PREPARATION OF GRANULAR COMPOSITIONS

Abstract: An agglomerated particulate product comprising zinc oxide, boric acid and at least 3 wt.% of 3ZnO.5B2O3.14H2O based on the total weight of zinc oxide, boric acid, and 3ZnO.5B2O3.14H2O is provided. This product may be used as a frit for the production of ceramics.

Description: PREPARATION OF GRANULAR COMPOSITIONS This invention relates to an agglomerated particulate product comprising zinc oxide and boric acid (H 3 BO 3 ) , to a process for the preparation thereof and to a frit comprising said product. The ceramics industry uses a mixture of silicon dioxide powder and other compounds such as zinc oxide and boric acid powders as a frit. These components in particulate form are typically mixed together, optionally with other ingredients such as colourants, and the composition is melted and applied to a product before it is fired. Processes for manufacturing particulate materials, such as spray-drying, often yield products in the form of small particles. Such products are frequently difficult to handle and typically are dusty and of low density. A particular problem has arisen in respect of zinc oxide powder. Such a powder has poor flowability. We have surprisingly discovered a process by which a composition comprising zinc oxide and boric acid can be produced in a form which has good flowability and which is suitable for use for forming a frit. The applicant has found that it is not possible to carry out a simple agglomeration technique using an intimate mixture of zinc oxide and boric acid. Carrying out a standard agglomeration technique on such a mixture followed immediately by the usual drying methods leads to caking after standing for short periods at high humidity. The resultant product cannot be used in frit manufacture. The applicant has now discovered that the process can be modified in an unexpected way to ensure that this caking does not occur. This modification entails allowing the components to undergo a controlled reaction after the agglomeration but before the product is dried. The product of the present invention mixes well with other frit components and may lead to improved quality frits, improved consistency of batches and consequently to better glazing. According to one aspect, the present invention provides an agglomerated particulate product comprising zinc oxide, boric acid and at least 3 wt% of 3ZnO.5B 2 0 3 .14H 2 0, based on the weight of zinc oxide, boric acid and 3ZnO.5B-0 3 .14H 2 0. The present invention also provides a process for preparing an agglomerated particulate product which comprises : (i) agglomerating an intimate mixture of zinc oxide particles and boric acid particles with up to 20 wt% water, based on the total weight of the particles, in the form of a fine spray or steam; (ii) agitating the agglomerated particles; and (iii) drying the agglomerated particles; wherein the temperature of the mixture during steps (i) and (ii) is at most 50°C and the temperature of the mixture during at least part of step (i) and/or step (ii) is at least 35°C, and the agglomerated product thereby obtainable. According to another aspect, the invention provides an agglomerated particulate product which comprises: (i) agglomerating an intimate mixture of zinc oxide particles and boric acid particles with up to 20 wt% water, based on the total weight of the particles, in the form of a fine spray or steam; (ii) agitating the agglomerated particles; and (iii) drying the agglomerated particles; wherein the temperature of the mixture during step (i) and (ii) is at most 50°C and the temperature of the mixture during at least part of step (i) and/or step (ii) is at least 35°C. The present invention also provides a frit comprising an agglomerated particulate product as defined above. In the process of the present invention a reaction takes place at the points of contact between the zinc oxide particles and the boric acid particles. The zinc oxide and boric acid react to form 3ZnO.5B 2 0 3 .14H 2 0. The initial zinc oxide and boric acid particles may be of any size. The zinc oxide particles may be significantly smaller than the boric acid particles. Suitably commercially available zinc oxide particles are used. For example the zinc oxide particles may have a D 50 size (i.e. 50 wt% below and 50 wt% above) of 1 to 5 μm. Commercially available boric acid particles may be used but preferably milled boric acid is used. Commercially available boric acid can be used to obtain a stable product visually similar to that using milled boric acid. However using milled boric acid, the conditioning (i.e. step ii) time is reduced, i.e. there is