 which relates to a method for preparing refractory blocks with a wollastonite compo¬ sition intended for use as lining in glass furnaces. According to the patent the cooling immediately after the casting should take place very slowly, but nothing is said with respect to the structure of the product. However, it is clearly stated that if it is desired to obtain a pro¬ duct with microcrystalline structure, it is necessary to add up to 5% fluor spar in the charge mixture, which in addition should also consist of unslaked lime (free of MgO) and particulate silica containing less than 0.2 and 0.3 percent Al^O,, respectively. In Austrian patent No. 332852 from 1969, which re- lates to a melting process for preparing synthetic mono- calcium silicate for use in ceramic industry, it is among other things a characteristic feature of the process to add A1_0 3 , more specifically in the form of clay in an amount up to 15% percent by weight in a charge mixture which otherwise consists of silica sand and burned lime in a stated ratio varying between 1 and 2, preferably 1.5, and to cool the melt in such a manner as to obtain a non- hydraulic, glassy final product or a non-hydraulic final product which is only partly crystallized. The melting method which accordingly may be said to be known, has also been considered as a possible alterna¬ tive to the sintering method as a process for preparing crystalline pseudowollastonite in both the above mentioned British patents. The patentee in British patent No. 1174919 asserts without any documentation that he has succeeded in the preparation of pseudowollastonite by a process which includes melting and subsequent crystalli¬ zation as the essential steps, but he states that the method is uneconomical. However, in British patent No. 1465635 it is stated that the melting process does not give a completely crystalline wollastonite directly, but a product which has a vitreous phase associated with a "crystalline phase formed by high temperature wolla¬ stonite". It is further stated that this "crystallo- vitreous" form limits the commercial and industrial uses of the product, and supplementary treatments are necessary to obtain conversion to crystalline form. In spite of these negative statements we have carried out three preliminary melting experiments described in Example 1 where the melting itself was carried out in an induction heated and open graphite crucible. To ensure that all charge and particularly all lime was completely dissolved in the melt, the latter was heated to 1600°C and kept at this level for about 1 hour with periodic stir¬ ring, before casting was performed (see experiments 1 and 2) or was allowed to solidify in the crucible (experiment 3) . In the resulting products which had a grayish blue, glassy dull appearance, clear signs of crystallinity could be seen with the bare eye. To our surprise the subsequent x-ray examinations demonstrated that we obtained dif- fractograms which unambiguously originated from one phase only and which we identified as representative of cyclo- wollastonite. Encouraged by this discovery we planned a more comprehensive work in order a) to test a melting process which is more suitable for industrial production than melting by indirect heating in crucibles , b) to ensure that the product formed really is crystal¬ line, as the x-ray diffractograms show, and that in addition there are no detectable amounts of glass phase which against expectations are not detected by x-ray diffraction, c) to examine whether there are special circumstances related to casting and the subsequent crushing and grin¬ ding which must be observed in order to obtain a final product in the form of a powder consisting of particles having rounded edges and no sign of fibrousness. According to the invention a process is provided for preparing synthetic wollastonite which does not contain any detectable phases other than crystalline cyclo- wollastonite and which upon crushing and/or grinding yields particles having an irregular, round-edged shape without any detectable fibrousness. The process is characterized by the use of purest possible CaO and SiO_ containing raw materials which are finely ground, preferably to a grain size less than 10 mm, mixed in the proportions stated below, and gradually charged to a melting furnace, fused to a homogeneous melt in the melting furnace by heating to above 1550°C, and cooled to a solidified product which is crushed/ground to the desired particle size, the amount of starting material being adjusted so that the molar ratio in the melt between s -° 2 an ~ divalent metal oxides, including CaO, is above 1- 0.014 • (% A1 2 0_) , where "% l^O " represents percent by weight of A1 2 0 3 Experiences gained during experiments described in Example 2 show that a conventional electric melting furnace is a suitable apparatus for preparing a melt having wollastonite composition. The furnace may be a single-phase furnace having a vertically positioned cen¬ tral graphite electrode and a carbon-lined