Reflective silica-based granules and powders for use in roofing and building materials and processes for making same - Patents.com

Abstract

A process for producing a white silica-based product, such as granules (e.g., roofing granules), comprising: grinding a feed material to form a ground feed material having a size of about 0.5 μm to about 50 μm; combining the ground feed material with a binder and water to provide an agglomerated mixture; and heating the agglomerated mixture at a temperature of about 1350° C. to less than about 1550° C. for a period of time to form the white silica-based product. The agglomerated mixture is selected such that cristobalite is the predominant silica crystal structure in the white silica-based product, and further, the white silica-based product has an L of 93 to 98 in the CIELAB color space. * It is heated sufficiently to show the value.

Description

[Background technology]

[0001] background Mineral granules can help maintain the integrity of roofs and other building products during exposure to outdoor environments. Specifically, the granules can protect and preserve roofs and building products from wear and abrasion caused by rain, snow, ice, and wind, and damage caused by the sun's ultraviolet radiation, and can also serve other purposes, such as providing an aesthetic appearance to the roof and reflecting solar energy and light. In some locations, building codes require roofs to have certain characteristics, such as a minimum solar reflectance. Mineral powders can be added to paints and surface coating materials to provide light reflectance for enhanced visibility, for example, for use as pavement lanes and road markings.

[0002] Various materials have been proposed to provide solar and light reflectance, but many of these materials do not meet other requirements for roofing, building, and coating materials. Still other materials proposed to provide reflectance are too expensive to incorporate into roofing and other building and coating materials. Summary of the Invention [Means for solving the problem]

[0003] overview In an embodiment of the present disclosure, a process for producing a white silica-based product includes grinding a feed material to form a ground feed material having a size of about 0.5 μm to about 50 μm; combining the ground feed material with a binder (e.g., sodium silicate) and water to provide an agglomerated mixture; and heating the agglomerated mixture at a temperature of about 1300° C. to less than about 1550° C. for a period of time to form the white silica-based product. The agglomerated mixture is characterized in that cristobalite is the predominant silica crystal structure in the white silica-based product, and further characterized in that the white silica-based product has an L of 93 to 98 in the CIELAB color space. * It is heated sufficiently to show the value.

[0004] In some embodiments, the feed material is derived from a waste sand by-product produced from a sand plant. In some embodiments, the binder is present in an amount of about 0.2% to about 10% by weight based on the total weight of the agglomerated mixture, or the binder is present in an amount of about 1% to about 5% by weight based on the total weight of the agglomerated mixture.

[0005] In some embodiments, water is present in an amount of about 10% to about 20% by weight based on the total weight of the agglomerated mixture, or water is present in an amount of about 12% to about 16% by weight based on the total weight of the agglomerated mixture.

[0006] In some embodiments, the agglomeration mixture further comprises sodium hydroxide.

[0007] In some embodiments, the binder and water are applied to the milled feed material via spraying. In some embodiments, the heating comprises heating the agglomerated mixture to a temperature of about 1350°C to about 1400°C. According to some embodiments, the heating is carried out for a time period of about 40 minutes to about 60 minutes. In some embodiments, the heating is effective to sinter particles of the agglomerated mixture into granules.

[0008] In some embodiments, the process further comprises cooling the white silica-based product to a temperature of about 100° C. or less. In some embodiments, the process further comprises applying at least one coating to the white silica-based product granules at a temperature of about 75° C. to about 100° C. In other embodiments, the process comprises applying at least one coating to the white silica-based product granules at a temperature of less than 50° C. In some embodiments, the process further comprises sieving the white silica-based product to select white silica-based granules having an average particle size of about 0.45 mm to about 2.5 mm.

[0009] According to some embodiments, the white silica-based granules have a specific gravity of about 2.32 to about 2.40. In some embodiments, sieving is effective to create an oversized white silica fraction and an undersized white silica fraction, and the process further includes combining the oversized white silica fraction and the undersized white silica fraction; and milling the combined fractions, thereby producing a white silica-based powder having a maximum mesh size of 325 mesh.

[0010] According to various embodiments of the present disclosure, the roofing material granules have an L of 93 to 98 in the CIELAB color space. * The white siliceous granules exhibit a high silica content, and cristobalite is the predominant crystalline structure of silica in the white siliceous granules.

[0011] In some embodiments, the roofing material granules have an average particle size of about 0.45 mm to about 2.5 mm. In some embodiments, the roofing material granules have a specific gravity of about 2.32 to about 2.40. In some embodiments, the roofing material granules have a functional coating on the outer surface of the white silica-based granules.

[0012] According to various embodiments of the present disclosure, the white silica-based granules have a visible light reflectance of greater than 75% in the wavelength range of 360 nm to 750 nm and an L of 93 to 98 in the CIELAB color space. * or that cristobalite is the predominant silica crystal structure; or that the granules have a specific gravity of about 2.32 to about 2.40. [Brief explanation of the drawings]

[0013] [Figure 1A] FIG. 1A is a photograph showing the feed material for Sample A described in the Examples.