faster formation of 3ZnO.5B 2 0 3 .14H-, there is generally less oversize product, i.e. > 500 μm, and the segregation index of the end product is lower, i.e. the tendency for the particles to segregate according to size is reduced. Preferably the initial boric acid D 50 size is reduced to 100 μm to 200 μm. "White Seal" (trade mark) grade ZnO is used by about 60% of frit manufacturers, and can be used in this invention as the initial zinc oxide. Other grades of ZnO may also be used, for example ceramic grade. The zinc oxide and boric acid may be present in any proportion. The relative proportions used will depend on the relative proportions desired in the final frit. Preferably, however, the zinc oxide and boric acid are present in a weight ratio of from 2:1 to 4:1, especially about 3:1 (determined as ZnO:B 2 0 3 ) . The 3ZnO.5B 2 0 3 .14H-0 is present in an amount of at least 3 wt%, preferably at least 5 wt%, and up to 20 wt %, more preferably from 5 to 20 (e.g. 5 to 10) wt%, based on the total weight of the zinc oxide, boric acid and 3Zn0.5B 2 0 3 .14H 2 0 in the agglomerated particulate product. There is no further advantage in having the compound or mixture present in an amount of much more than 5 wt% . If the 3Zn0.5B 2 0 3 .14H 2 0 is present in an amount of at least 3 wt% the product is stable, i.e. when subjected to 100% Relative Humidity for one week it shows no sign of caking, that is to say it is still free flowing. The particle size of the agglomerated particles is such that the agglomerated particles have good flowability. In general the particle size is from lμm to 500μm. Desirably the composition does not contain particles having a particle size of less than lOμm, preferably less than 25μm. A preferred particle size is from 25μm to 500μm. The D so size of the agglomerated particles is preferably from 80μm to 130μm, more preferably about llOμm. If the composition produced contains particles having a size which is too small or large, such particles may, if desired, be removed by simple sieving. Any oversized product may be pulverized and added back to the composition. Any undersized product may be recycled or dissolved in the water used to agglomerate the particles . The product of the present invention generally consists of only the zinc oxide and boric acid and the reaction product thereof. However, other components may be added before, during or after granulation, for example colourant materials, anti-caking agents or other component(s) of frits. In the process of the present invention an intimate mixture of zinc oxide particles and boric acid particles is initially prepared. The mixture must be intimate since otherwise the zinc oxide agglomerates by itself and the desired reaction does not occur to the extent necessary. Following the formation of the intimate mixture agglomeration is carried out in a standard way by adding water in the form of a fine spray or steam. Techniques for the agglomeration of small particles to provide larger particle size products are well known. A comprehensive review of processes for agglomeration and which can be used in the process of the present invention is given in Kirk-Othmer' s Encyclopedia of Chemical Technology, 3rd Edition (21) pp 77-105. The agglomeration may be carried in any apparatus suitable for the mixing of dry particulate materials adapted so that a liquid granulating agent can be sprayed on or otherwise added to the particles . Conventional granulation equipment can be used. An intensive mixer such as an Eirich or Lδdige mixer may, for example, be used. A jacketed Winkworth mixer may also be used. By proper adjustment of the process variables, such as the rate and amount of water added, speed of rotation of the mixer, product residence time within the reactor and placement of spray to achieve maximum contact of the water with feedstock particles to be granulated, an agglomerated product containing a high proportion of the agglomerates of the desired particle size can be obtained. Undersize and/or oversize particles can be removed, for instance by screening, if a narrow particle size distribution is desired for the agglomerated product. The time required for the desired degree of agglomeration is dependent on the amount of water added and the rate of agitation. A suitable agglomeration time is about 10 minutes. The particle size of the agglomerated particulate product is dependent on the amount of water added, the rate of addition and the speed of agitation. One skilled in the art is readily able to determine appropriate agglomeration conditions. In view of the requirement to have a separate agitation step after the