furnace pot as counter-electrode as described in Example 2, or it may be a single-phase furnace having 2 electrodes, or optionally a more conventional 3-phase furnace having 3 electrodes. The furnace crucible may in the latter cases be cerami- cally lined, or it may be surrounded by water-cooled or oil-cooled panels so that the melt forms its own lining by solidifying on the panels. Graphite may equally well be replaced by cheaper self-baking carbon electrodes of the Søderberg type, or it may be chosen to replace these with plasma burners suitable for transferred arc operation - if it is essential to melt in a non-carbon environment. In the example, which actually describes the beginning of a melting campaign, the process is carried out as a series of batchwise melting steps comprising a charging period, wherein the charge-materials are added, preferably continuously, so that the bath all the time is kept well covered by a solid charge, a melting down period, and finally a period of tapping before the charging for the next step is initiated. It is particu¬ larly important to carry out the process in this manner if the process is performed in a tilting furnace wherein the melt is withdrawn through a spout. By using a stationary furnace and bottom tapping, as in the example, and if the furnace is drained completely at each tapping, it is of course equally important to carry out the process in this manner in order to ensure that all the material charged is completely transformed and mixed with the melt before the bath is drained. This does not prevent a large furnace from being charged continuously and bottom tapped inter¬ mittently, provided that the tapping is carried out in such a manner that unmelted and undissolved material is retained in the furnace. During all stages of the melting cycle, except from the start with an empty furnace, the electrode is kept submerged in the bath so that the energy is essentially supplied as ohmic heat in the bath itself. The thermal efficiency of the furnace is highest when the bath is covered, and lowest when the bath is open and the temperature is highest immediately before tapping - while at the same time the thermal load on electrode holders, roof etc. is then greatest. In cyclic melting it is therefore desired to make the melting down period as short as possible without affecting the quality of the product with respect to homogenity and consistency of the composition, and it is desired to keep the temperature as low as possible, preferably below 1650°C, desirably at a maximum of 1600°C, but above 1550°C. The choice of raw material and the sizing are important parameters to take into consideration in addition to those which only concern the melting technology, which among other things include electrode submersion, bath movement and the possibility to influence this, for instance by using gas for stirring. Of the two types of raw materials used in the examples, viz. quartzite and burned lime, the Si0 2 ~source is the one which has the lowest melting point and ini¬ tially forms the molten phase together with dissolved CaO. We observed during the crucible experiments in Example 1 that lime particles, which as such have a very high melting point, and which also in the presence of a Si0 2 - rich melt always will be surrounded by a layer of high- melting Ca 2 SiO.-phase, are dissolved more slowly than quartzite and that coarse lime particles may stay floating on the bath surface for a relatively long time after all the quartz had been dissolved. This was the reason why we in the subsequent example decided to choose the size 0-6 mm for the CaO components which then were somewhat finer than the quartzite, with a size grading of 0-10 mm. With respect to the kinetics of solution it is advan¬ tageous to use fine grained charge components, but there is a lower limit set by the desire to avoid uncontrolled and in particular selective dust losses which may have as a result that the product composition is changed in an uncontrolled manner. The risk for this to take place increases with increasing gas evolution in the charge which is connected with possible residual content of C0 2 and presence of chemically bound water in added lime - which should advantageously be kept at a low and control¬ led level. The dust losses were insignificant in the experiments in the pilot furnace in Example 2 where we used thoroughly burned lime which was only slightly hydrated, but the situation may readily become different in a large industrial furnace which is possibly spray charged and which also may not be fully closed, and where the gas velocity only due to natural convection may be considerable. The lowest permissible grain size in the charge mixture must therefore be subject to a practical consideration in each case where several factors are to be taken into consideration, such as the constructional design of the furnace, the design of the vent system, the method of charging and the total content of gas evolved in the charge. Upwards the grain size