[0014] [Figure 1B] FIG. 1B is a photograph showing the resulting crude material of Comparative Sample D formed from heat treatment of the feed material of Sample A described in the Examples.

[0015] [Figure 1C] FIG. 1C is a photograph showing the resulting granules of Sample E formed from the feedstock of Sample A described in the Examples.

[0016] [Figure 2] FIG. 2 is a photograph showing the resulting granules of Sample F formed from the feedstock of Sample B described in the Examples.

[0017] [Figure 3] FIG. 3 is a graph of % reflectance (Y-axis) as a function of wavelength (X-axis; in nm) for various samples described herein. DETAILED DESCRIPTION OF THE INVENTION

[0018] Detailed Description The present disclosure provides white, and in some embodiments, ultra-white, silica products in powder or granule form that are highly reflective and suitable for use as roofing granules and powders for coatings and building materials. A commonly preferred ultra-white filler for many of these products is cristobalite because of its high whiteness and relatively low cost. While naturally occurring, cristobalite used to make man-made products is typically produced by heating silica sand, i.e., sand that is essentially silica with a quartz crystal structure, until essentially all of its quartz content is converted to other forms of silica, including cristobalite. This is usually done by heating the silica sand to ≥1550°C for ≥1 hour in a rotary kiln adapted to tumble the sand as it moves from the furnace inlet to its outlet. This results in an L of 97 or higher in the CIELAB color space. *An ultra-white cristobalite product is produced that exhibits a .alpha.-1.0% (.alpha.) value and contains at least about 85% by weight cristobalite, based on the total weight of crystalline silica, and less than 1% by weight quartz, based on the total weight of crystalline silica. The ultra-white cristobalite product also typically contains some amount of amorphous silica, typically about 5% by weight or more, based on the total weight of silica contained in the ultra-white cristobalite product. However, to obtain an ultra-white product, high-quality sand must be used as the input.

[0019] The present disclosure provides a process for producing a white silica-based product, such as granules (e.g., roofing granules), comprising: grinding a feed material to form a ground feed material having a size of about 0.5 μm to about 50 μm; combining the ground feed material with sodium silicate and water to provide an agglomerated mixture; and heating the agglomerated mixture at a temperature of about 1300° C. to less than about 1550° C. for a period of time to form the white silica-based product. The agglomerated mixture has an L of 93 to 98 in the CIELAB color space, such that cristobalite is the predominant silica crystal structure in the white silica-based product. * The process allows for the production of white or extra-white silica-based products from alternative sand input sources.

[0020] Silica Feed Material The silica feedstock subjected to the heat treatment process of the present invention will typically contain greater than or equal to about 98 wt. % silica, including greater than or equal to 99.5 wt. % silica, based on the total weight of silica contained in the silica feedstock, with about 95% or more of the silica being quartz silica. In this context, "quartz silica" refers to silica having a quartz crystal structure. Thus, the feedstock will typically contain greater than or equal to about 93 wt. % (about 98 wt. % x 95% = about 93 wt. %) quartz silica, including greater than or equal to 94.5 wt. % (about 99.5 wt. % x 95% = about 94.5 wt. %). In various embodiments, the silica feedstock comprises between about 98 wt. % and about 99.8 wt. % silica, and between about 93 wt. % and about 99 wt. % quartz silica. Thus, while the processes described herein utilize quartz silica sand of reasonable purity, as discussed above, it should be understood that the processes do not require the highest purity quartz silica sand as a feedstock.

[0021] Silica sand ore processing typically begins with a series of steps performed to separate the desired quartz ore from bulk impurities. These steps typically include crushing (for hard rock or sandstone), scrubbing, washing, hydrosizing, and deslimming. Flotation may also be necessary if the ore contains heterogeneous materials such as feldspar, garnet, and mica. Magnetic separation can also be used to remove magnetic or paramagnetic particles. The cleaned ore is then typically dried and classified by size, typically by bulk dewatering using cyclones and / or pile drainage, heat drying (e.g., less than about 1% water by weight), and sizing through screens or sifters.

[0022] However, in the embodiments described herein, the feed material may be raw silica sand that already has a desired relatively high level of silica purity or that can be easily and inexpensively cleaned by scrubbing, washing, hydrosizing, deslimming, etc. to achieve this relatively high level of silica purity.

[0023] In this context, "raw silica sand" is understood to mean a naturally occurring free-flowing sand in which at least 95% by weight of the silica present has the quartz crystal structure.

[0024] Additionally, "naturally occurring" is understood to mean that, prior to the inventive heat treatment process, the silica feedstock has not been treated to be chemically converted into another material, as occurs, for example, when quartz silica is converted into alkali metal silicate, alkoxysilane, or hydrolyzed silica, or physically converted into another material, as occurs, for example, when granular silica is converted into silica sol or water glass, or when granular silica is fired / sintered at a temperature high enough to change the phase structure of the silica from crystalline to amorphous and / or change the shape of the silica particles to more spherical. Thus, "naturally occurring" means that the granular product is found essentially in nature, as occurs, for example, in the case of sea sand, quarry sand, and sand obtained by crushing sandstone.

