NL2027774A - Method of manufacturing glass frit products - Google Patents
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Abstract
The present invention relates to a method of manufacturing a range of different glass frit products. The method enables a range of glass frit products to be manufactured to a variety of target chemical compositions and particle size distributions in an efficient manner, including switching between different glass frit products while ensuring that processing parameters can be efficiently modified for each glass frit product.
Description
METHOD OF MANUFACTURING GLASS FRIT PRODUCTS Field of Invention The present invention relates to a method of manufacturing a range of different glass frit products. The method enables a range of glass frit products to be manufactured to a variety of target chemical compositions and particle size distributions in an efficient manner, including switching between different glass frit products while ensuring that processing parameters can be efficiently modified for each glass frit product.
Background Methods of manufacturing glass frits are well known. Typically, metal compounds (e.g. oxides) in powdered form are mixed in defined quantities, melted in a kiln, quenched to form a glass material and milled to form a glass frit product have a target chemical composition and particle size distribution. Glass frit products may comprise a single glass frit in terms of chemical composition and particle size distribution, a mixture of two or more glass frits which differ in terms of either one or both of their chemical composition and particle size distribution, or an ink or paste including one or more liquid components in addition to the solid glass frit material. Glass frit products may additionally comprise other components such as one or more pigments in order to form coloured enamels and/or one or more crystalline compound additives which have functional performance characteristics in the glass frit product. The chemical composition and particle size distribution of a glass frit product is highly engineered for its end application and the performance characteristics of a glass frit product can be highly sensitive to small changes in chemical composition and particle size distribution. As such, even small changes in chemical composition and particle size can be highly detrimental to performance. In recent years, the range of applications for glass frits has grown, and the performance specifications in end applications have become more demanding.
In addition, there has been a drive to use less toxic compositions such as lead-free glass materials.
Further still, there has been a drive to use more energy efficient glass frits, e.g. ones which melt at a lower temperature and thus require less energy to process.
The drive towards less toxic, lower melting point, more environmentally friendly glass frits with improved performance characteristics for a wider range of end applications has led to a rapid increase in the number of different glass frit formulations and an increase in the complexity of such {formulations, with each individual formulation being highly tailored for a particular end application.
The aforementioned drivers present several challenges to glass frit manufacturers.
One such challenge is that a wide range of processing techniques are required to produce a wide range of highly engineered glass frit products with precisely defined chemical and physical characteristics.
For example, in order to form glass frit products a range of processing techniques are utilized including: melting; quenching; dry mixing; wet mixing; dry milling; bead milling; wet milling; Jet milling; drying; sieving; sintering; calcining; pasting; and triple roll milling.
These different techniques, and combinations or sequences of these techniques, are utilized to progress raw materials through various intermediate product stages until a final glass frit end product is achieved with precisely defined chemical and physical characteristics.
Optimizing each processing technique, and combinations or sequences of such techniques, for different glass frit products is difficult and time consuming.
Furthermore, when manufacturing a range of different glass frit products having a variety of target chemical compositions and particle size distributions, switching between different glass frit products while ensuring that processing parameters can be efficiently modified for each glass frit product, and variants thereof, is demanding. Further still, given that each glass frit product passes through multiple processing steps during its manufacture, if a glass frit product deviates from its precisely defined specifications it is not straight forward to identify the source of an error, and suitable corrective measures to implement.
It is an aim of the present invention to at least partially address this problem when manufacturing a range of different glass frit products. Summary of Invention It has been found that following the drive to replace many lead-based glass frit products with lead-free alternatives, manufacturing issues can arise when targeting precisely defined chemical and particle size distribution specifications for the lead-free glass frits. Relative to traditional lead-based glass frits, modern lead-free glass frit systems are more difficult to process and more sensitive to variations in processing conditions. This may be due, at least in part, to the fact that traditional lead- based glass frit systems tend to vary less in terms of their density as well as other physical and chemical characteristics (e.g. hardness, friability, reactivity, etc) when compared to lead-free glass frit systems.
As described in the background section, if a glass frit product deviates from its precisely defined specifications then the source of an error and suitable corrective measures need to be implemented within the manufacturing process.