particles have been agglomerated, a batch operation may be used, although it is possible to carry out the process of the present invention in a continuous operation. The water for the agglomeration is used in an amount of up to 20% by weight based on the total weight of the particles. Desirably the water is used in an amount of 8 to 15 wt%, preferably 8 to 12 wt%, more preferably about 10 wt% . If used in the form of a fine spray the water desirably has a temperature of 15 to 25°C. Preferably the water is atomized, for example having a droplet size of about lOOμm. It may, if desired, contain dissolved or suspended components, for example zinc oxide, boric acid, a colourant, an anti-caking agent or other component (s) used in a frit. The granulated product is then agitated in the second step. The product may be agitated in the reactor used for agglomeration, or may be discharged into another vessel provided with means of agitation such as a stirrer, for instance a Z-blade/ribbon blender or a rotary drum blender. The agitation is carried out for a sufficient time to ensure that at least 3 wt%, preferably at least 5 wt%, of 3ZnO.5B 2 0 3 .14H,0 is formed. Agitation may be carried out, for instance, for from Vi to 2 hours (e.g. 1 to 2 hours) , preferably 45 mins to 1 hour. Although agitation may be carried out for longer, there is no advantage in this. If the agitation is carried out in the granulation vessel, the vessel must be provided with a variable speed agitator. For example the mixture may be stirred at high speed for agglomeration, but stirred at a lower speed, for example 200 rpm, during the agitation step. It is necessary for an appropriate stirring speed to be used to ensure that the particles are kept moving but do not break up. The range may be 100 to 300 rpm. Suitably however the range is 5 to 50 rpm, preferably 10 to 30 rpm, more preferably about 10 rpm. It is essential that the temperature of the mixture of zinc oxide and boric acid reaches at least 35°C during at least part of step (i) and/or (ii) in order to form the 3ZnO.5B 2 0 3 .14H 2 0. The temperature should not, however, exceed 50°C, and generally should be at most 42O, preferably at most 40 C, in order to avoid dehydration of the components. The temperature can be raised during the agglomeration step (i) by, for example, external heating or by agitation. High speed agitation will raise the temperature of the mixture. The temperature may be kept at 35°C or above for all or only part of the granulation step. For instance, the granulation can be started at room temperature (20°C) and stopped once a desired temperature, for example 35°C, has been reached after all the water has been added. The temperature of the agglomerated product during the agitation step (ii) may also be at least 35°C. The temperature may be kept at 35°C or above for all or part of the agglomeration step. If the temperature of the agglomerated product is at least 35°C at the end of the agglomeration step, the agglomerated product may simply be allowed to cool in the agitation step. In order to avoid breakup of the agglomerated particles high speed agitation cannot be used during the agitation step. Therefore if it is desired to increase the temperature or maintain an increased temperature during the agitation step, it is necessary to provide an external source of heat. After the agitation step, the agglomerated particulate product is dried. A suitable drying temperature is 60 to 80 °C (product temperature) . Drying may be carried out for, for instance, around 10 minutes. Although the boric acid starts to loose water to form metaboric acid above 60°C, the decomposition is avoided due to the short duration of the drying step. The purpose of the drying step is to remove all the water apart from water of crystallization which is present in the agglomerated particulate product. In order to obtain a temperature of 60 to 80°C for the material, a dryer may, for example, be used which has an inlet temperature of 110°C and an outlet temperature of 80°C. Any dryer such as a fluid bed, rotary, tray or vibratory dryer may be used. The agglomerated particulate product may optionally be cooled to less than 25 J C before the drying step. Generally speaking, if the drying step to be used is a batch drying, there is no need to cool prior to drying. On the other hand if a continuous dryer is to be used, it is generally preferred to have cooled the agglomerated product first. The purpose of cooling is to slow down or stop further formation of 3ZnO.5B 2 0 3 .14H 2 0, which may lead to caking of the wet product. The present invention is further described with reference