WO2016011668A1

WO2016011668A1 - Method for producing ceramic tiles using coal combustion waste

Info

Publication number: WO2016011668A1
Application number: PCT/CN2014/083058
Authority: WO
Inventors: Alex Koszo; Peter Ma; Erik Severin
Original assignee: Shandong V-Tong Science And Technology Co., Ltd.
Priority date: 2014-07-25
Filing date: 2014-07-25
Publication date: 2016-01-28
Also published as: CN107207352A

Abstract

The present invention relates to a process of producing ceramic tiles using coal combustion products, comprising the steps of size classification, declustering, magnetic separation, mixing, pressing, aggressive drying, oxidation of impurities, and firing. The method uses standard equipment. The drying step aggressively causes steam channels to open in the material, which are used by escaping CO₂ when carbon impurities are oxidized.

Description

METHOD FOR PRODUCING CERAMIC TILES USING COAL COMBUSTION WASTE

Field of the Invention

The invention relates to an improved process for the production of ceramic tiles using industrial wastes, particularly coal combustion wastes.

Background of the Invention

Disposal of the products from coal combustion is a long-term problem associated with burning coal to generate steam in electrical power stations. In power stations across the world, over a billion tons of coal ash is generated each year. There is a desperate need for useful, high-value outlets of this waste material. Low-value uses include roadway fill, asphalt extender, and flow-able fill for mineshafts. These outlets are not sufficient to absorb all the coal combustion waste that is generated each year and they are low in economic value. Additionally, these applications only apply to regions experiencing infrastructure growth. A small fraction of this waste is suitable for mid- value applications such as extenders for cement, but this uses only the finest material and is not applicable to all coal ash types.

The present invention relates to a process of producing ceramic tiles using coal combustion products such as fly ash. Fly ash is fine ash produced in the burning of coal to make electricity that rises up the exhaust stack to be captured by electronic precipitators. The finest fly ash is utilized by cement companies, but the coarser material is simply landfilled. The other ashes produced when burning coal are bottom ash and boiler slag.

Some ceramic tiles contain waste material that is generated in the manufacturing of tiles themselves, such as grinding dust and broken or unfired tiles. This material is put back into the original feedstock and used to extend body composition material. However, no tiles are commercially being made from waste that is unrelated to ceramic tile production. Ceramic tiles for interior and exterior floors and walls are a large market, and one without commercially viable environmentally friendly products. Tiles using this inventive method, made from waste material (particularly waste material unrelated to the tile industry itself) will be welcomed because they will be considered the first environmentally sustainable ceramic tile solution that can be made with commercially viable profit margins.

The products produced by the inventive process are ceramic materials of different size and shape, glazed or unglazed, planar or curved as required. The main applications for the products of this process are floor and wall tiles, building cladding, roof tiles, or pressed bricks and street pavers. These products can be unglazed for industrial use or decorated with glaze for aesthetics. The products are distinguished by their environmentally beneficial body composition since they are made from recycled material from industries other than the ceramics industry.

Summary of the Invention

The main object of this invention is to provide an improved process to produce ceramic tiles using industrial wastes, such as coal combustion products, whereby the cost of raw materials are reduced in production.

Another object of the inventive process is to enable production of ceramic tiles while reducing the depletion of scares ceramic raw material resources.

Another object of the invention is to enable production of ceramic tiles from waste streams unrelated to the ceramics industry thereby reducing overall pollution in the environment.

The disclosed process comprises the steps of treating the ash in the following way: particle size classification, de-clustering particles, magnetic separation, admixing ingredients, spray drying, hydraulic pressing to form green tiles, aggressive drying to form steam channels, heating to intermediate temperatures to burn-out organic impurities via the formed steam channels, and then final firing of the tiles.

Specifically, the invented method uses a set of process steps that allow using intermediate percentages (about 30%-50% by weight) of coal ash in the body material to make ceramic tiles using standard production equipment.

Description of Figures

Figure 1. A general flow chart of the process for producing ceramic tiles using coal ash.

Figure 2. Standard Firing Curve in the prior art.

Figure 3. Example Firing Curve in the inventive method.