of the CaO component should be limited to about 10 mm due to the dissolution rate. In order to avoid that fluctuations in the product compo¬ sition may occur due to segregation in the storage and transportation system of the melting furnace, the upper grain size of the Si0 2 -component should also not be essentially different. In contrast to the rotary kiln process the melting process does not present any other specific requirements as to the physical form of the raw materials than this. If the size grading is initially acceptable, the handling of the raw material consists in that they are weighed separately, batchwise or continuously, mixed in proper proportions and transported into the furnace where they are distributed over the melt surface in a suitable and known manner without any essential occurence of any type of selective accumulation or loss on the way. Factors which will then be decisive for the choice of raw ateri- als for the melting process, are availability, price, chemical composition and chemical stability. All these factors were taken into consideration in the choice of CaO-source in the present experiments where we started with ordinary well-burned lime from an adjacent, large and good limestone deposit which distin¬ guishes itself by a low contamination level with respect to A1 2 0 3 , Ti0 2 and MgO. It is of course with respect to melting technology fully possible to replace burned lime with limestone as a source of CaO, but the gas evolution from the charge will then be considerable, roughly cal- 3 culated as 200 Nm C0 2 /ton of product, and the specific energy consumption, which for a big furnace and by using burned lime may probably get down towards 600 kWh/ton, will increase to about 900 kWh/ton. Evolved CO ? will then also cause flank attacks on carbon electrodes and possible lining materials of carbon. Burned lime is therefore a preferred CaO-source in the melting process. The quartzite used as Si0 2 -source in the present investigations was chosen because it is available as a cheap and suitably localized waste product although basically a more pure material would be preferable, parti¬ cularly with respect to the content of A1 2 0 3 and MnO. These contaminations are entirely recovered in the product and contribute to reduce its quality, while contaminations of Fe 2 0 3 and Ti0 2 under the prevailing reducing melting conditions were found predominantly to be reduced to metallic form and were to a minor extent found as oxide in the wollastonite melt. However, the metal phase formed under such conditions will get little opportunity to separate out in the form of a uniform melt phase which may readily be isolated from the oxide melt by tapping, but is found as small metal globules suspended in the oxide melt from which they may be removed by magnetic separation after crushing. This introduces the need for a separating step making the process more expensive, and it is accor¬ dingly preferred that the raw materials have a low content of reduceable oxides, preferably below 0.4%. The melting process is suitably carried out according to the invention by fusing the added raw materials com¬ pletely in an oxide melt phase which upon tapping and subsequent cooling and solidification outside the furnace crystallizes and forms the purest possible cyclo- wollastonite as the only phase. Ideally the melt must upon tapping consist of Si0 2 and CaO in a mixing ratio which expressed on molar basis is 1, i.e. N g . 0 /N c Q = 1. If this ratio is > 1 there will according to the theory in addition to cyclowollastonite be formed a certain propor¬ tion of Si0 2 -rich glass phase, which is considered to be undesired, but not directly destructive for the quality of the final product. However, if the ratio N g . Q /N c Q < 1 the dicalcium silicate phase is formed, and this is a just as detrimental component in products of melt wollastonite as it is in synthetic wollastonite prepared by sintering in rotary kilns. It is an important feature of the process according to the invention that the process is adapted to industrial operation, i.e. that ordinary contaminations of the type FeO, MnO, MgO and A1 2 0 3 , which necessarily will be brought in by using readily available and cheap raw materials, and fluctuations in the melt composition, which is frequently a result of incidental operational disturbances and un¬ certainty with respect to the chemical analysis of the raw materials, may be tolerated without any reduction of the product quality, for instance by the formation of detri¬ mental dicalcium silicate phase. This is attained by considering the most important contaminations in the calculation of the charge and specifying the mixing ratio so that the melt composition with sufficient margin will have a ratio between Si0 2 and divalent metal oxides ex¬ pressed by the term: wherein N„Me Λ/ o ~ is the sum of the molar fractions of all divalent metal oxides occuring in the oxide melt in ad¬ dition to CaO. In examples 1 and 2, where the stated molar ratio has a