to the accompanying drawing showing, schematically, a plant for the production of agglomerated particulate product according to the present invention. Referring to the drawing, the plant comprises a source of boric acid 1, a mill 2 and a bin for holding milled boric acid 3 together with a bin for holding particulate zinc oxide 4. Weigh belts 5 and 6 respectively move material from bins 3 and 4 to an agglomerator/conditioner 7, suitably a jacketed Winkworth mixer. A pipe 8 for the introduction of water to the agglomerator/conditioner 7 is provided. Also the agglomerator/conditioner 7 is provided with an external heater 9. A line 10 from the agglomerator/conditioner 7 communicates to a blender/cooler 11 which is provided with a cooler 12. Product from the blender/cooler 11 passes to a wet bin 13, through a screw feeder 14 into a continuous dryer 15, which is suitably a vibratory fluid bed dryer. Water is discharged from the dryer at 16 and fines are removed along line 17. Product from the dryer passes along line 18 to a screen 19 and subsequently to batch bins 20 for despatch. The line 17 from the dryer 15 is provided with a cyclone 21 whereby any non-fines material may be returned along line 22 to line 18 and undersize material along line 23 via dust socks 24 to holder 25. Additionally oversize material from the screen 19 can pass along line 26 to holder 25. Holder 25 is connected via a mill 27 and line 28 to a hopper 29 from whence material can be introduced into agglomerator/conditioner 7. In use boric acid from source 1 is milled in mill 2 to the required particle size and passed to bin 3 from when is introduced, via weigh belt 5, into agglomerator/- conditioner 7 and zinc oxide from bin 4 is introduced along line 6 and ground and recycled material may be introduced from hopper 29. An intimate mixture of the boric acid and zinc oxide ingredients is formed in the agglomerator/conditioner 7 and then water from line 8 is sprayed into the agglomerator/- conditioner 7. Once the agglomeration step has been completed, the rate of stirring in the agglomerator/conditioner 7 is reduced and the agglomerated product is conditioned, with agitation, with reaction of zinc oxide and boric acid. Supplementary heating may be provided as required by use of heater 9. The agitation is preferably carried out over a period of 45 minutes to 1 hour at a stirring speed of 10 to 30 rpm, most preferably about 10 rpm and under temperature conditions similar to those in the agglomeration step. The conditioned product from the agglomerator/- conditioner 7 is passed along line 10 to the blender/cooler 11 provided with a cooler 12. In blender/cooler 11 the product is stirred slowly to prevent agglomeration and cooled e.g. down to less than 25°C with a view to preventing or suppressing further reaction. The cooled product from blender/cooler 11 passes through wet bin 13 and screw feeder 14 to dryer 15 in which the maximum product temperature is generally of the order of 70 J C. Dried product is removed from the bottom of dryer 15 via line 18 to a screen 19, from which agglomerated particulate product of the required size is passed into batch bins 20 for subsequent despatch. Water is removed from the dryer 15 at 16 and fines removed along line 17 are separated in cyclone 21 and undersized material passes along line 23 via dust socks 24 to holder 25. Any entrained non-fines material from cyclone 21 is recycled along line 21 to line 18 and hence to the screen 19. Oversize material from the screen 19 is recycled along line 26 to holder 25. The material from holder 25 is milled in mill 27 and passed along line 28 to hopper 29 from whence it is recycled to agglomerator/conditioner 7. The agglomerated particles may be used in a frit. A frit is made by mixing various compounds to obtain a composition which provides the desired glaze properties. Thus, for instance, for single fast fired tiles, the frit typically has the following composition (expressed as oxides although the components are not necessarily present in oxide form) : Si0 2 50-60 wt% A1 2 0 3 4-7 wt% B 2 0 3 (as H 3 B0 3 ) 3-6 wt% CaO 8-10 wt% MgO 1-3 wt% Na 2 0 0-1 wt% K 2 0 2-4 wt% ZnO 8-10 wt% BaO 0-2 wt% The raw materials of the frit may be minerals or raw products, such as quartz, feldspars (Na or K) , carbonates (Ca or Mg) , zinc oxide, alumina, kaolins, zirconium silicates and borates in the form of boric acid, colemanite, ulexite or sodium tetraborate. The agglomerated particulate product of the present invention desirably has a ZnO:H 3 B0 3 weight ratio (determined as ZnO:B 2 0 3 ) of that which is used in the frit. If the product does not have