Systems and Methods to Treat Flue Gas Desulfurization Waste to Produce Ammonium Sulfate and Calcium Carbonate Products

Abstract

Disclosed herein are system and methods for producing a high purity ammonium sulfate product as well as either a lower or a high purity calcium carbonate product by reacting flue gas desulfurization (FGD) gypsum feedstock in batch or continuous mode using synthesized ammonium carbonate from ammonia and carbon dioxide gases. The high purity calcium carbonate is obtained by using a precipitation catalyst, filtering out the impurities, then precipitating a high purity calcium carbonate. Alternatively, the lower purity calcium carbonate may be dissolved in acid, impurities filtered out, then a high purity calcium carbonate is precipitated out using a soluble carbonate salt.

Claims

1. A processing system, comprising: a mixer for mixing ammonia gas, carbon dioxide, and flowing water resulting in an ammonium carbonate reagent solution; one or more reactors, wherein the one or more reactors comprises at least a first reactor, for reacting calcium sulfate with the ammonium carbonate reagent solution to form a reacted slurry; a filter for separating calcium carbonate from the reacted slurry resulting in a filtrate of ammonium sulfate liquor and a calcium carbonate residue; a dryer for drying the calcium carbonate residue to produce a calcium carbonate product; an evaporator followed by a crystallizer for forming ammonium sulfate crystals from the filtrate resulting in the ammonium sulfate crystals and processed liquor; a centrifuge for separating the processed liquor from the ammonium sulfate crystals resulting in separated ammonium sulfate crystals and saturated ammonium sulfate liquor, wherein the saturated ammonium sulfate liquor is recycled to the evaporator and evaporator condensate is recycled to the first reactor; and a dryer for drying the separated ammonium sulfate crystals resulting in ammonium sulfate product.

2. The system of claim 1, wherein the ammonia gas is injected into the mixer one of before and after the carbon dioxide.

3. The system of claim 1, wherein mixer is one of in-line and stirred tank reactor.

4. The system of claim 1, wherein conductivity is measured to determine ammonium carbonate concentration in the ammonium carbonate reagent solution, and wherein conductivity in the range of 80-90 milli-siemens per centimeter indicates ammonium carbonate concentration of 15%.

5. The system of claim 1, wherein pH is monitored in the mixer and wherein the pH is in a range of 8.7-9.0.

6. The system of claim 1, wherein between 140% and 150% stoichiometric amount of ammonium carbonate reagent solution is used to react with the calcium sulfate.

7. The system of claim 1, wherein the ammonium sulfate product is greater than 99% pure.

8. The system of claim 1, further comprising: an agglomerator to increase particle size of the ammonium sulfate product for flowability; and an additive to reduce the absorption of water by the ammonium sulfate product.

9. The system of claim 1, wherein the calcium carbonate product is 90-99% pure.

10. The system of claim 1, wherein the carbon dioxide is sourced from power plant emissions.

11. A processing method, comprising: mixing ammonia gas, carbon dioxide, and flowing water with a mixer resulting in an ammonium carbonate reagent solution; feeding the ammonium carbonate reagent solution and calcium sulfate into one or more reactors, wherein the one or more reactors comprises at least a first reactor, to form a reacted slurry; filtering the reacted slurry resulting in an ammonium sulfate filtrate and a calcium carbonate residue; drying the calcium carbonate residue to produce a calcium carbonate product; using an evaporator followed by a crystallizer to forming ammonium sulfate crystals from the ammonium sulfate filtrate resulting in separated ammonium sulfate crystals and processed liquor; using a centrifuge to separate the processed liquor from the ammonium sulfate crystals resulting in separated ammonium sulfate crystals and saturated ammonium sulfate liquor, wherein the saturated ammonium sulfate liquor is recycled to the evaporator and the evaporator condensate is recycled to the first reactor; and drying the separated ammonium sulfate crystals resulting in ammonium sulfate product.

12. The method of claim 10, wherein the ammonia gas is injected into the mixer one of before and after the carbon dioxide.

13. The method of claim 10, wherein the mixer is one of in-line, and stirred tank reactor.

14. The method of claim 10, wherein conductivity is measured to determine ammonium carbonate concentration in the ammonium carbonate reagent solution, and wherein conductivity in the range of 80-90 milli-siemens per centimeter indicates ammonium carbonate concentration of 15%.

15. The method of claim 10, wherein pH of the ammonium carbonate reagent solution is monitored in the mixer and wherein the pH is in a range of 8.7-9.0.

16. The method of claim 10, wherein between 140% and 150% stoichiometric amount of ammonium carbonate reagent solution is used to react with the calcium sulfate.

17. The method of claim 10, wherein the ammonium product is greater than 99% pure.

18. The method of claim 10, further comprising: using an agglomerator to increase particle size of the ammonium sulfate product for flowability; and adding an additive to reduce the absorption of water by the ammonium sulfate product.

19. The method of claim 10, wherein the calcium carbonate product is 90-99% pure.

20. The method of claim 10, wherein the carbon dioxide is sourced from power plant emissions.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0015] A more complete understanding of the systems, methods, processes, and/or apparatuses disclosed herein may be derived by referring to the detailed description when considered in connection with the following illustrative figures. In the figures, like-reference numbers refer to like-elements or acts throughout the figures.