[0025] Unless otherwise indicated, all mesh sizes disclosed herein refer to Mesh (US).

[0026] Due to variations in feed materials and precision associated with measurements, "about" in reference to the above concentrations is understood to mean that the numbers have an accuracy of ±3% by weight, more typically ±2% by weight, ±1% by weight, and possibly even ±0.1% by weight.

[0027] Unless otherwise indicated, "white silica-based product" includes white or extra-white silica-based granules according to the present disclosure, white or extra-white silica-based powders according to the present disclosure, or combinations thereof.

[0028] Finally, "cleaned" and "cleaning" when referring to raw silica sand are understood to mean the removal of non-siliceous components from the surface of the raw silica sand grains. This does not refer to and excludes processes in which silica in the form of water-soluble compounds is extracted from the raw silica sand grains and the silica is subsequently recovered from the extracted water-soluble compounds.

[0029] In either case, it should be understood that the feed material may contain some small amounts of impurities, typically equal to or greater than about 0.2%, equal to or greater than about 0.5%, equal to or greater than about 0.6%, equal to or greater than about 0.7%, equal to or greater than about 0.8%, or equal to or greater than about 0.9% by weight. Additionally, in most cases, the feed material will not contain more than 2.0% or more than 1.0% by weight of these impurities. Thus, in various embodiments, the feed material may comprise from about 0.2% to about 2.0% by weight, from about 0.5% to about 2.0% by weight, from about 0.6% to about 2.0% by weight, from about 0.7% to about 2.0% by weight, from about 0.8% to about 2.0% by weight, from about 0.9% to about 2.0% by weight, from about 0.2% to about 1.5% by weight, from 0.5% to about 1.5% by weight, from about 0.6% to about 1.5% by weight, based on the total weight of the feed material. %, about 0.7% to about 1.5%, about 0.8% to about 1.5%, about 0.9% to about 1.5%, about 0.2% to about 1.0%, about 0.5% to about 1.0%, about 0.6% to about 1.0%, about 0.7% to about 1.0%, or about 0.8% to about 1.0% by weight of impurities, including any and all ranges and subranges therein.

[0030] In an embodiment, the silica feed material is raw silica sand having a particle size of 30 to 170 mesh (US). However, feed materials having other particle sizes can also be used. Thus, the silica feed material can have a bulk particle size of 2.5 to 8 mesh (US), a grit or coarse sand particle size of 8 to 70 mesh (US), or a fine sand particle size of 70 to 170 mesh (US). Silica feed material having a particle size of 140 to 200 mesh (US) can also be used, depending on the particular embodiment. It should be understood that the milling process described herein below can enable the use of silica feed materials having a wide range of particle sizes.

[0031] In various embodiments, waste sand fractions, often found in a wide variety of sand plants, can be used as feedstock. Many industrial sand plants are operated to produce sand fractions of various particle sizes and / or particle size distributions to make various products, such as glass, proppant, play sand, concrete sand, etc. As a result, high-quality, high-purity sand fractions with limited commercial particle sizes are often produced as by-products during the manufacture of commercial products. These sand fractions are not all used, or in some cases must be disposed of as waste, such as by burying them underground or otherwise returning them to the mine, beach, or other geological location from which they were originally obtained. Thus, these sand fractions, hereinafter "waste sand by-products," either have no value at all or, more commonly, have negative value due to the cost of disposing of them as waste.

[0032] In various embodiments, the feedstock is subjected to a milling step. During the milling step, the feedstock is milled to a maximum mesh size of approximately 325 mesh. Milling can be performed using any suitable equipment and protocols known and used in the art. In embodiments, milling is effective to form angular powder particles having an average particle size of about 10 to 15 μm. For example, milling is effective to produce particles having a particle size of about 0.5 μm to about 50 μm.

[0033] Following milling, the milled feed material is combined with a binder and water in an agglomeration step. In various embodiments, the milled feed material can be combined with water and a binder, a flux, or both a flux and a binder in the agglomeration step. In various embodiments, the binder is a sodium-containing binder, such as, for example and without limitation, sodium silicate. In embodiments, the flux is a sodium-containing flux, such as sodium hydroxide. Without being bound by theory, it is believed that certain binders, such as sodium silicate, can act as both a binder (e.g., to bind particles together) and a flux (e.g., to change the crystal structure of silica from quartz to cristobalite and / or tridymite under conditions of lower temperature than would otherwise occur). In embodiments, the binder is present in an amount of about 0.2 wt % to about 10 wt %, based on the total weight of the final product. For example, the binder may be present in an amount of about 0.5% to about 10%, about 0.5% to about 7.5%, about 0.5% to about 5.0%, about 1.0% to about 10%, about 1.0% to about 7.5%, about 1.0% to about 5.0%, about 1.5% to about 10%, about 1.5% to about 7.5%, about 1.5% to about 5.0%, about 2.0% to about 10%, about 2.0% to about 7.5%, about 2.0% to about 5.0%, about 2.5% to about 10%, about 2.5% to about 7.5%, or about 2.5% to about 5.0% by weight of the final product, including any and all ranges and subranges therein.