For example, batches of certain types of glass frit product may be periodically rejected by quality control due to the specific surface area of the glass frit being too high. Analysis has found that this is caused by superfine particles of glass frit in the product. Furthermore, batches of certain types of glass frit product may be periodically rejected by quality control due to discolouration. Yet another problem is batches of certain types of glass frit product being periodically rejected due to deviations from specifications caused by leaching of glass frits during manufacture. Additionally, problems can arise when manufacturing multiple different glass frit types due to cross-contamination between the different glass frit types passing through the same manufacturing equipment.
One source of all of these problems has been traced to the wet ball milling step of the manufacturing process. In particular, superfine particles in the milled glass frit can be produced by inefficient milling or over-processing during the wet balling milling stage of manufacture. Inefficient milling or over-processing during the wet balling milling stage of manufacture has also been identified as a cause of discolouration of the glass frit product. Furthermore, it has been found that leaching of frits occurs more in larger wet ball mills compared to smaller wet ball mills, likely caused by hotter temperatures generated in the larger wet ball mills. Further still, it has been found that cross- contamination between different glass frits can occur in the wet ball milling equipment. As such, a need has been identified to provide a more reliable wet balling milling procedure for processing a plurality of different glass frit types to avoid these problems.
The present invention thus relates to a method of manufacturing a range of different glass frit products which include wet ball milling as part of their manufacturing process. Wet ball milling of glass frit materials is known. However, it has been found that there are optimal operating conditions for wet ball milling of glass frits which vary according to the raw materials being milled and the desired final glass frit product. Processes have been developed for setting operating conditions for the different types of glass frit product being milled. Furthermore, it has been found that certain glass frits are more problematic in causing contamination of subsequent glass frits to be processed in the same mill. As such, it has been found to be advantageous to categorize the glass frits to be milled into different contamination categories and use different cleaning procedures when switching between glass frits according to their contamination categories.
According to the present specification a method of 5 manufacturing a plurality of different glass frits using wet ball milling is provided, the method comprising: (a) introducing a glass frit, water, and a plurality of milling balls into a ball mill; (b) rotating the ball mill to mill the glass frit until a target particle size distribution is achieved; (c) removing the glass frit from the ball mill when the target particle size distribution has been achieved; {d) periodically cleaning the ball mill; and (e) repeating steps (a) to (d) for the plurality of different glass frits, wherein a predefined ratio of water to glass frit is introduced into the ball mill in step (a), the predefined ratio of water to glass frit being determined empirically for each glass frit, wherein mass of the glass frit and volume of water to be introduced into the ball mill in step (a) is calculated for each glass frit based on the density of the glass frit, the predefined ratio of water to glass frit, a total solid volume of the milling balls, and a total interior volume of the ball mill, such that when the glass frit, water, and milling balls are introduced into the ball mill they together fill the ball mill to a fill level which is between 60 and 704 by volume of the total interior volume of the ball mill, wherein prior to milling the plurality of glass frits they are categorized into contamination groups, wherein step (d) of periodically cleaning the ball mill is performed based on the contamination group categorization including: (dl) cleaning the ball mill when switching between different glass frit types within the same contamination group; and (d2) cleaning the ball mill using a more rigorous cleaning protocol when switching between glass frits in different contamination groups, and wherein ordering rules are defined to prohibit direct switching between milling of glass frits of incompatible contamination groups regardless of cleaning protocols.
One important feature of the milling method is the finding that for more efficient milling of glass-frits, particular where lead-free glass frits are to be milled, the fill level of the mill should be between 60 and 70% by volume of the total interior volume of the ball mill. That is, the volume of the glass frit, water, and milling balls together should equate to between 60 and 70% of the interior volume of the ball mill. Previously when milling lead-based glass frits, a fixed mass of glass frit was added into a ball mill regardless of the specific composition of the glass frit. However, it has been found that variations in the milling characteristics of lead-free glass frits have required that the mass of glass frit introduced into a mill should be varied accounting for changes in density between the different glass frits in order to ensure that a target fill level of between 60 and 70% by volume is achieved for all the different glass frits being milled and without varying a pre-defined ratio of water to glass frit.
The predefined ratio of water to glass frit is such that between 0.30 and 0.60 litres {or Kg), optionally between 0.40 and 0.60 litres (or Kg), optionally between
0.43 and 0.55 litres (or Kg), of water may be added per Kg of glass frit. Alternatively, this ratio can be defined in terms of the mass of glass frit per litre (or Kg) of water.