the desired ratio, the product may be added to the remaining constituents of the frit to provide the required amount of ZnO, and then further borate, for example in the form of boric acid, may be added to reach the required amount of B 2 0 3 . The frit is melted and quenched, and then used in the usual manner. The invention is further illustrated in the following Examples. EXAMPLES Example 1 l,488g of granular technical grade boric acid having a maximum particle six below 1mm and a D so of 250μm and 2,512g of "White Seal" grade zinc oxide having a maximum particle size of less than 5μm were intimately mixed for one minute in an 8 litre working volume Eirich mixer at 950 rpm. The rate of agitation was increased to 1900 rpm, and 424g water was added in the form of a spray over 6-7 minutes (about 65g/minute) . The thus-obtained agglomerated product was then transferred to a rotating drum, agitated at 200 rpm and allowed to cool over a period of 120 minutes. The product was then dried in a fluid bed dryer at 80°C for 10 minutes, and subsequently screened to remove particles having a size of greater than l,200μm. The results are shown in Table 1. Example 2 Example 1 waε repeated except for the following changes: • 330g water was added during the agglomeration step; • low speed agitation (800 rpm) was used during the agglomeration step; • the temperature of the mixture during agglomeration was maintained at 35°C for all of the agglomeration step by heating. The results are shown in Table 1. Example 3 Example 1 was repeated except for the following changes : • 440g water was added during the agglomeration step; • low speed agitation (800 rpm) was used during the agglomeration step; • the temperature of the mixture during agglomeration was maintained at 35°C for all of the agglomeration step by heating; • the temperature of the mixture during agitation was maintained at 35°C for all of the agitation step by heating. The results are shown in Table 1. Comparative Example 1 Example 1 was repeated except for the following changes : • 44Og water was added during the agglomeration step; • low speed agitation (800 rpm) was used during the agglomeration step such that the temperature of the mixture throughout the agglomeration step was 20- 21°C. The results are shown in Table 1. Example 4 50 kg of milled boric acid having a maximum particle size of <600 μm and a D 50 of 76 μm, and 85 kg of White Seal grade zinc oxide having a maximum particle size of <5 μm plus 13 kg of recycled product (maximum particle size <500 μm) were intimately mixed at 30 rpm in a Winkworth mixer having a working volume of approx. 200 litres (actual volume 322 litres) . The rate of agitation was increased to 80 rpm and 20 kg of water were added in the form of a spray over a period of ten minutes (2 kg/minute) . Agitation at 80 rpm was continued for a further five minutes and then intermittently for 30 seconds every five minutes over a period of 40 minutes . The rate of agitation was reduced to 30 rpm and sufficient solid carbon dioxide was added such that after a further ten minutes, the temperature of the product was <25 ' C. The product was discharged from the mixer through a coarse screen (5 mm) into a storage hopper prior to drying in a continuous dryer. The conditions in the dryer were " such that- , the inlet temperature was 140-156'C; the air exit temperature 48-56'C and the product temperature <70 J C. The product was screened to remove particles having a particle size of >500μm. The results are shown in Table 1. TABLE 1 •plus 13 kg recycled material The mean flow rate is determined by allowing a - 15 - known volume of powder to flow through a calibrated orifice and recording the flow time using the following apparatus and procedure : APPARATUS 1. Plastic funnel 2. Glass tube- 50 cm in length and 3.5 cm internal diameter. The tube is calibrated using water to give upper and lower marks so that the volume between the marks is 287 ml. 3. Brass orifice with diameter 2.25 cm. 4. A metal slide for blocking the bottom of the tube whilst it is filled with powder. 5. Stopwatch calibrated in tenths of a second. PROCEDURE 1. Fit the 2.25 cm diameter orifice to the tube. 2. Close the orifice with the metal slide and fill the tube with the powder under test using the funnel. 3. Open the orifice. 4. Start the stopwatch when the powder passes the upper graduation mark. 5. Stop the stopwatch when the powder passes the lower graduation mark. 6. Note the elapsed time. 7. Retain the powder and repeat 2 to 6 twice more. CALCULATIONS The mean flow rate is calculated from the volume between the two marks and the time is given in ml/s.