calculated border value of 0.99, the real composition has varied between a molar ratio of 1.07 and 1.12. X-ray diffraction analyses and subsequent light microscope examinations, described under examples 2-4, show that melt mixtures of this type upon cooling and solidification yield cyclowollastonite as the only detect¬ able phase. Accordingly, a molar ratio of 1.07 is recom¬ mended as a guiding figure for an industrial process so that there is sufficient margin for incidental deviation in both directions. According to the structure investigations which have been made during this work, and a selection of which has been reported in examples 1-4, the crystallization takes place completely independent of the cooling rate and independent of whether the solidification takes place in a hot or cold graphite chill mould, in a large cast iron chill mould with stationary bath, or whether the bath is stirred, for instance as a consequence of oxygen being injected during the solidification, in water or in cold copper chill moulds having thick walls, but the macro- and micro-structure of the material is of course influenced by whether the cooling takes place slowly or rapidly and whether the bath is stationary or is stirred. By slow cooling without forced stirring large pillar- shaped crystals are formed with marked transverse nickings, while by forced stirring a coarse tabular struc¬ ture of short prismatic grains is formed. Corresponding structures are also found in materials which have been cooled directly in contact with, cold, compact copper surfaces, but it is found that the individual grains apparently have a fibrous substructure which probably in the reality is flake-shaped/lamellar. With a slightly slower cooling a characteristic structure occurs, con¬ sisting of fibre aggregates arranged in rosette. The mean lowest diameter of individual fibres has in this connec¬ tion been measured to about 10 μ , which corresponds to the maximum grain size in the desired powder product to be prepared by starting from fused wollastonite. It is only in the casting skin against compact copper that one finds material in which the mean value of the lowest crystal dimension starts to become comparable with the mean par¬ ticle diameter in powder products of wollastonite. It was accordingly not unexpected that fused wolla¬ stonite prepared according to the description in Example 2 upon crushing and grinding in a jet mill yielded a powder product consisting of round-edged grains without any trace of fibrousness, and it was also not unexpected that the same material after remelting, rapid solidification, crushing and grinding in different types of mills as described in Example 4, still yielded round-edged powder particles without any sign of formation of needle-shaped grains. The above illustrates that two of the purposes of the invention, viz. to prepare a synthetic raw material which does not contain any detectable phases other than cyclo- wollastonite and which upon crushing and grinding yields particles having an irregular round-edged shape without detectable fibrousness, may be realized without the neces¬ sity of specifying further how the melt should be cast or cooled or how the subsequent crushing and grinding should take place. This means that there is freedom of choice of casting technique as well as of equipment and procedures for subsequent crushing and grinding to the desired fineness. Powder prepared by direct grinding of fused wolla¬ stonite produced according to the process, has according to measurements reported in Example 2 a whiteness or brightness which does not satisfy the requirements of the market for certain applications. Reduced whiteness may be due to the presence of socalled dark oxide contaminations such as MnO and FeO, but it may also be due to a certain content of elementary carbon in the material, in the order of magnitude of about 0.1% C. The latter component may be effectively removed by annealing the powder in air, and thereby the whiteness is improved considerably. However, the oxide contaminations must be eliminated by choosing purer starting materials for the melting, or with respect to FeO, by carrying out the melting under sufficiently reducing conditions. Reducing melting conditions may also be attained in a carbon-free environment, for instance by adding metallic silicon or ferrosilicon in small amounts. Example 1 Raw materials of quartzite and burned lime having a chemical composition and grain size distribution as i.a. disclosed in table 1, were weighed and mixed in 3 equal batches of about 3.5 kg where the amount ratio was adjusted to give a SiO — -ratio CaO on molar basis which was slightly above 1. Table 1 Chemical analysis for raw materials and resulting products from crucible experiments, expressed in weight% The batches were heated to 1600°C in open graphite cru¬ cibles which were heated by induction and kept at the temperature for about 1 hour until everything had melted, while at the same time the bath was carefully homogenized by periodic