In this case, between 1.5 and 6.0 Kg, optionally 2.0 and 6.0 Kg, optionally between 2.5 and 5.5 Kg, of glass frit can be introduced into the ball mill per litre (or Kg) of water. The specific ratio of glass frit and water is determined empirically in the laboratory before transferring the glass- frit product recipe to manufacturing and is subsequently fixed in the manufacturing process for a particular frit recipe. As such, it is important to note that the target fill level for the ball mill is achieved while maintaining the pre-defined water/frit ratio for each type of glass frit. The specific value for the water to glass frit ratio will be dependent on the characteristics of a particular 5 glass frit, including the particle size distribution and/or specific surface area of the raw glass frit introduced into the mill, and/or the friability of the raw glass frit, and/or the target particle size distribution after milling. The optimal ratio of water/frit can be determined via milling trials in the laboratory prior to transfer of the final glass frit recipe to production but it has been found that it will typically fall within the aforementioned ranges, particularly for lead-free glass frit recipes of the type used in automotive enamel applications.
The optimal amount of milling balls introduced into the ball mill is such that they will have a total weight which varies with the total interior volume of the ball mill being utilized. It has been found that approximately 1 Kg of milling balls per litre volume of the ball mill works well for milling glass frit products. A ratio of the total weight of the milling balls to the total interior volume of the ball mill may lie in a range 0.9 to 1.2 Kg per litre. The milling balls may each have a diameter of between 20 to 40 mm, optional between 25 and 35 mm, and a density lying in a range 1.5 to 2.5 Kg/m3, optionally between 2.0 and 2.2 Kg/m?3. In production, the mass (and associated number and volume) of milling balls is fixed for each individual mill and is dependent on the volume of the mill. As such, it is important to note that the target fill level for the ball mill is achieved while maintaining the pre-defined mass of milling balls in a particular mill in addition to maintaining the pre-defined water/glass frit ratio for each type of glass frit. That is, the interior volume of the mill and the volume of milling balls is fixed for a particular mill, the ratio of water/frit is also fixed, and the target fill level of 60-70% is achieved by calculating the mass of frit and volume of water to be added which will achieve the target fill level based on the density of the particular glass frit to be processed while maintaining the pre-defined water/frit ratio.
Typical glass frits processed using the methodology as described herein have bulk densities ranging from 1.2 KgL-! to 2.5 KgL-!. That is, the highest density glass frits have a density greater than twice the density of the lowest density glass frits.
Put another way, the highest density glass frits have a density which is more than 1 KgL-! larger than the lowest density glass frits.
As previously indicated, the density of modern glass {frits varies significantly according to their composition and thus the mass of glass frit which should be introduced into the ball mill for milling also varies significantly for the different frit types.
This approach is a significant departure from the approach taken when processing traditional lead-fits which typically were all processed with the same mass of glass frit material being introduced into a wet ball milling process.
The ball mills are typically rotated at a rotation frequency of between 15 and 40 revolutions per minute (rpm). The rotational frequency is dependent on the size of mill being utilized with the rotational frequency being higher for smaller mills.
For example, the rotational frequency is typically between 30 and 45 rpm when the interior volume of the ball mill lies in a range 300 to 500 litres, between 20 and 30 rpm when the interior volume of the ball mill lies in a range 1000 to 2500 litres, and between 15 and 20 rpm when the interior volume of the ball mill lies in a range 3000 to 5000 litres.
In production, the rotational frequency may be fixed for each individual mill for simplicity of operation.
However, it is also possible to tune the rotational frequency for a particular glass frit product.
In this regard, it has been found to be useful to reduce the rotational frequency during milling.
That is, the ball mill is rotated at a rotation frequency which is set at a starting frequency and then reduced in one or more steps during milling.