Claims: CLAIMS 1. An agglomerated particulate product comprising zinc, oxide, boric acid, and at least 3 wt% of 3ZnO.5B 2 0 3 .14H-0 based on the total weight of zinc oxide, boric acid and 3Zn0.5B 2 0 3 .14H 2 0. 2. A product according to claim 1 which comprises from 5 to 20 wt% 3ZnO.5B 2 0 3 .14H 2 0. 3. A product according to claim 1 or 2 wherein the zinc oxide and boric acid are present in a weight ratio of from 2:1 to 4:1 (determined as ZnO:B 2 0 3 ) . 4. A product according to claim 3 wherein the zinc oxide and boric acid are present in a weight ratio of about 3:1. 5. A product according to any one of the preceding claims wherein the agglomerate particle size is from lμm to 500μm. 6. A product according to claim 5 wherein the agglomerate particle size is from 25μm to 500μm. 7. A process for preparing an agglomerated particulate product which comprises: (i) agglomerating an intimate mixture of zinc oxide particles and boric acid particles with up to 20 wt% water, based on the total weight of the particles, in the form of a fine spray or steam; (ii) agitating the agglomerated particles; and (iii) drying the agglomerated particles; wherein the temperature of the mixture during steps (i) and (ii) is at most 50°C and the temperature of the mixture during at least part of step (i) and/or step (ii) is at least 35°C. 8. A process according to claim 7 wherein fre 0 to 15 wt% water in the form of a fine spray having a temperature of 15 to 25°C is used. 9. A process according to claim 7 or 8 wherein the temperature of the mixture during steps (i) and (ii) does - 17 - not exceed 42°C. 10. A process according to claim 7, 8 or 9 wherein the agglomerated particles which have been agitated in step (ii) are cooled to less than 25 " C before drying in step (iii) . 11. A process according to any one of claims 7 to 10 wherein the particles are dried at a temperature of from 60 to 80°C for from 10 to 20 minutes. 12. An agglomerated particulate product obtainable by a process which comprises: (i) agglomerating an intimate mixture of zinc oxide particles and boric acid particles with up to 20 wt% water, based on the total weight of the particles, in the form of a fine spray or steam; (ii) agitating the agglomerated particles; and (iii) drying the agglomerated particles; wherein the temperature of the mixture during step (i) and (ii) is at most 50°C and the temperature of the mixture during at least part of step (i) and/or step (ii) is at least 35°C. 13. A product according to claim 12 wherein from 10 to 15 wt% water in the form of a fine spray having a temperature of 15 to 25°C is used. 14. A product according to claim 12 or 13 wherein the particles are dried at a temperature of from 60 to 80 C for from 10 to 20 minutes. 15. A frit comprising a product as defined in any one of claims 1 to 6 , and 12 to 14.

Inventors: HAYATI IGAN, SIMON JOHN MARTIN, HAYATI, IGAN, SIMON, JOHN, MARTIN
Applicants: HAYATI IGAN, SIMON JOHN MARTIN, UNITED STATES BORAX INC, HAYATI, IGAN, SIMON, JOHN, MARTIN, U.S. BORAX INC.
Application year: 1997
Main classification: C03C1
IPC codes: C03C1