[0034] Water may be present in an amount of about 10% to about 20% by weight, based on the total weight of the final product. For example, water may be present in an amount of about 10% to about 20% by weight, about 11% to about 18% by weight, or about 12% to about 16% by weight, including any and all ranges and subranges therein. Water may be added to the agglomerated mixture as a separate component, as part of the binder (e.g., when the binder is added in aqueous form), or a combination thereof. For example, in embodiments, the binder is added as a 40% by weight active sodium silicate solution in water, and supplemental water is also added to the milled feed material.

[0035] Without being limited to any particular method, in various embodiments, binder and water are applied to the ground feed material via spraying or pouring a dilute aqueous binder into a granulator, including, but not limited to, an Eirich mixer. Regardless of the application method, the ground feed material, binder, and water are mixed and granulated to provide an agglomerated mixture, sometimes referred to herein as granules. In other embodiments, water and binder, flux, or both binder and flux are applied to the ground feed material and mixed to form granules. Although agglomeration can be carried out according to any suitable method known and used in the art, in various embodiments, agglomeration can be carried out at ambient temperature.

[0036] Heat Treatment According to various embodiments, the agglomerated mixture (i.e., granules) flows into a rotary kiln where it is heated for a time and at a temperature sufficient to convert the crystalline structure of a substantial amount of its silica content from quartz to cristobalite.

[0037] As previously indicated, the use of heat to convert quartz to cristobalite has been practiced industrially for many years. In various embodiments, a similar heating process is carried out, except that substantially lower processing temperatures and substantially shorter processing times are used, thereby requiring substantially less energy. According to various embodiments, it has been discovered that this approach can produce white or ultra-white silica-based products with whiteness levels approaching, and in some cases equal to, the whiteness levels exhibited by industrially produced cristobalite, even while using substantially less thermal energy.

[0038] Thus, various embodiments provide a white or extra-white silica-based product as a significantly lower cost alternative that performs essentially similarly, or even exactly like, industrial cristobalite in many applications.

[0039] Thus, in various embodiments, the agglomerated mixture is heated to a temperature of about 1550°C or less for about 1 hour (i.e., about 60 minutes) or less. In some embodiments, the agglomerated mixture is heated to a temperature of about 1300°C to about 1550°C, about 1325°C to about 1550°C, about 1350°C to about 1550°C, or about 1375°C to about 1550°C for a time period of about 40 minutes to about 60 minutes. While the temperature may vary depending on the particular embodiment, generally, the temperature may be high enough to achieve a predominantly cristobalite silica composition. However, it is believed that grinding the feed sand produces particles that may be converted to cristobalite at lower temperatures than required according to conventional methods. Furthermore, the temperature may be sufficient to sinter the particles into cohesive granules, thereby providing mechanical integrity to the granules. Heating may be carried out in a continuous manner, for example, in a rotary kiln, although other heating methods are possible and contemplated.

[0040] The white or ultra-white silica-based product is then cooled to a temperature of about 100°C or less, about 95°C or less, about 75°C or less, about 50°C or less, or room temperature (e.g., about 23°C). In embodiments, the white or ultra-white silica-based product may be cooled via an air quench process using forced convection of ambient air to facilitate cooling. In some embodiments, one or more coatings may be applied during the cooling process. For example, a waterproof or other functional coating may be applied to the white or ultra-white silica-based product while it is at a temperature of about 75°C to about 100°C, and the coated silica may then be cooled to room temperature. As another example, a functional coating may be applied to the white or ultra-white silica-based product while it is at a temperature of less than 50°C. For example, a functional coating may be applied to the white or ultra-white silica-based product at a temperature of about 15°C to about 50°C.

[0041] White silica fine granules In various embodiments, the white or extra-white silica-based product can be sieved to select granules having a specific size. Sieving can be performed by any suitable method known and used in the art, including, but not limited to, the use of vibrating screens. In various embodiments, the granules are selected to have a particle size based on -8 / +35 mesh. In some embodiments, the granules have an average particle size of about 0.45 mm to about 2.5 mm. However, in other embodiments, granules having larger or smaller particle sizes can be used depending on the particular application. For example, if the granules are intended for use in roofing applications (e.g., application to the surface of shingle or flat-top roofs), the granules can be selected to have a standard size #11 granule size or another standard size commonly used in the roofing industry. For other applications, such as when the granules are used as a pigment filler, coarser or finer granules can be used. As described in more detail below, in some embodiments, granules coarser and / or finer than the granule size for a particular embodiment can be further processed.

[0042] The shape of the white or extra-white silica-based granules is typically round, although it is contemplated that the agglomeration process described above may be modified to achieve granules having other shapes.