Prior Art

Published patent application US 2003/0183988 Al describes a process for the production of ceramic tiles using a blend of iron ore slime, fly ash, blast furnace slag, additional aluminum silicates, and a select group of mineral additives. In addition to requiring a mixture of materials, which may not be readily available, the application teaches a drying step of 10 - 15 hours, which would cause this process to be uneconomical.

US 2576565 teaches the use of fly ash and boiler slag to make ceramic tiles. The invention utilizes the burning out of excess carbon from the mixture; however, the invention requires boiler slag that can act like a flux on the material. In addition, the invention teaches the use of raw ash with high levels of carbon, therefore only a controlled ratio of fly ash to boiler slag can be used in the tile body material. Fly ash with higher carbon content must be used at a lower ratio to the boiler slag.

According to the invention, high carbon content leads to an undesirably porous and weak tile. To achieve proper oxidation of the organic impurities, it teaches that long firing times of 24— 36 hours (and up to 4 weeks in one example) at about 1000 °C are required to sufficiently burn out carbon before vitrification. These long firing times are expensive and inefficient.

US 5175134 discloses a ceramic tile using sludge slag, which is the ash residue from burned organic sludge that has been melted, ground up, and then formed into a clay-like materials by admixing alkali oxide additives to help reduce the firing temperature. This process is inefficient and expensive because the slag ash is first melted, then ground, and then heated again in a firing kiln. Additionally, the added ingredients (such as borax) need to be mixed in, melted, and then ground to give a chemically consistent frit.

US 5521132 teaches ceramic material made from waste coal and municipal waste incinerator ash. The invention requires particular and expensive admixtures be blended with the ash, such as sodium tetraborates, calcium containing triple superphosphate, and dolomitic lime. These additives limit the practicality of this invention, both in terms of material sourcing and economics.

US 5935885 teaches a process in which fly ash containing organic material, metallic contaminates, and glass-forming material is oxidized to combust the organic material and then is completely melted (requiring temperatures up to 1550 °C), formed, and pulverized to form uniform glass cullet and stored. To make tiles, the cullet is heated until molten again and poured into molds. The power, time, and equipment needed for complete melting, forming cullet, and then re-melting, and forming makes this an uneconomical process.

US 6342461 teaches the use of fly ash and clay to bind heavy metal waste that is found in electric arc furnace dust, steel slag, aluminum dross, and paper ash to make ceramic materials. These other components aid in the formation of the tiles by lowering the melting point of the mixture and increasing plasticity of the body and green strength of the dried material. These additives are required for the feasibility of this invention.

US 5227047 teaches a process of beneficiating fly ash using techniques that include low-gauss (≤ 10,000 G) magnetic separation and flotation. However, these separations are carried out in separate dedicated devices that are not part of a contiguous process. Additionally, the iron and carbon need to be reduced to an impractically low level. To achieve this, they started with waste material that was already low in iron (0.4%) and carbon (1.6%). Additionally, this patent does not address size separation. US 3533819 and 3769054 teach dry separation techniques that include air classifying in order to obtain coarse material to be used in artificial aggregate formation for use in concrete. These techniques make use of size separation, but they utilize the larger material instead of the smaller. The patent teaches nothing about removing other impurities in the waste material, nor does it teach how to optimize the process to obtain the desired small particles used in clay replacement materials.

The above processes have other disadvantages in addition to the ones described. Therefore, the industry must continue to seek improved processes to make ceramic tiles from recycled material (especially from waste generated outside the industry). Such a process should use less energy, lower the cost of production, and provide high quality ceramic tiles made from an abundant waste material - such as coal ash.

Detailed Description

The incoming coal ash is classified by size using an air classifier or an automatic screening system. Other methods of classification are also possible and could be used by one skilled in the art without compromising the inventive nature of this process. This removes the bulk of impurities leaving behind less contaminated aluminum silicates. Regardless of the manner in which the ash is classified, the final ash product after classification will be sized under about 50 microns, for example, about 45 microns or less, have carbon content below about 3% by weight, and iron content below about 5% by weight.

After classification, the ash is homogenized with water (for example, about 60% by weight solids and about 40% by weight water) in a mixer or mill. The ash slurry is then sent through an iron separator at 20,000 Gauss to remove iron species. Alternately, if the iron content is low (below about 3%) a dry magnetic separation step can be used.