This has been found to promote fine grinding, so the glass frit particle size reaches specification faster and with less energy. The total number of rotations and/or the milling time required to achieve the target particle size distribution for a glass frit is pre-determined empirically. This may be done via milling trials in the laboratory prior to transfer of the glass frit recipe to production. However, it has been found that due to production variables, the need to achieve precisely defined specifications for the particle size distribution of a glass frit product, and the sensitivity of certain glass frit compositions to small process variations, a pre-defined milling time (or associated number of rotations) is not sufficiently reliable to ensure that the desired particle size distribution is achieved. Accordingly, one or more samples of glass frit can be removed from the ball milling during the milling process and analysed to check the particle size distribution of the glass frit, the milling process being stopped when it is found that the particle size distribution corresponds to the target particle size distribution. As this sampling process can be time-consuming, in practice a combination of these approaches can be taken. That is, a predefined milling time can be determined empirically, and then sampling of the glass frit to check the particle size distribution is only performed in production when nearing the pre-determined milling time for achieving the target particle size distribution. For efficiency reasons, a full particle size distribution analysis may not be performed as part of this sampling process. It has been found that measuring the D90 particle size of the glass frit is sufficient, at least for most glass frit products, to check the particle size distribution of the glass frit during milling. Additionally, or alternatively, the specific surface area of the glass frit can be measured.
While the aforementioned operating procedures for the wet ball milling process improve efficiency of the process and reduce the possibility of batches of glass frit material being rejected, cross-contamination between different glass frit products can still be an issue. It has been found that certain glass frits are more problematic in causing contamination of subsequent glass frits to be processed in the same mill. Conversely, it has been found that certain groups of glass frits are relatively compatible with each other and do not cause significant cross-contamination issues when processed in series through the same mill. For example, the plurality of glass frits may include two or more of: bismuth containing glass frits; lithium containing glass frits; zinc containing glass frits; lead containing glass frits; strontium containing glass frits; other transition metal containing glass frits; sulfur containing glass frits; glass frits to be added to white only powders; and glass frits to be added to black only powders.
As previously indicated, a key feature of the milling method of this specification is to categorize the plurality of glass frits to be milled into contamination groups and implement modified cleaning protocols and milling schedules based on this categorization. In terms of cleaning protocols, the ball mill is cleaned using a standard cleaning procedure when switching between different glass frit types within the same contamination group, but a more rigorous cleaning protocol is used when switching between glass frits in different contamination groups. For example, the standard cleaning procedure when switching between different glass frit types within the same contamination group may comprise a single rinsing or flushing step (with water) whereas the more rigorous cleaning protocol used when switching between glass frits in different contamination groups may comprise multiple rinsing/flushing steps to avoid cross-contamination. While multiple rinsing steps could be applied when switching between any glass frits, this is time consuming to implement. As such, the aforementioned approach provides a more efficient production process while still avoiding cross-contamination issues. As an alternative or additional method of rigorous cleaning when switching between contamination groups, particularly when switching between a highly contaminating group and one which is sensitive to contamination, is to introduce a cleaning charge (e.g. water and an anti-settling agent) into the mill and then run the mill for a period of time (e.g. 5000 rotations) before draining and flushing/rinsing.
It has also been found that certain glass frit groups are so incompatible that they should not be processed directly after each other in the same mill, regardless of cleaning protocols. As such, ordering rules are defined to prohibit direct switching between milling of glass frits of incompatible contamination groups regardless of cleaning protocols.
Finally, after removal of the glass frit from the ball mill it is dried to remove water from the glass frit and yield a dry glass frit powder. A filter is utilized in the drying apparatus to prevent release of the glass frit during drying. It has been found that the type of filter which is used should be selected according to the contamination group categorization of the glass frit in order to prevent release of the glass frit into the atmosphere and also to alleviate cross-contamination issues. As such, the filter in the drying apparatus may be changed to a different type when switching between glass frits in different contamination groups.
The present specification thus provides a wet ball milling methodology for manufacturing a plurality of glass frit products, including a variety of lead-free glass frits, which has improved flexibility, efficiency, predictability, and reliability.
Brief Description of the Drawings For a better understanding of the present invention and to show how the same may be carried into effect, certain embodiments of the present invention will now be described by way of example only with reference to the accompanying drawings, in which:
Figure 1 shows a schematic cross-sectional illustration of a ball milling apparatus; Figure 2 shows a schematic illustration of a ball milling apparatus including milling balls, glass frit, and water; Figure 3 shows a graph of particle size over time when milling at a constant rotational frequency; Figure 4 shows a graph showing a standard milling profile keeping the rotational frequency of the mill constant (e.g. 45 rpm) and the use of variable frequency in which the rotational frequency of the mill is stepped down from 45 rpm to 35 rpm and then further stepped down from 35 rpm to 25 rpm during processing; Figure 5 shows a temperature profile for the standard (std) fixed frequency profile and for the variable profile in which the rotational frequency is stepped down; and Figure 6 shows a graph of particle size versus milling time for a standard milling procedure using fixed rotational frequency and two trials using a step-down frequency approach.
