|Title of Invention||
AN APPARATUS AND METHOD FOR PRODUCING CALCITEE AND ARAGONITE PILYMORPHS OF PRECIPITATED CALCIUM CARBONATE FROM HIGH PH ASH WATER
|Abstract||This invention identifies a solution process for the production of Precipitated Calcium Carbonate (PCC) from high PH ash water. PCC is formed by reaction of the dissolved calcium hydroxide present in ash water under multi stage reaction with carbon dioxide. This method will enable production of high purity PCC of calcite and aragonite polymorphs and more consistent crystal size.|
|Full Text||4. FIELD OF THE INVENTION
This invention relates to an apparatus and method for the production of caicite and aragonite polymorphs of Precipitated Calcium Carbonate (PCC) using high PH ash water.
One finished calcium product of particular interest is precipitated calcium carbonate, which is salable in a growing market at high value. A method for the manufacture of PCC from waste ash water would enjoy market, environmental benefits and economic advantages over currently practiced technology.
PCC is used in paper manufacturing for increasing the brightness of paper, as filler material in plastics manufacture, as additive in inks and paints, etc on large scale. In general, the value of the PCC product increases as the size of the CaC03 particles decreases. The value also increases with increasing brightness and with increasing dryness. The caicite form of the CaC03 crystal is preferred to the aragonite form because of the greater thermal stability and brightness of the caicite form.
Pulverized coal fired boilers/ furnaces are used to produce steam for driving the steam turbine generators in coal fired power stations and for process requirements in process industry. Both fly ash and bottom ash are produced in the process of burning coal. Generally the ash (if not used in dry form) is mixed with water to create ash water slurry and pumped to an ash disposal area. The water from ash (i.e. ash water) is drained or decanted from the ash disposal pond as waste effluent. The PH of ash water is as high as 13 since quick lime / calcium oxide present in dry ash got dissolved in ash water. Thus the dissolved Ca(OH)2 in ash water is present and also rendering it as wastewater.
It is, therefore, an object of the invention to utilize ash water and to provide a method for the production of precipitated calcium carbonate by utilizing the dissolved calcium hydroxide present in ash water.
OBJECTS OF THE INVENTION
It is an object of the invention to utilize or maximize the value of ash water to provide a method for the production of PCC by utilizing the dissolved calcium hydroxide present in ash water.
It is further an object of the invention to provide a method for the control of the crystal size and morphology of precipitated calcium carbonate produced from lime solutions.
It is further an object of the invention to provide a method for making a variety of higher-value PCC products from ash water.
It is further an object of the invention to provide a method for making waste ash water suitable for utilization such as make up water to open re-circulation cooling water systems and as feed water to produce de-mineralized water.
It is further an object of the invention to provide a method for making waste ash water suitable for discharging in to natural stream meeting the statutory pollution stipulations.
SUMMARY OF THE INVENTION
The invention involves the utilization of ash water for production of high-value calcium materials.
(a) Filtering the high pH ash water to remove suspended and floating solids.
(b) This filtered ash water is passed from step (a) through a channel with diverging funnel cross section to achieve different desired flow velocities along the funnel length. Generally Carbon Dioxide gas with 95% purity is used. Exhaust gas from fuel fired boilers / gas turbines / IC engines which consist of Carbon Dioxide gas is also used if flue / exhaust gas is available nearby. The flue gas is water washed to remove dust particles and the sulfur gases (SOx) The flue gas / CO2 gas is injected from the bottom in multiple stages along the ash water flow path in the funnel to react with dissolved calcium hydroxide in the ash water producing slurry of PCC.
The size of PCC crystals depends on the ash water velocity and temperature of water at the point of CO2 injection/reaction. Also the crystals growth of PCC can be monitored / regulated by injecting additional quantity of CO2 in the down stream of ash water which has lesser flow velocity to facilitate crystal growth. In this method, the water velocity, CO2 mass injection both for crystal formation and crystal growth can be regulated to achieve any type of PCC crystals ranging from aragonite to calcite. In this method, calcite form crystals are formed when the ash water temperature is below 25.degree. C and aragonite crystals are formed above this temperature.