The ash slurry is then mixed with a standard mix of clay and binder in a mixer or a mill. The final mixture comprises, for example, of about 40% by weight ash, about 50% by weight standard clay, and about 10% by weight high plasticity clay (such as bentonite or other clays with an Atterberg's plasticity index above 25). Additional additives can include feldspar or other minerals that modulate the sintering temperature in the final tile. To this mixture is added about 1% by weight of an organic binder. Possible organic binders include polyvinyl alcohol, superplasticizers, methylcellulose, carbomethoxy cellulose, or dextrin. Other binders will be known to those skilled in the art. The amount of binder is empirically determined for each mixture to impart sufficient green strength in the pressed items.

To these ingredients is added enough water to make the final water percentage about 35% solids by weight. The mixture is homogenized in a mixer or a mill for 10— 15 hours (typically about 13 hours). The slurry is then spray-dried using the standard single-effect method using air for drying at about 400°C. The dry granules can be put into storage until use. The material is delivered to a standard ceramic hydraulic press in the standard way using conveyors and other equipment. The press machine is a standard machine used in the standard way. Each pressing consists of three separate actions of the hydraulic ram: the first two presses are used to compact and de-air the ceramic body, which allows the final press (at a pressure of about 300 kg/ cm²) to form an article with little trapped air, high green strength, and few laminations.

After pressing, the green tiles are conveyed to a dryer where they are aggressively dried to cause steam channels to open in the body material. The aggressive drying is adjusted so that it is not so aggressive as to burst the tile, yet aggressive enough to cause the formation of steam channels that will be later used by escaping gases as organic material burns out of the tile.

As an example of this aggressive drying, a tile goes into a dryer at about 90°C and stays at that temperature for about 4 - 5 minutes, whereupon it is heated to 175°C - 225°C, preferably about 200°C, over 4 - 5 minutes. The tile stays at this temperature for about 5 - 10 minutes. This drying procedure forces steam out of the tile body forming micro-channels in the green body. These channels are utilized later when the tile is heated to about 450°C and then to about 850°C to oxidize the carbon impurities to C0₂. The channels allow the CO_zto escape without causing bloating in the tile body.

After the formation of steam channels, the temperature is raised sufficiently to burn out the easily oxidized carbon impurities. The tile is heated to 400°C - 500°C, preferably about 450°C, over about 4 - 5 minutes and it is kept at that temperature for an experimentally determined period, but typically about 10 - 20 minutes to burn out easy to oxidized carbon. This heating step can be performed in the drying oven if the equipment allows, or it can be the first temperatures zone in the firing kiln. The rate of this oxidation of carbon to C0₂ must be slow enough not to overwhelm the steam channels and thus cause bulges or blisters. The resulting C0₂ uses the pre-formed steam channels to escape.

After the initial carbon oxidation, tile is conveyed into the kiln (if the first oxidation was performed in the dryer) for further carbon oxidation and then final vitrification. The second stage of carbon oxidation is done in the kiln, and is sufficiently hot to burn out the remaining carbon impurities. For example, the tile temperature is raised to 900 - 1000°C, preferably about 950°C, and it is kept at that temperature for an experimentally determined period, but typically about 5 - 10 minutes, in order to burn out the last carbon impurities. The resulting C0₂ again uses the pre-formed steam channels to escape.

After this lengthened pre -heat zone, the remainder of the firing curve is similar to other firing curves used in ceramic manufacturing, but with the addition of a custom cool-down period. As in a standard firing curve, the tiles are heated to 1170 - 1220°C (for example 1200°C) over about 4 - 6 minutes (a heating rate of about 50 - 75 degrees per minute) and held at this temperature for about 5 - 10 minutes. Controlled cooling is then performed in an industry standard way but with an additional hold at 635°C for three minutes to allow crystal changes to occur within the tile body. The remaining cooling can be uncontrolled.

Tile shaping, QA/ QC, and packaging stages follow the main firing and the standard methods are used. The total time of firing is between about 60 to about 80 minutes (not including uncontrolled cooling time), preferably about 70 minutes. This is comparable to traditional ceramic tiles firing curves. One skilled in the art will know these techniques and procedures.

In an embodiment, the body composition mixture comprises about 30 - 50 % by weight ash, about 40 - 50% by weight standard clay and about 10 - 20% by weigh high plasticity clay and water.

In an embodiment, the mixture further comprises binder, preferably, organic binder, more preferably, liquid organic binder.