[0043] White or extra-white silica-based granules have a bulk density of approximately 75 lb / ft 3 For example, in various embodiments, the bulk density of the white or extra-white particulate silica-based filler is less than about 30 lb / ft 3 ~about 75lb / ft 3 , about 35lb / ft 3 ~about 75lb / ft 3 , approximately 40 lb / ft 3 ~about 75lb / ft 3 , approximately 45 lb / ft 3 ~about 75lb / ft 3 , or about 50 lb / ft 3 ~about 75lb / ft 3 In contrast, conventional granules of a similar size have a yield of about 80 lb / ft3 ~120lb / ft 3 In various embodiments, the white or extra-white particulate silica-based filler has a specific gravity (via helium pycnometer) of about 2.32 to about 2.40, depending on the temperature and hold time during heat treatment. This contrasts with a specific gravity of about 2.65 for the feed material of certain embodiments. Measurement of the specific gravity of the granules via water pycnometer showed a reduction of about 20% compared to the feed material, suggesting a large void volume within the granules produced by this method.

[0044] As already mentioned, white or extra-white silica-based granules have the same L value as industrial cristobalite, even though the heat treatment conditions used to make them are substantially less severe than those used to make industrial cristobalite. * Very high L values, close to, and in some cases equal to, * In various embodiments, the white or extra-white silica-based granules have an L of about 93 to about 98, e.g., about 94 to about 97, about 94 to about 98, about 95 to about 98, about 96 to about 98, about 93 to about 97, about 93 to about 96.5, about 93 to about 96, about 93 to about 95, or about 95 to about 96.5. * Additionally, in various embodiments, the white or extra-white silica-based granules exhibit an average visible light reflectance of between about 75% and about 85%, e.g., between about 77% and about 83%, or between about 79% and about 82%.

[0045] White silica powder As described above, in various embodiments, the white or extra-white silica-based product is sieved to select white or extra-white silica-based granules having a specific size. Thus, in addition to the selected white or extra-white silica-based granules, an oversized white or extra-white silica fraction (e.g., particles having a particle size greater than 8 mesh or approximately 2.5 mm) and an undersized white or extra-white silica fraction (e.g., particles having a particle size less than 35 mesh or approximately 0.45 mm) are produced.

[0046] Thus, in some embodiments, following heat treatment and sieving, the oversized white or extra-white silica fraction and the undersized white or extra-white silica fraction are combined for further processing. Similar to the white or extra-white silica-based granules described above, the oversized white or extra-white silica fraction and the undersized white or extra-white silica fraction are primarily cristobalite and have a specific gravity (by helium pycnometer) of about 2.32 to about 2.40, depending on the temperature and holding time during heat treatment. The combined fractions can then be ground or milled to produce a powder having a maximum mesh size of 325 mesh. The grinding or milling can be carried out according to any method known and used in the industry and can be the same or different from the grinding or milling method utilized prior to heat treatment. For example, in some embodiments, the combined fractions are milled using a ball mill with alumina media to a particle size distribution such that about 97% of the resulting powder distribution mass is finer than approximately -325 mesh. Conventional air classification can be used to control the upper size of the white or extra-white silica-based powder to be less than 325 mesh. In other words, in various embodiments, the white or extra-white silica-based powder has a particle size of approximately 44 microns or less.

[0047] In some embodiments, the fine granules described herein above can be milled in place of or in addition to one or both of the white or extra-white silica fractions to produce a white or extra-white silica-based powder. For example, following sieving, the selected fine granules can be milled using a ball mill to produce a white or extra-white silica-based powder. In other embodiments, the sieving step can be omitted, and the white or extra-white silica-based product can be milled using a ball mill to produce a white or extra-white silica-based powder.

[0048] The white or extra-white silica-based powders can be used in a variety of applications, including as fillers or feed materials for other granulation processes. Other applications are also contemplated and known in the art.

[0049] artificial products The white or extra-white silica-based granules and / or white or extra-white silica-based powders can be used to produce a wide variety of man-made products, including both solid shaped articles, as well as binders, sealants, coatings and adhesives.

[0050] Examples of solid-shaped articles include artificial stone, such as that used to make synthetic kitchen countertops, artificial rock, such as that used for fireplaces and building exterior finishes, tile, brick, and white architectural concrete. Such products typically contain one or more fillers or aggregates and at least one or more binders, which may be cement, resin, or both. White or extra-white silica-based products (powder or granules) can replace these fillers and / or aggregates. Portland cement (usually containing whitening agents such as titanium dioxide, calcium carbonate, or cristobalite) is the most common binder, but other binders, including pozzolanic binders, other hydraulic lime-based cements, and the like, can also be used. Furthermore, in some embodiments, white or extra-white silica-based granules can be used as granules for roofing shingles. For example, white or extra-white silica-based granules can be used as specialized granules for roofing shingles, providing increased solar reflectivity compared to conventional standard roofing granules. Additionally or alternatively, white or extra-white silica-based granules and powders can be used to replace fillers used in asphalt or other building products, such as wallboard, paving, and architectural coatings. Reflective powders can be added to paints and coatings for use as lane and other road markings.