Moreover in this method the crystal size is uniform without wider variation since the chemical process is continuous type and using the readily available dissolved lime in the water. This method is in contrast to the conventional PCC manufacture where fresh solid form lime has to dissolve in to water to be available for reaction with CO2. The conventional method is of cyclical type reaction (with respect to time) taking few minutes. The claimed method is continuous, instantaneous and uniform in reaction with respect to time facilitating production of uniform crystal size PCC.
(c) Introducing flocculation aids into the resultant slurry of calcium carbonate crystals of step (b) to promote weak association of the calcium carbonate crystals.
(d) dewatering the associated calcium carbonate crystals by gravity separation in which the resultant of step (c) is introduced into a quiescent environment sufficient to allow settling of the associated calcium carbonate crystals to form an essentially clear solution and to form a thickened slurry of the associated calcium carbonate crystals.
(e) filtering or centrifuging the thickened slurry resulting from step (d) to further remove water to form a thickened slurry or an essentially solid filter cake of calcium carbonate crystals.
(f) Optionally drying the resultant of step (e) by evaporating the water from the calcium carbonate crystals in the filter cake under conditions to remove moisture there from.
In current practice, CaC03 is not produced using ash water. Instead, precipitated CaC03 is made by reacting CO2 gas with hydrated lime produced from mined materials that are already free of large amounts of contaminants. These hydrated lime sources usually have a Ca(OH)2 content exceeding 96 wt. % or more. In summary, the hydrated lime is prepared by a special method to ensure its quality and reactivity by the following reaction sequence:
This is a cumbersome method and requires substantial energy to convert the high quality limestone (CaCCO3) to lime (CaO) in reaction (1). Additionally, calcium must be transported in several stages beginning at the mine, then to a central process facility and to the place where it is precipitated.
The CaCO3 of the invention is of high market value and also solves an historic problem with disposal of polluted ash water.
Another point of departure is the use of multi point reaction system to form the precipitated calcium carbonate. In conventional crystalIizers, the reaction system is generally mixed or stirred in a batch-type system. In the present invention, a multi stage chemical reaction is used to exploit the kinetics of calcium carbonate crystal formation and to improve the consistency in crystal growth.
The proposed multi stage chemical reaction permits a better/ accurate control over the crystal size distribution in the product. This is possible because the residence time and reactant concentrations can be controlled throughout the crystal growth process. The result is a size distribution where the size of smallest crystals differs from the largest crystals by a factor of only about five. This compares better with currently available commercial products wherein the size distribution range is factor of about ten.
The multi point chemical reaction also allows the crystal to be grown in different predetermined reaction systems during its growth. This is done by achieving reaction in two or more stages. Each stage may have a different amount of CO2 and/or a different system of additives to control crystal growth and morphology. Greater control over the system may also be provided by having a plurality of carbon dioxide injection sites along the length of ash water flow path.
As a solution system, the present invention provides advantages over conventional slurry or solid systems. The calcium hydroxide available in ash water is totally in solution that in turn can be used to form a high-purity, high-value calcium carbonate product.
In the method of the invention the calcium hydroxide solution is highly dilute. (The solubility of Ca(OH)2 is only 0.185 g/100 g water at 0 degree C) Therefore, when the solution is reacted to precipitate a less soluble calcium product, such as the reaction with carbon dioxide to produce calcium carbonate, the precipitated product is separated from any impurities that may be dissolved in the ash water. In the preferred embodiment, precipitated calcium carbonate is fractionally precipitated away from more soluble salts that remain in solution. Thus, by practice of the present invention a very pure product can be produced.
Another advantage of a solution reaction system and compared to a slurry reaction system, is that the reaction time of CO2 with calcium hydroxide in solution is much faster than with solid calcium hydroxide in a slurry. The reaction times in the present invention are of the order of seconds, compared with tens of minutes up to an hour or more with slurry systems.
Other advantages of solutions system over slurry system are the better control over the progress of chemical reaction thus producing uniform crystal structure of the precipitate. This is because in the multi stage reaction system, at any point along the ash water flow path from initial introduction of carbon dioxide until the complete growth of the crystals, the progress of the reaction can be precisely known/ monitored. This advantage is not possible in slurry system. In slurry system the reaction rate of the calcium hydroxide and the carbon dioxide will depend upon the rate of dissolution of the calcium hydroxide slurry into solution, which cannot
be precisely predicted or controlled. In contrast, in the multi stage reaction system, the available calcium hydroxide is completely in dissolved condition and properties of the system, such as the reactions rates, concentrations, etc., are more precisely determinable.