In an embodiment, the organic binder includes, superplasticizer, polyvinyl alcohol, methyl cellulose, carbomethoxy cellulose, or dextrin.

In an embodiment, the organic binder preferably is in an amount of about 0.1 - 1% percent by weight.

In an embodiment, the high plasticity clay comprises bentonite.

In an embodiment, the drying step is conducted aggressively enough to form steam channels in the tile body but not so aggressive to cause bloating.

In an embodiment, the carbon oxidizing steps at 400 - 500°C and at 900 - 1000°C are conducted slowly enough to allow C0₂ to escape through produced steam channels (experimentally determined but typically about 10 minutes each). For the lower temperature step, instead of a hold at the desired temperature, a slow rise in temperature (over 15 min) can be conducted if the level of carbon is low.

Examples

1000 kg of coal ash was classified by size to remove all material that would not pass through a 325- mesh screen (i.e. all particles smaller than 45 microns). The ash was transferred to a ball mill fitted with alumina balls and to this was added 650 liters of water. The slurry was tumbled in the ball mill for one hour to de-cluster the material. The slurry was then passed through a 35,000 Gauss magnetic separator. The slurry was transferred to another ball mill and to it was added 1250 kg of standard clay, 250 kg high-plasticity clay, and 25 kg of binder. An additional 250 liters of water was added such that the final slurry was about 35% water and about 65% solids. These ingredients were milled for 13 hours to form a homogeneous mixture.

The mixture was granulated by spray drying in air using the standard equipment. The dry granules were conveyed to the hydraulic press using the standard system. The press was a standard ceramic tile press.

The press was automatically filled with the dry granules and pressed in a continuous manner. The pressing consisted of three presses of the top punch into the material. The first two presses were incomplete presses (no pressure developed) that were used to compact the material. The final press reached a pressure of 280 kg/ cm² and formed the final tile with sufficient green strength to be further processed.

The tiles were conveyed into the dryer where they were dried at 250°C for 10 minutes, the tiles were conveyed to the kiln where they were heated to 450°C for 15 minutes, then the temperature was raised to 950°C for 10 minutes, then they were fired at 1200°C in the standard method.

The tiles produced met all international criteria for ceramic tile. The American Society for Testing and Materials (ASTM) code is provided. The details of the methods are well known and are publically available from ASTM:

Abrasion resistance (ASTM C1027): Class 4 and above

Water Absorption (ASTM C373): < 3 % for vitreous tile

Breaking Strength (ASTM C648): >250 lbf

Claims

1. A process of producing ceramic tiles using coal combustion products, comprising the steps of: size classification, de-clustering, magnetic separation, mixing ingredients, pressing, aggressive drying, oxidation of impurities, and firing.

2. The process of claim 1, wherein the coal ash is size classified or reduced to be smaller than 50 microns.

3. The process of Claim 1, wherein the ingredients are homogenized by milling together in a ball mill.

4. The process of claim 1, wherein in the drying step, aggressive drying is conducted at about 90°C for about 4-5 minutes, and 250°C for about 5 - 15 minutes.

5. The process of any of claims 1-3, wherein the mixture comprises about 30 - 50 % by weight of coal ash, about 250 - 350% by weight of standard clay, about 10 - 20% by weight of high plasticity clay, 10 t-15% feldspar, and water.

6. The process of any of claims 1-3, wherein the mixture further comprises organic binder.

7. The process of claim 5, wherein the organic binder includes superplactisizer, polyvinyl alcohol, methyl cellulose, carbomethoxy cellulose, or dextrin or any combination thereof.

8. The process of claim 5 or 6, wherein the organic binder is in amount of about 0.1 - 1% percent by weight.

9. The process of claim 4, wherein the high plasticity clay is bentonite.

10. The process of any of claims 1-3, wherein the drying is aggressive and forms open micro- channels for the leftover impurities to escape during carbon oxidation and firing.

11. The process of any of claims 1-3, wherein the tile is heated to 400 - 500°C to oxidize lightly bound organic impurities and then heated to 800 - 900 to oxidize the remaining organic impurities.

12. The process of claim 10 wherein the heating steps are conducted at 400 - 500°C and at 800 - 900°C for about 10 minutes each.

. A ceramic tile produced by the method of any one of claims 1-11