stirring with a graphite rod. In Experiment 1 the melt was cast in a thin-walled and pre-heated graphite crucible in which the melt was allowed to cool in air. In Experiment 2 the melt was cast in a thick-walled and cold graphite chill mould. In Experiment 3 the majority of the melt was cooled and solidified in the melt crucible of the induction furnace, while a smaller melt sample was quenched in water. The termperature course under Experiment 3, as regis¬ tered by means of a thermocouple placed in the middle of the melt, has been illustrated on Fig. 1, where the ordinate is the temperature in °C, and the abscissa is the time in hours, and a platau is observed on the cooling curve at 1484°C which marks the solidification tempera- ture. The cooling rate for the other melts was not measured, but it is assumed that it was highest in Experiment 2, medium fast in Experiment 1 and slowest in Experiment 3 if we disregard the water-cooled sample, which of course had the highest cooling rate of all. Visually no difference could be seen between the products from the three experiments. The material had a dull, glassy fracture surface with a greyish blue colour tone, and to our surprise large extended crystals could be clearly seen in the structure. Even the quenched sample had a characteristic crystalline appearance. Crushed samples were examined by means of x-ray diffraction, and resulted in nearly identical diagrams for all the samples including the quenched one, as will appear from the x-ray diffractogram on Fig. 2. If this is compared with the data given for cyclowollastonite (α - CaO • Si0 2 ) in the JCPDS-register (Litt: Powder diffraction file inorganic faces, 1982, file No. 31-300, JCPDS card 1981. Published by: JCPDS international centre for diffraction data) we find very good conformity at low d-values, while at high d-values there is a certain lack of conformity both with respect to the intensity and location of the x-ray reflexes. The x-ray reflex at the d-value 3.764 is for instance not detectable in the diagrams which represent Experiments 1 and 2, but is barely visible on Fig. 2 which relates to Experiment 3. Extended annealing of the latter sample in air did not result in any change in the x-ray diagram of the sample. Accordingly, the conclusion is drawn that the lack of conformity is caused by contaminations present in the cyclowollastonite structure in our samples. However, there are no traces of other crystalline phases in any of the diagrams, and nor are there any traces of glass phase which would normally be recognized by a displacement of the base line over a certain range in the x-ray diagram. The melting in all these experiments has been carried out under strongly reducing conditions, and accordingly the analyzed iron is present in divalent form, not as Fe 2 0 3 as indicated in Table 1. It will be seen that the iron-content in the products is lower than in the starting materials, which is due to the fact that more than the half of the iron has been reduced to metallic form and is found as small globules in the bottom of the crucible. The ratio between Si0 2 and divalent metal oxides on a molar basis has according to the analysis in Table 1 varied between 1.07 and 1.12. Example 2 The same raw materials as used in example 1, with the exception that limestone with a size distribution between 0 and 6 mm, had now been chosen, were mixed in approxi¬ mately the same proportions as in Example 1 and melted in a single phase electric furnace with transformer capacity of 150 kVA. The furnace used was stationary and equipped with a vertical adjustable center electrode of graphite and with an open carbon-lined furnace crucible as counter- electrode and with a tapping hole in the bottom. The melting campaign comprised two experiments. The first one started with cold furnace by short circuiting the electrode against the bottom, whereafter a charge of 2-3 kg was placed around the electrode tip. A molten pool was rapidly formed in the arc crater, and more charge could be added so that the bath became more or less covered by solid charge, and the electrode very soon became submerged in the melt. This continued until all charge, i.e. 80 kg, had been added, whereafter the cover layer was melted down, and the bath temperature was measured and adjusted up to about 1700°C before the melt was tapped from the bottom and directly into a 60 liter conical cast iron chill mould, where the melt was allowed to cool without further treatment. The charging to Experiment No. 2 started immediately after the 1st tapping, accordingly on a hot furnace. Otherwise the procedure was as for Experiment 1, with the exception that during the tapping the melt was treated with oxygen which was blown in through a water-cooled copper nozzle which was submerged in the chill mould and which was operative as long as the bath was fluid. In both experiments the furnace was operated on constant voltage, about 40 volts alternating current and a load of about 80 kW. The operation was very stable and