Detailed Description As described in the summary section, the present specification is directed to a production methodology for manufacturing a range of different glass frits using wet ball milling.
Figure 1 shows a schematic cross-sectional illustration of a ball milling apparatus.
The ball mill 100 comprises a cylindrical body 102 and two opposing end plates 104, 106. A sealable port 108 is provided to introduce glass frit, water, and milling balls into the ball mill and remove said components therefrom.
In Figure 1 the port 108 is illustrated in the cylindrical body 102. However, alternatively it could be provided in one of the end plates 104, 106 or by providing that one of the end plates is removable.
The interior surface of the ball mill is lined with a hard-ceramic material 110 (e.g. aluminium oxide) to protect the metal body of the ball mill during operation. This lining 110 is periodically replaced (e.g. every 2 or 3 years) to prevent failure of the lining. The total interior volume V of the ball mill is the cavity defined by the inner surface of the lining 110. This may be calculated geometrically based on the interior radius R of the cylinder and the interior length L as V = pR2 x L. Alternatively, the volume can be measured by, for example, filling the mill with water and measuring the volume of water required to fill the mill. This measuring approach may be desirable if the mill becomes deformed from a perfectly cylindrical shape. In operation the ball mill is mounted horizontally as illustrated and rotated around its longitudinal axis (dotted line in Figure 1).
Figure 2 shows a schematic illustration of two different views of a ball milling apparatus 200 which has been loaded with milling balls 202, glass frit 204, and water 206. It should be noted that the volume occupied by the milling balls within the interior cavity of the ball mill will be a combination of the volume of the milling balls themselves and the volume of cavities formed between the milling balls. Similarly, the volume occupied by the glass frit within the interior cavity of the ball mill will be a combination of the volume of the glass frit particles themselves and the volume of cavities formed between the glass frit particles. The water percolates through the cavities between the particles of glass frit and the cavities between the milling balls. As such, only excess water sits above the surface of the glass frit layer as illustrated. There will also be a certain amount of glass frit which will percolate into cavities between the milling balls. Once the materials within the interior cavity have settled in this manner then they will occupy a proportion of the total interior volume of the ball mill. Since excess water is generally utilized, i.e. the volume of water introduced is larger than the volume of the cavities between the glass frit particles and milling balls, and it can be assumed that the water completely fills the cavity volume, then the volume occupied by the combination of water, glass frit and milling balls can be calculated as the sum of the solid volume of milling balls, the solid volume of glass frit, and the volume of water introduced. As indicated in the summary section, one important feature of the milling method is the finding that for more efficient milling of glass-frits, particular where lead- free glass frits are to be milled, the fill level of the mill should be between 60 and 70% by volume of the total interior volume of the ball mill (when the water has percolated through the glass frit and milling balls and thus fills cavities therebetween). That is, the volume of the glass frit, water, and milling balls together should equate to between 60 and 70% of the interior volume of the ball mill. Previously when milling lead-based glass frits, a fixed mass of glass frit was added into a ball mill regardless of the specific composition of the glass frit. However, it has been found that variations in the milling characteristics of lead-free glass frits have required that the mass of glass frit introduced into a mill should be varied accounting for changes in density between the different glass frits in order to ensure that a target fill level of between 60 and 70% by volume is achieved for all the different glass frits being milled and without varying a pre-defined ratio of water to glass frit or the quantity of milling balls in a particular mill.
While the fill level should always fall between 60 and 70% by volume (or between 0.6 and 0.7 defined as a fill fraction of the mill volume), the actual optimized fill level for each type of frit and each ball mill can be optimized within this range. That is, an optimum filling level for each product and mill combination can determined via a combination of lab scale and production scale empirical tests within this range. Once this optimum filling level is determined, we can calculate what mass of glass frit material and volume of water must be added to the rotary drum wet ball mill apparatus to meet this optimum filling level.
The relationship between the fill fraction of the mill and the associated volumes of water, milling balls {solid volume excluding cavities) and volume of glass frit (solid volume excluding cavities) in the mill is described by the equation below.