[0051] According to various embodiments, the white or extra-white silica-based product can be used to replace some or all of these conventional silica-based products, as it has been found to perform similarly to these materials in terms of the desired whiteness and physical properties of the man-made products, potentially at a lower cost, depending primarily on other factors such as raw material supply, location, transportation costs, etc. Additionally, the L value of the white or extra-white silica-based product of the present invention approaches and can even be equal to the L value of conventionally formed industrial cristobalite, potentially providing additional advantages in terms of product appearance. [Example]

[0052] The following examples are included for illustrative purposes and are not intended to limit the general concepts of the invention described herein.

[0053] Example 1

[0054] First, the optical properties of various feed materials were recorded. The feed material for Sample A was raw coarse sand that was chemically "dirty" (e.g., contained more than 2% by weight of contaminants). The feed material for Sample B was -100 mesh size sand that was 99% by weight SiO2. The feed material for Sample C was SILVERBOND 325 ground crystalline silica, a high-purity quartz feed material commercially available from Covia, containing approximately 99.5% by weight SiO2 and ground to -325 mesh size. The optical properties of the dry materials were measured using an X-rite 9600 reflectance spectrophotometer using a 25 mm diameter optical glass cuvette. The results are reported in Table 1. Bulk density (loose) was measured according to ASTM C-29 and is reported in grams per milliliter (g / mL).

[0055] [Table 1]

[0056] Comparative Sample D was prepared by subjecting Sample A feedstock to heat treatment. Specifically, without first grinding and agglomerating Sample A feedstock into fine particles, a quantity of Sample A sand was combined with 0.2 wt. % sodium hydroxide and heated to 1525°C for 60 minutes in a rotary furnace to convert the quartz-derived SiO2 to cristobalite. Optical properties and bulk density were measured as described above and are reported in Table 1. Photographs of Sample A and Comparative Sample D are provided as Figures 1A and 1B, respectively.

[0057] As can be seen from Figures 1A and 1B, black spots appear in the coarse cristobalite coarse sand after heat treatment. These black spots are iron oxides, which appear yellowish in the Sample A feed but are believed to change from one form to another during heating, resulting in black spots. Mica and other minerals present as contaminants in the Sample A feed also appear as spots and are present as black spots in Comparative Sample D. Thus, although the cristobalite itself is white and appears lighter compared to the feed material, the black spots can render the product unsuitable for its intended use. Simply heating the feed material does not reduce the appearance of the black spots.

[0058] Samples E, F, and G were then prepared by granulating the feed materials of Samples A, B, and C, respectively. Specifically, a certain amount of Samples A, B, and C was ground to approximately -325 mesh, which is equivalent to a range of approximately 0.5 μm to 50 μm and has an average particle size of approximately 10 to 15 μm, granulated with 5 wt. % water glass (40% sodium silicate), and fired in a rotary kiln at 1350 °C for 60 minutes. Samples E and F were granulated using a laboratory-grade granulator, while Sample G was granulated using an Eirich granulator. Optical properties and bulk densities were measured as described above and are reported in Table 2. Photographs of granulated Samples E and F are provided in Figures 1C and 2, respectively.

[0059] [Table 2]

[0060] Comparing Figures 1C and 1B, it can be seen that milling and agglomeration are effective in eliminating the appearance of black spots in the granules. For Sample E, the treatment increased the brightness of the product (e.g., L) compared to both Comparative Sample D and the feed material of Sample A. * Similarly, the treatment increased the lightness (L * The values ​​in Table 2 thus demonstrate that the processes described herein are effective in producing a brighter product that does not have black spots and that is not subjected to a cleaning or contaminant removal process.

[0061] Furthermore, comparing Figures 1C and 2 and the values ​​in Tables 1 and 2, it can be seen that although the material in Sample F is brighter than the material in Sample E, the feed material from which Sample F is made is of higher quality than the feed material from which Sample E is made. However, the resulting granules in Sample E have an L greater than 92. * values ​​are shown, suggesting that these granules are at least comparable to conventional solar reflective roofing granules.

[0062] Although the granulation process significantly reduces the bulk density of Samples E-G compared to the feed material of Samples A-C, it is believed that the bulk densities of Samples E and F may be significantly lower due to the use of a laboratory granulation process instead of using a commercial grade granulator as in Sample G.

[0063] L * , a * , and b * In addition to the values, the average visible light reflectance for each of the above samples was measured over wavelengths from 360 nm to 750 nm. The % reflectance (Y-axis) as a function of wavelength (X-axis) is shown graphically in Figure 3. The average visible light reflectance for each of the samples is reported in Table 3.