While, the preferred calcium-containing compound produced by the present invention is calcium carbonate, the present method is suitable for the production of certain other calcium compounds that can be produced from Ca(OH)2. Such compounds are those that have a very low solubility in water, specifically those less soluble than Ca(OH)2. Examples of such compounds include, but are not limited to those shown below in Table A. Also shown is a possible reagent for reaction with Ca(OH)2 to produce the compound. The reagents shown are illustrative.
TABLE A Examples of Calcium Products from Calcium Hydroxide in ash water
Calcium Product Reagent
Calcium Carbonate (PCC) CO2
Calcium Fluoride HF
Calcium Oxalate Oxalic Acid
Calcium Diorthophosphate H3PO4
An economic criterion for production of calcium-based products using ash water is that they have a high market value in relationship to the cost of reactants. The price/cost relationship establishes a measure of the potential value. In the present method, the source of the Ca(OH) 2 reactant is generally the waste ash water. Accordingly, the production cost depends mostly on the cost of the other reactants. It has been found that as a general rule that a higher mass contribution from the Ca(OH)2 and a lower mass contribution from other reactants will produce a more commercially viable product. For example, for CaC03, the first product in Table A, the contribution of Ca(OH)2 to the final product, CaC03, is 56 wt. %. In contrast, for the other calcium products the contribution is lower. Thus, more of the value for these calcium products that derives from the anionic reactant. The method is commercially viable only if the calcium product has a higher value than the reactant used with the calcium hydroxide.
The order of the listing in Table A shows the ranking of higher mass contributions to lower mass contributions. Heading the list is calcium carbonate. Calcium carbonate has afforded this advantage because of the relatively low molar mass of the anion (C03.sup.-2) and the fact that effectively one of the oxygen atoms contained in the Ca(OH)2 is used in the final product. In other words, more calcium-based product is made from a given amount of reactant (CO2) than any other commonly traded product. In addition, the reactant, CO2 is easily and cheaply obtained as a by-product of combustion of fossil fuels.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a process flow diagram illustrating a calcium carbonate process of the invention.
FIG. 2 is a general arrangement of CO2 reactor of a Multi stage injection used in the method of the invention. FIG. 3 shows the interior view of the CO2 reactor.
DETAILED DESCRIPTION OF THE INVENTION
The calcium hydroxide in the filtered ash water solution is made to react with carbon dioxide gas to form PCC. This is done by contacting the solution with carbon dioxide using multi stage CO2 injection in a controlled way in the CO2 reactor. In order to achieve sufficient residence time in the reactor, the reactor may be lengthened. The unreacted CO2 is collected from the top surface of the water and reused in the reactor with the help of a blower.
The reason for the multi stage reaction system is to exploit the reaction kinetics of the system to preferentially produce relatively small, uniformly sized CaC03 crystals. It has been found that if there is a large driving force or fluid velocity, crystal formation/nucleation is taking place, e.g., by a high concentration of both reactants, new crystals are preferentially formed as opposed to a growth of existing crystals. On the other hand, if the driving force / water velocity is small, e.g., one reactant (CO2) is gradually introduced over the length of the reaction, crystal growth is preferred to nucleation.
Thus, the best reaction for uniform and small crystals is to start with a high water velocity for crystal formation, followed by a smaller velocity for crystal growth. This is achieved by in multi stage reactor where water velocity is high at the start of reaction and reduces later for crystal growth. When first contacted when the CO2 and Ca(OH)2reactants are mixed at a maximum concentration, the reaction is very rapid, forming a large number of very small crystals. The ash water stream, now of reduced velocity in the down stream, favors crystal growth. Growth of the initially formed crystals is favored, rather than new crystal formation. By adding and dispersing CO2 in the proper stoichiometric amount and maintaining uniform flow, the crystals are of uniform and small size.
In contrast, in a continuous stirred-tank reactor system, the reactants are continuously back mixed, mixing solution depleted of reactants with new solution entering the system. The driving force for nucleation is only modest and significant crystal formation and crystal growth occurs throughout the residence time of the reacting solution. Thus, the crystal distribution is wide, ranging from small newly formed crystals to large, older crystals. This leads to wide variations in the progress of reaction and results in crystals of wide size distribution.