without sign of arc formation, except from start-up periods with an empty furnace. The specific energy requirement may most properly be estimated on the basis of operation data for Experiment 2 where approximately 60 kWh were used to melt a charge of 80 kg, i.e. 750 kWh/ton. This is approximately twice the theoretical requirement and was as expected for a small furnace having a low thermal efficiency. The two tappings had approximately the same chemical composition, 53% Si0 2 , 45.2% CaO, 0.17% Fe 2 0 3 and 0.75% A1 2 0 3 , and the x-ray diagrams were practi¬ cally identical as it will appear from Fig. 2. Thin polished test samples (thin sections) were made from compact lumps of both product types, and these were examined in light microscope connected to a picture analy¬ zer. Fig. 3 is the product from Experiment 1 which consisted of long prismatic, partly fibrous, possibly lamellar crystals which generally were several mm long. With increasing magnification as illustrated on Fig. 4, it will be seen more clearly that there are nickings in the form of hairline cracks across the longitudinal direction of the crystals. The mean characteristic crack distance was for the present sample measured to 100 μm, while the mean thickness of the crystals was measured to 56 μm. It was therefore not unexpected that this material upon crushing and subsequent grinding in a jet mill to < 10 μm yielded powder particles having a round-edged shape with¬ out any sign of fibrousness. Pictures taken by means of a scanning electron microscope (SEM) confirm this, see Fig. 5 which illustrates an arbitrarily chosen area of the sample support at 2000x magnification. The material from Experiment 2, where the melt solidified with stirring, has acquired a short prismatic, almost tabular structure, cf. the picture on Fig. 6. Upon crushing and grinding in a jet mill this material yielded the same particle form as the long prismatic material from Experiment 1. 20-30 kg of the material from Experiment 1 was crushed to -1.5 mm and ground in a jet mill of the ALPINE type to a grain size <10 μm. The obtained powder product was examined and characterized according to standardized methods, and the results are found in Table 2. Table 2 Characteristic data of powder product prepared from melt wollastonite Chemical composition Si0 2 % 53.1 CaO % 45.1 Fe 2°3 % °' 2 A1 2 0 3 % 0.8 No free Si0 2 or CaO. Physical and chemical properties Shape "Aspect" ratio L/D Minus 8 μm Average particle size Surface area (BET) Specific weight Bulk weight Brightness (ISO) (may be varied) * Hardness (Mohs scale) Melting point Heat expansion coefficient pH in 10% slurry Moisture (105°C) Oil absorption The brightness of the material may be influenced, for instance by annealing ground powder in air. It may for instance be mentioned that a product sample which after grinding in a jet mill had an ISO brightness of 70.3%, acquired an improved brightness to 84.9% after annealing the sample in air for 12 hours at 800 C. Example 3 In order i.a. to investigate more thoroughly whether a rapid cooling and solidification may suppress the crys¬ tallization and result in undesired glass formation, about 1 kg of the material from Experiment 1 in Example 2 was melted in an induction heated graphite crucible. After some holding time at 1550°C most of the homogeneous melt phase was poured directly into a cold, vertical and rectangular chill mould of compact copper having a slot width of 20 mm, while the rest was poured onto a horizontal plate of compact copper, where the melt was allowed to solidify in a thin layer. A thin section of solidified material was prepared with orientation parallell with and perpendicular to the solidification front which here is assumed to have moved parallell with the vertical chill mould wall in the first case and parallell with the horizontal plate in the second case. Reference is made to Fig. 7-10 which illustrate the material which was cooled at the fastest rate, viz. where the melt was allowed to solidify in a thin layer directly against a cold copper surface. Fig. 7 and 8 illustrate with different magnification a homogeneous area located some distance above the boundary surface against copper and consisting of fibrous or lamellar aggregates varying between 100 μm and 2 mm in length. Individual fibres in these agglomerates have a mean thickness of 6 μm, and there is a relatively long distance between observable cracks across the longitudinal axis of the fibres. Near the boundary surface against copper, where the solidifi¬ cation rate has been highest, a granular structure with grain size in the range 200-800 μm, is initially observed with low magnification, cf. Fig. 9. However, increased magnification reveals that the tendency of the cyclo¬ wollastonite to form its characteristic microstructure is present as it can be clearly seen on Fig. 10 that indivi- dual grains have a well arranged fibrous or lamellar substructure. The mean fibre thickness was here measured to 4 μm. In the interior of the chill mould