[0064] [Table 3]

[0065] As shown in Table 3 and Figure 3, the average visible light reflectance is comparable between the granulated materials in Samples E and F, and the reflectance is significantly higher than the reflectance of the feed material for Samples A and B (not shown in Figure 3). Compared to granulated Samples E and F, the average visible light reflectance is significantly lower for Comparative Sample D, which was neither milled nor granulated. Sample G exhibited the highest average visible light reflectance value, which can be attributed to the use of a commercial-grade granulator. It is believed that the commercial-grade equipment used to prepare Sample G resulted in rounder, more consistent granules compared to the laboratory-grade equipment used to prepare Samples E and F.

[0066] Example 2

[0067] Next, the effect of the flux on the cristobalite product was further investigated by varying the amount of sodium silicate. For each sample, 5 lb (2268 g) of quartz silica powder with an average particle size of 325 μm was added to an Eirich mixer, and the appropriate amount (see Table 4) of technical-grade sodium silicate (40% solution) was weighed into a 500 mL graduated cylinder. The graduated cylinder was filled to 500 mL with distilled water, and the contents of the graduated cylinder were added to the Eirich mixer. The Eirich mixer was closed and operated at "low" speed for approximately 1 minute, followed by an additional 2 minutes at "high" speed. The center blade was then stopped, and the bowl was rotated for an additional minute. Approximately 125 g of additional quartz silica powder was then sprinkled in, and the bowl was rotated for an additional minute before the mixture was removed from the mixer.

[0068] [Table 4]

[0069] The mixture was dried overnight in a convection oven with the fan running but no heat. The resulting mixture was screened to obtain granules having a size of -8+35 (i.e., larger than 35 mesh and smaller than 8 mesh). Approximately 250 g of the screened granules was placed in a platinum dish and placed in a box furnace set at a predetermined temperature (see Table 5) for 1 hour. The dish was then removed from the oven and allowed to cool to room temperature.

[0070] [Table 5]

[0071] As shown in Table 5, for each sodium silicate concentration between 2 wt% and 8 wt%, and for each temperature between 1300°C and 1400°C, the samples exhibited L values ​​between 93 and 98. * Value, a between 1.0 and 1.6 * Value, b between 3.3 and 4.8 * Values ​​were shown with a brightness greater than 79 and a specific gravity of about 2.32 to about 2.40.

[0072] X-ray powder diffraction (XRPD) analysis was performed on each of the samples in Table 5 to determine the morphology of the resulting granules. Each sample was weighed out and divided into three splits of 4 g. Each split was mixed with 20 wt% (1 g) corundum powder and milled to ensure optimal particle size distribution. A Philips X'Pert Pro PW3040 diffractometer equipped with a variable divergence slit was set to a 16 mm exposure area, a 15 mm incident beam mask, and primary and secondary Soller slits of 0.04 radians. Samples were loaded into a sample holder with an inner diameter of 27 mm. Measurements were performed using Cu-Kα radiation (40 kV and 40 mA, step size: 0.026° 2θ in the range 5–64° 2θ, total measurement time: 62 min) in Bragg-Brentano geometry using continuous scan mode with a rotating sample holder (8 rpm).

[0073] The BGMN program was utilized for Rietveld refinement using the Profex graphical user interface. The BGMN program is described in detail in J. Bergman et al., "BGMN - a new fundamental parameters based Rietveld program for laboratory X-ray sources, its use in quantitative analysis and structure investigations," CPD Newsletter 20, no. 5 (1998): 5-8, the entire contents of which are incorporated herein by reference. Structural parameters for the analyzed phases were obtained from the Crystallography Open Database and the BGMN structure file repository. For each specific phase, the lattice, peak broadening parameters, and preferred orientation (where warranted) were refined within physically allowed parameters. Corundum was used as an internal standard to approximate the amorphous fraction in the samples.

[0074] XRPD analyses were performed in triplicate and the average weight percentages of each of cristobalite, quartz, tridymite, and amorphous silica relative to the total amount of silica for each sample are provided in Table 6.

[0075] [Table 6]

[0076] As shown in Table 6, the combination of sodium silicate and heat treatment at temperatures between 1300°C and 1400°C was sufficient to convert crystalline quartz material to silica containing primarily cristobalite (e.g., greater than 60 wt% cristobalite relative to the total amount of silica). Each of the samples also contained less than 5 wt% quartz (minimum less than 1 wt%, or even less than 0.5 wt%), 2 wt% to 22 wt% tridymite, and 15.5 wt% to 25 wt% amorphous silica relative to the total amount of silica. The sample containing 2 wt% sodium silicate (Formulation 1) treated at 1300°C contained approximately 3.8 wt% quartz, demonstrating that increasing the amount of sodium silicate, increasing the temperature, or both was sufficient to convert more than 99 wt% of the quartz. However, cristobalite was the predominant crystalline form for all of the samples tested.

[0077] In many cases, the commonly used commercial term refers only to the crystalline portion of the roofing material, and in such cases, the cristobalite portion of all crystalline silica contents of the test samples was greater than 73%, and in samples formed using 2%, 4%, and 6% sodium silicate, cristobalite ranged from 85.3% to 96.6% of the crystalline content of the samples, as shown in Table 7.