The higher valued form of calcium carbonate is calcite, which is preferentially formed at lower temperature, 30.degree. C. or less, preferably between 5.degree. C. and 25.degree. C. The reaction stream may also be treated with known additives to aid desirable crystal formation, association, and/or size distribution. The solubility of calcium carbonate is much lower than that of calcium hydroxide. Thus, the calcium carbonate readily precipitates from the solution forming slurry in the reacted solution.
Immediately after reaction, the calcium carbonate slurry is preferably treated with conventional additives to prevent fusing of the crystals, or disproportionate growth, i.e., the re-dissolving of small thermodynamically unstable crystals and the re-precipitation of the re-dissolved CaC03 on other crystals until they become more thermodynamically stable and to promote association of the crystals.
It is also contemplated in the invention to use crystal growth modifiers to control the size and morphology of the crystals. These materials are incorporated in the crystal or interfere with the surface so as to interfere with growth kinetics and growth morphology. Preferably, the process is controlled to produce a crystal size range from 0.1 microns to 5 microns, although narrower ranges are possible by practice of the invention, e.g., from 0.1 to 0.3, or 1 to 2 microns.
The calcium carbonate slurry is then concentrated by introduction into a settling environment, using conventional settler/clarifier and thickener technology. The rate of settling may be enhanced by use of cationic, anionic or non-ionic flocculating agents. The dewatered or concentrated sludge is then filtered by conventional means to form essentially solid filter cakes, which are purified calcium carbonate.
The dewatered particles in the filter cake are then dried to form an anhydrous product. The drying is by any suitable means, such as a ring dryer, drum dryer or spray dryer. Before drying the dewatered particles may optionally be wet-milled by conventional techniques to disassociate or disperse the particles. In addition, the particles may be coated by known additives before drying, including but not limited to, dispersants. The purity of the calcium carbonate product meets the highest industrial standards, e.g., for use as whiteners in paper making, paints, pigments, plastics, and the like. The calcium carbonate is bagged or shipped
by conventional techniques according to industry practice.
In general, the value of the CaCO3 product increases as the size of the CaC03 particles decreases. The value also increases with increasing brightness and with increasing dryness. The calcite form of the CaCO3 crystal is preferred to the aragonite form because of the greater thermal stability and brightness of the calcite form.
An effective method is one, which takes these objectives into account and is so configured as to provide the smallest and whitest particle and to dry the product as price/cost benefits allow.
A method of the invention was conducted as laboratory study, essentially as described in the description above.
The calcium carbonate product was analyzed physically and chemically from a series of runs by conventional techniques. Typical results are summarized in Table B;
While this invention has been described with reference to certain specific embodiments and examples, it will be recognized by those skilled in the art that many variations are possible without departing from the scope and spirit of this invention, and that the invention, as described by the claims, is intended to cover all changes and modifications of the invention which do not depart from the spirit of the invention.
1. An apparatus for producing Calcite and Aragonite polymorphs of Precipitated Calcium
Carbonate (PCC) using high PH ash water, wherein
the apparatus is devised using multi stage CO2 gas injection in continuous production process to exploit the kinetics of calcium carbonate crystal formations as herein described to produce the calcite and aragonite polymorphs of Precipitated Calcium Carbonate (PCC).
2. A method for producing Calcite and Aragonite polymorphs of Precipitated Calcium
Carbonate (PCC), the method comprising;
(a) Filtering the ash water to remove floating and suspended solids.
(b) Reacting the filtered ash water with a reagent to form a purified precipitated calcium product with solubility less than calcium hydroxide as herein described, wherein the reagent and the calcium product are chosen from the list shown in the table below;
Calcium Product Reagent
Calcium carbonate Carbon Dioxide (CO2)
Calcium Fluoride HF
Calcium Oxalate Oxalic Acid
Calcium Diorthophosphate H3PO4.
and calcium hydroxide is sourced for producing purified precipitated calcium products from ash water which is an industrial wastewater.
|Indian Patent Application Number||90/DEL/2007|
|PG Journal Number||31/2009|
|Date of Filing||15-Jan-2007|
|Name of Patentee||NALLAPANENI SASIDHAR|
|Applicant Address||F.NO. 104, 152/2/2, CHINAR RESIDENCY WHEELER ROAD FRAZER TOWN BANGLORE-5|
|PCT International Classification Number||C01F11/00|
|PCT International Application Number||N/A|
|PCT International Filing date|