cast material there are found fibre or disc aggregates of the same appearance as shown on Fig. 7 and 8, but there the indivi¬ dual fibres had a mean thickness of 11 μm measured in a section parallell with the longitudinal axis of the fibres and 15 μm perpendicular to the longitudinal axis. The pictures on Fig. 7-10 confirm clearly that the melt mixture used in this work crystallizes and forms a structure which is characteristic to. cyclowollastonite even under the most extreme cooling conditions which may be contemplated by using conventional casting technique. Only under such extreme cooling conditions the fibre dimensions in the dried product begin to approach the particle dimensions which desirably should be prepared in industrially interesting powder products of wollastonite where the grain size generally is <10 μm. The chances that smaller fibrous crystallites may be crushed free and survive the crushing and grinding steps necessary to obtain such a product, are minimal. Example 4 In order to check whether rapidly solidified material against expectations may give rise to fibrousness in powder products prepared by starting with melt wollastonite, additional 7 kg of the material from Experiment 1 in Example 2 were melted in a graphite crucible and cast in a cold, compact, copper chill mould with dimension 600x300x10 mm, where it by means of a thermocouple located in the bottom of the chill mould i.a. could be determined that the temperature during the first 60 seconds after the filling dropped from 1600°C to 1100°C. The product consisted of approximately rectan¬ gular fibre or disc aggregates of a type corresponding to the one illustrated previously on Fig. 9 and 10, and where the mean fibre thickness was measured to 5.5 μm. This material was initially crushed in a laboratory jaw crusher and then rough-ground in 3 rounds in a rolling crusher before it was split up in samples for further grinding in different types of laboratory mills, including planet mill, ball mill, vibrator mill, attrition mill and jet mill. The resulting products were then inspected in a stereo microscope with 3Ox magnification, and no sign of fibre formation was observed in any of them. Surprisingly, it was found that the ball mill yielded a product with the highest aspect ratio. Measurements with VIDEOPLAN of about 280 relatively coarse grains from this product where the highest mean diameter was 20 μm, gave an aspect ratio of 2.3.

Claims: CLAIMS 1. Process for preparing synthetic wollastonite which does not contain any detectable phases other than crystal¬ line cyclowollastonite and which upon crushing and/or grinding yields particles having an irregular, round-edged shape without any detectable fibrousness, c h a r a c t e r i z e d i n that the purest possible CaO- and Si0 2 -containing raw materials are finely ground, preferably to a grain size less than 10 mm, mixed in the proportions stated below, gradually charged to a melting furnace, melted to a homogeneous melt in the melting furnace by heating to above 1550°C, and cooled to a solidified product which is crushed/ground to the desired particle size, the amount of starting material being adjusted so that the molar ratio in the melt between Si0 2 and divalent metal oxides, including CaO, is above 1 - 0.014 • (% Al 2 0 3 ) , where "% A1 2 0 3 " represents percent by weight of A1 2 0 3 . 2. Process according to claim 1, c h a r a c t e r i z e d in that the molar ratio is below 1.12, preferably about 1.07. 3. Process according to claims 1-2, c h a r a c t e r i z e d by using such starting materi¬ als that the amounts of Si0 2 plus CaO in the melt is at least 98% by weight. 4. Process according to any of claims 1-3, c h a r a c t e r i z e d in that the melting takes place at reducing conditions. 5. Process according to claim 4, c h a r a c t e r i z e d in that the reducing conditions are obtained by means of carbon and/or ferrosilicon. 6. Process according to any of claims 4 and 5, c h a r a c t e r i z e d in that the melting is per- formed in an electric melting furnace with one or more graphite electrodes or self-baking carbon electrodes. 7. Process according to any of claims 4-6, c h a r a c t e r i z e d in that the addition of raw materials to the melting furnace is performed under a cover of solid raw materials. 8. Process according to any of the preceding claims, c h a r a c t e r i z e d in that readily reduceable oxide components in the raw materials, which during the melting process are reduced to metallic form, are removed from the melt and/or are removed during the subsequent preparation of a powder product. 9. Process according to any of the preceding claims, c h a r a c t e r i z e d in that the product is annealed after solidification and/or after crushing/- grinding.

Inventors: LOE LARS, SCHOENING CHRISTIAN, TUSET JOHAN KRISTIAN SKEI, LOE, LARS, SCHOENING, CHRISTIAN, TUSET, JOHAN, KRISTIAN, SKEI
Applicants: WOLLASTONITT AS, A/S WOLLASTONITT
Application year: 1991
Main classification: C01B33
IPC codes: C01B33