/ / / Volume Occupied Fill Fraction of Mill = ree. Volume of Empty Drum ‚ } } Volume of Glass + Volume of Water + Volume of Milling Balls Fill Fraction of Mil = ————————— ______- __ — Volume of Drum Verass + Vwater + V, Fill Fraction of Mill = “Glass _ Water © “Balls Vorum The volume that the glass occupies in the mill can be determined based on the density of the glass particles, the empty void space between particles, and the mass of the glass added to the mill.
Determine the bulk volume of the glass: Volume _ MaSSguik Glass Bulk Glass DensitYguik Glass Void Fraction between glass particles: Volume i Void Fraction as _ Space between glass particles Vo lumeg, x Glass Determine the actual solid volume of the glass: Volume qs = Volumegyik grass X (1 — Void Fraction gq) Veiass = Vaut class X (1 — Void Fraction giass) Determine the corresponding mass of the glass:
MasSgiass = Volume qos X Denisty crass The volume that the milling balls occupy in the mill can be determined based on the density of milling balls, the empty void space (gaps) between the balls and the mass of the balls added to the mill. Determine the bulk volume of the balls: Volume _ MASSpuik pans Bulk Balls = Tyoairns VASES Densitygu paus Void Fraction between milling balls: Volume illing ball Void Fraction, — Space between milling balls Volumeguik paus Determine the actual solid volume of the balls: VoluMepaus = Volumegy, x pans X (1 — Void Fraction gaus) Vgaus = Veuik aus X (1 — Void Fraction gatis) The volume of that the water occupies can be calculated based on the mass ratio of glass to water set during the development stage of the product.
The mass ratio between water and glass 1s set during development: MassSgiass Mass RatiOgiass: == Glass:Water MassSywater Determine the volume of the water:
Volume = __ MaSSewass OO water Mass RatiOgiass:water X Density Water V. _ Meiass Wat — re TTT wer M RatiOygter:Glass X Pwater
The relationship between the fill level, or fill fraction, of the mill and the associated volumes of water, milling balls and glass in the mill is described by the equations:
. . . Volume Occupied Fill Fraction of Mill = —— Volume of Drum | / / Volume of Glass + Volume of Water + Volume of Milling Balls Fill Fraction of Mill — es ee Volume of Drum
V, Velass +Vwater + V, Equation 1: Fill Fraction of Mill = SOceupled _ Glass | Water | Balls Vporum Vorum The relationship between Mass, Density and Volume can be described by:
M ‚ Equation 2: Vyateria = _ Material PMaterial Substitute Equation 2 into Equation 1 for each material:
Metass | Mwarer | Maus Equation 3: Fill Fraction of Mill = Pets Duster Pais Vprum The ratio of water to glass is defined during the development of the product and is key to achieving the product characteristics:
. _ My ater RatiOyasswater:MassGlass T Mor Glass In terms of Myer:
Equation 4: My ager = RatiOyasswWater:MassGlass x Mgtass Substitute Equation 4 into Equation 3 to determine the relationship between the fill fraction and the mass of glass to be added to the mill: Metass + RatiOyasswater:MassGlass: Matass + Mpgaus Equation 5: Fill Fraction of Mill = Patass Pwater a Vorum Rearranging in terms of Mgiass gives:
: : Mpaus Fill Fraction of Mill Vorm — Bere Equation 6: My uss = == 2e 1 + Ratioysswarer:massciass (oo + Hessen assess) Glass Pwater The mass of the glass can therefore be calculated based on the fixed, known quantities of fill fraction, mill drum volume, mass and density of the milling balls, density of the glass and water, and the mass ratio of water to glass.
Once the mass of glass to be added to the mill is calculated then the mass of water can be calculated using the fixed mass ratio of water to glass:
My ater = RatiOyasswater:Masstlass: Maiass This can alternative be expressed as a volume of water based on the density of the water: Mater Vater = ——— Pwater
As such, the mass of glass frit and the volume of water to be added to a mill to achieve the desired fill fraction between 0.6 and 0.7 (i.e. 60 to 70% by volume) can be calculated for a range of glass frits and a range of different mills while keeping a desired ratio of water to glass frit for each type of glass frit.
Parameters which are known or which can be determined via in-house measurements include Pater: PGlass: and Pgaus- Equipment specific parameters which can also be measured or taken from equipment specifications include Vorm and Mpaius: Parameters which are empirically determined for each type of glass frit to give desired product characteristics include Fill Fraction of Mill (set during trials to determine the optimum filling level) and RdtiOygsswater:Massctass (Set during development of the glass frit product). Using these parameter values and the aforementioned equations it is possible to calculate the mass of glass frit and volume of water to be added to any of the ball mills in production to achieve optimal wet ball milling of the glass frits. A worked example is set out below by way of illustration.