[0078] [Table 7]

[0079] Granule strength was also tested. Granules were prepared and screened as described in Table 4 above to obtain granules with a size range of -8 to +35, and calcined at temperatures of 1300°C, 1350°C, or 1400°C. Approximately 20 g of each sample was then measured and dispensed into the bottom of a crush cell using a crush cell funnel. The plunger was inserted into the bottom of the cell and allowed to settle. The filled crush cell was then placed on the lower press surface, and the filled cell was placed on top of the crush cell plunger. The Monogram benchtop controller was then started, tared, and the lever was pumped until the filled cell contacted the upper surface. The lever was then pumped until the controller registered 1,000 lb of force. The sample was subjected to a force for 5 minutes; the lever was gently tapped as needed to maintain a force between 1,000 lb and 1,050 lb throughout the five minutes. When the timer expired, the force was released, and the crush cell was carefully opened.

[0080] The sample was poured onto a #35 mesh screen with a hard pan at the bottom. The screen was placed on a sieve shaker at speed 5 for 1 minute, turning as necessary to ensure good coverage of the sample on the screen. The sample on the screen was then discarded and the weight of the fines in the hard pan was measured and recorded. Each sample was repeated in triplicate. The average percent of fines produced is reported in Table 8 below. The percent of fines produced was calculated by dividing the mass of fines by the initial sample mass and multiplying by 100.

[0081] [Table 8]

[0082] As shown in Table 8, the percentage of fine particles produced by the fracture of fine grains decreased as the amount of sodium silicate increased, regardless of the calcination temperature, indicating that the fine grains became stronger as a result of the increased amount of sodium silicate.

[0083] While only a few embodiments of the present invention have been described above, it should be recognized that many modifications can be made without departing from the spirit and scope of the invention. All such modifications are intended to be included within the scope of the present invention, which is limited only by the claims that follow.

Claims

1. A process for producing white silica-based products, A step of grinding the supply material to form ground supply material having a size of approximately 0.5 μm to approximately 50 μm; The process involves combining the ground feed material with sodium silicate and water to prepare a coagulated mixture; A step of forming the white silica-based product by heating the aggregated mixture to a temperature of about 1300°C to less than 1550°C over a certain period of time, wherein the aggregated mixture is such that cristobalite is the dominant silica crystal structure in the white silica-based product and the white silica-based product has a color temperature of 93 to 98 L in the CIELAB color space. ＊ As shown in the value, the process is to heat thoroughly and A process that includes this.

2. The process according to claim 1, wherein the sodium silicate is present in an amount of about 0.2% to about 10% by weight relative to the total weight of the agglomerated mixture.

3. The process according to claim 1, wherein the water is present in an amount of about 10% to about 20% by weight relative to the total weight of the coagulated mixture.

4. The process according to claim 1, wherein the agglomerated mixture further comprises sodium hydroxide.

5. The sodium silicate and the water are sprayed The process according to claim 1, applied to the ground feed material.

6. The process according to claim 1, wherein the heating includes heating the agglomerated mixture to a temperature of about 1350°C to about 1400°C for a period of time of about 40 minutes to about 60 minutes.

7. The process according to claim 1, wherein the heating is effective in sintering the particles of the aggregated mixture into fine particles.

8. The white silica-based product is cooled to a temperature of approximately 100°C or less. Applying at least one coating to the fine particles of the white silica-based product at a temperature of approximately 75°C to approximately 100°C. The process described in any prior claim, further including the process described in any prior claim.

9. The process according to claim 1, further comprising applying at least one coating to the fine particles of the white silica-based product at a temperature below 50°C.

10. The process according to claim 1, further comprising sieving the white silica-based product to select white silica-based fine particles having an average particle size of about 0.45 mm to about 2.5 mm.

11. The aforementioned sieving is effective in producing oversized white silica fractions and undersized white silica fractions, and the process is A step of combining the oversized white silica fraction and the undersized white silica fraction; The process involves grinding the combined fractions to produce a white silica-based powder having a maximum mesh size of 325 mesh. The process according to claim 10, further comprising:

12. In the CIELAB color space, L values ​​of 93 to 98 ＊ Roofing material granules containing white silica-based granules exhibiting a value, wherein cristobalite is the dominant crystalline structure of silica in the white silica-based granules.

13. The roofing material granules according to claim 12, wherein the roofing material granules have an average particle size of about 0.45 mm to about 2.5 mm.

14. The roofing material granules according to claim 12, wherein the roofing material granules have a specific gravity of about 2.32 to about 2.

40.

15. The roofing material granules according to claim 12, wherein the roofing material granules have a functional coating on the outer surface of the white silica-based granules.

16. White silica-based fine granules, Visible light reflectance greater than 75% in the 360 ​​nm to 750 nm wavelength range, and below: L 93-98 in the CIELAB color space ＊ value; or The silica crystal structure is predominantly cristobalite; or The fine particles have a specific gravity of approximately 2.32 to approximately 2.

40. Showing at least one of the following: White silica-based fine granules.

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