As previously indicated, the mass of glass to be added to a wet ball milling apparatus is given by the following equation: Fill Fraction of Mill. Vp, pm — Mpaus Motass Tae (—— + MasswWater:MassGlass PGlass Pwater Example values of parameters which are known or determined via in house measurement: ° Pwater = 1kg/L ° Patass = Skg/L ° Ppaus = 3k9/L Example values of equipment specific parameters:
° Vorum = 3000L ° Mpaus = 3000kg Example values of parameters empirically determined by laboratory trials to give desired product characteristics ° Fill Fraction of Mill = 0.6 ° RatiOygsswater:MassGlass = 0-4 Substituting the example values into the equations for calculating the required mass of glass and water gives: : : : Mpaus Fill Fraction of Mill. Vp, ym — === M | — : PBalls Glass ( 1 + RatiOmasswater:MassGlass PGlass Pwater 3000k
0.6 X 3000L — rnd 3kg/L Mowass = TVA GET + TKD Skg/L 1kg/L 3000k
0.6 x 3000L — Shalt Mgiass Ga - 1333kg (5Eg7T, + Tkg7D) My ater = RatiOyasswater:Massttass: Maiass = 0.4 X 1333kg = 533kg A software tool has been developed such that for any specific ball mill and glass frit product, the mass of glass frit and volume of water to be added into the mill can be calculated to optimize the milling process and ensure that target specification are met in an efficient manner. The benefits of using this methodology including: milling times are reduced; production operational efficiency and production capacity 1s increased; energy usage is reduced; increased flexibility to accommodate a range of glass frit products as well as new products; increased equipment lifespan; and reduction in failed quality control testing e.g. due to discolouration or specific surface area deviating from specifications.
While the aforementioned operating procedures for the wet ball milling process improve efficiency of the process and reduce the possibility of batches of glass frit material being rejected, cross-contamination between different glass frit products can still be an issue. After discharge of powder from the mill there will be residual powder on the walls of the mill and the surfaces of the milling balls. Most of the powder can be removed by flushing the mill with water. Where there is a high risk of contamination, multiple flushes can be implemented, and this will reduce the powder content further. Yet another more thorough cleaning method is to apply a full cleaning charge. A full cleaning charge comprises running the mill with a cleaning fluid (e.g. water and an anti-settling agent). This may be done for 5000 rotations before draining and flushing the mill as normal. Full cleaning charges are time consuming and thus are only applied when necessary.
A more complete procedure for cleaning the mill to remove residual powders from the mill is to remove the milling balls, clean the mill, and then return fresh milling balls. However, this is time consuming and may lead to damage or loss of milling balls. It has also been identified as a hazardous task for the operator given the number, hardness, and weight of the milling balls. As such, it is advantageous to implement a procedure which allows the milling balls to be retained within the mill while avoiding cross-contamination issues. At the same time, it is advantageous to avoid implementing multiple flushes or full cleaning charges where possible as this is time consuming and utilizes large quantities of water which is not environmentally friendly.
As described in the summary section, it has been found that certain glass frits are more problematic in causing contamination of subsequent glass frits to be processed in the same mill. Conversely, it has been found that certain groups of glass frits are relatively compatible with each other and do not cause significant cross-contamination issues when processed in series through the same mill. As such, and as previously indicated, a key feature of the milling method of this specification is to categorize the plurality of glass frits to be milled into contamination groups and implement modified cleaning protocols and milling schedules based on this categorization. By following a cleaning and frit processing protocol it is possible to minimise milling ball changes, water usage and cleaning time. Contamination groupings may include two or more of the following groups:
1. Glass frits which contain Li and Zn but no Bi or Pb.
2. Glass frits which contain Zn and one or more other transition metals but no Bi or Pb.
3. Glass frits which contain Bi and Li and one or more transition metals but no Pb or S.
4. Glass frits which contain bismuth but no Pb or S.
5. Glass frits which contain Zn but no Bi, Li, or Pb.
6. Glass frits which contain Bi and Li but no Pb.
7. Glass frits which include an additional seed material.
8. Glass frits which include Sr and only consist of black powder.
9. Glass frits which include Pb.
10. Glass frits which include Pb and only consist of white powder.
11. Glass frits which contain Li but no Bi, Zn, or Pb.
12. Glass frits which contain Zn but no Bi, Li, or Pb and only consist of white powder. When using these groupings, the baseline cleaning rules are that when changing between products within a Group then clean the mill with 1 flush and when changing between products in different Groups then clean the mill with 3 flushes. In this regard, the volume of water per flush is dependent on the mill size as follows: Quantity of water per flush: 500 kg mill 250 L 1000 kg mill 500 L 1500 kg mill 750 L 2000 kg mill 1000 L Several additional rules apply to avoid cross- contamination in addition to the aforementioned baseline rules, including one or more of the following: Changing between products within Groups 6 and 7 - use 3 flushes.
Changing from any group to Group 5 requires a full cleaning charge.
Products from Group 3 should not be milled directly after Group 2 or Group 7.
Always plan to mill Group 8, 9 and 12 products after Group 5. If this is not possible, then use a full cleaning charge.
Changing from Group 6 and 7 to any other Group requires a full cleaning charge.
Use a full cleaning charge when changing out of Groups 8, 9 or 12.
Changing from group 10 or 11 to any other group requires two full cleaning charges. This methodology provides an efficient production process while avoiding cross-contamination between different glass frits processed through the same milling apparatus.
In addition to the above, another advantageous modification to improve the efficiency of the milling process is to vary the rotational frequency of the mill during the milling process. In production, the rotational frequency may be fixed for each individual mill for simplicity of operation. However, it is also possible to tune the rotational frequency for a particular glass frit product. In this regard, it has been found to be useful to reduce the rotational frequency during milling. That is, the ball mill is rotated at a rotation frequency which is set at a starting frequency and then reduced in one or more steps during milling. This has been found to promote fine grinding, so the glass frit particle size reaches specification faster and with less energy. In this regard, particle size reduction occurs by impact at high speeds and by friction/attrition at low speeds.
The milling mechanism at a constant rotational frequency can be seen from a typical graph of particle size over time as illustrated in Figure 3. Once the curve flattens, it requires relatively more time and energy to reach lower particle sizes. This section would benefit from more attritional fine grinding, which is favoured at low speeds. Therefore, it has been found to be advantageous to reduce the rotational frequency during this time, such that glass frit particle size reduction is more rapid and gives more efficient milling with a lower energy requirement.
Figure 4 shows a graph showing a standard milling profile keeping the rotational frequency of the mill constant (e.g. 45 rpm) and the use of variable frequency in which the rotational frequency of the mill is stepped down from 45 rpm to 35 rpm and then further stepped down from 35 rpm to 25 rpm during processing. Temperature and particle size can be monitored using these two approaches.
Figure 5 shows the temperature profile for the standard (Std) fixed frequency profile and for the variable profile in which the rotational frequency is stepped down. From the graph it can be seen that the temperature across the milling cycle is much more stable using the frequency step-down approach, as well as being significantly lower than the standard fixed frequency approach.
Figure 6 shows a graph of particle size versus milling time for a standard milling procedure using fixed rotational frequency and two trials using a step-down frequency approach. As can be seen from the graph, the step-down approach leads to a significant reduction in the milling time required to achieve a target D90 particle size.
Accordingly, the step-down rotational frequency approach has several advantages due to reductions in temperature and reduction in processing time including: i. The process safety risk to operators is reduced thanks to less steam generation and pressure build up in the chamber, which can be explosively released when removing the lid. ii. Reduced product susceptibility to leaching. iii. Reduced leaching of alumina from the milling balls. iv. Shorter processing time and reduced energy requirements.
Vv. Less high energy impact on mill lining, increasing time between relining, and assisting in prolonging equipment life.
The present specification thus describes several methodologies which lead to an improved production process for wet ball milling of a plurality of different types of glass frit. While these methodologies are advantageously combined, it is also envisaged that any one of the of the process methods described in this specification could be provided separately from the others. For example, the fill fraction methodology, the cleaning protocol methodology, and the variable rotational frequency methodology may be applied in combination or separately. While this invention has been particularly shown and described with reference to certain examples, it will be understood to those skilled in the art that various changes in form and detail may be made without departing from the scope of the invention as defined by the appended claims.
Claims (15)
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