[0016] FIG. 1 depicts an embodiment of a production plant for implementing an FGD gypsum conversion process.

[0017] FIG. 2 is a table showing the composition of an FGD gypsum feedstock used in preliminary testing.

[0018] FIG. 3 depicts a particle size distribution analysis for the FGD gypsum feedstock used in preliminary testing.

[0019] FIG. 4 depicts kinetic data for varying reagent additions in preliminary testing of the FGD gypsum conversion process.

[0020] FIG. 5 depicts crystallized ammonium sulfate product assays for ammonium sulfate product generated in preliminary testing of the FGD conversion process.

[0021] FIG. 6 depicts example test conditions and results from preliminary testing of the FGD conversion process.

[0022] FIG. 7 depicts calculated final product generated in preliminary testing of the FGD conversion process.

[0023] FIG. 8 depicts a schematic of a pilot production plant operating in continuous mode.

[0024] FIG. 9 depicts calculated gypsum conversion with changing conditions in the pilot production plant depicted in FIG. 8.

[0025] FIG. 10 depicts discharge sulfur assays from the pilot production plant depicted in FIG. 8.

[0026] FIG. 11 depicts exemplary ammonium sulfate and calcium carbonate products produced by the pilot production plant depicted in FIG. 8.

[0027] FIG. 12 depicts a composition of an ammonium sulfate product produced by the pilot production plant depicted in FIG. 8.

[0028] FIG. 13 depicts an embodiment of a calcium sulfite oxidation process added to FIG. 1 to treat the FGD gypsum feedstock prior to feeding into the FGD gypsum conversion process.

[0029] FIG. 14 depicts an embodiment of an acid dissolution calcium carbonate whitening process.

[0030] FIG. 15 depicts a whitened calcium carbonate product produced by the calcium carbonate whitening process depicted in FIG. 14.

[0031] FIG. 16 depicts an example embodiment of a process for using a catalyst to separate impurities from calcium carbonate product produced by the FGD conversion process.

[0032] Elements and acts in the figures are illustrated for simplicity and have not necessarily been rendered according to any particular sequence or embodiment.

DETAILED DESCRIPTION

[0033] Although the disclosure described herein is susceptible to various modifications and alternative iterations, specific embodiments thereof have been described in greater detail herein. It should be understood, however, that the detailed description of the systems and methods is not intended to limit the disclosure to the specific embodiments disclosed. Rather, it should be understood that the disclosure is intended to cover modifications, equivalents, and alternatives falling within the spirit and scope of the disclosure. In the following description, and for the purposes of explanation, numerous specific details, process durations, and/or specific formula values are set forth in order to provide a thorough understanding of the various aspects of exemplary embodiments. However, it will be understood by those skilled in the relevant arts that the apparatus, systems, and methods herein may be practiced without all of these specific details, process durations, and/or specific formula values. It should be noted that there are different and alternative configurations, devices, and technologies to which the disclosed embodiments may be applied. The full scope of the embodiments is not limited to the examples that are described below.

[0034] In the following examples of the illustrated embodiments, references are made to the accompanying drawings which form a part hereof, and in which is shown by way of illustration various embodiments in which the systems, methods, processes, and/or apparatuses disclosed herein may be practiced. It is to be understood that other embodiments may be utilized and structural and functional changes may be made without departing from the scope.

[0035] Headings are for organizational purposes only and are not intended to be limiting. Embodiments described under the various headings herein are interoperable with embodiments under other headings.

[0036] Overview

[0037] Disclosed herein are systems and methods for reacting flue gas desulfurization (FGD) gypsum (calcium sulfate) feedstock, in either batch or continuous mode, with ammonium carbonate reagent to produce commercial products wherein the commercial products comprise ammonium sulfate and calcium carbonate. The systems and methods described herein are highly beneficial to the coal industry in that they produce higher value products from coal waste. The primary reaction is shown in equation 1 below.

CaSO.sub.4.2H.sub.2O (insoluble)+(NH.sub.4).sub.2CO.sub.3 (soluble).fwdarw.(NH.sub.4).sub.2SO.sub.4 (soluble)+CaCO.sub.3 (insoluble)+2H.sub.2O(1)

[0038] FIG. 1 depicts an embodiment of a production plant 100 for implementing an FGD gypsum conversion process resulting in at least two commercial products. In the depicted embodiment, FGD gypsum (calcium sulfate) feedstock is fed, either in batch or continuous mode, into a reactor cascade 105 (comprising reactors 110, 111, 112, and 113) with ammonium carbonate reagent, which may be synthesized from ammonia and carbon dioxide gases or supplied as a powder. In some embodiments, the FGD gypsum feedstock may be fed to the system using a quantitative powder feeder or a gravimetric feeder optionally coupled to a screw feeder (not shown). In some embodiments, the FGD gypsum feedstock is in powder form. In embodiments where the FGD gypsum feedstock is moist it may require drying prior to feeding to avoid blockages in the feeder. In some embodiments, the FGD gypsum feedstock may be dried to 7% by weight or less moisture content.

[0039] The number of reactors in the reactor cascade 105 may vary depending on throughput required, the size and type of reactors, and the reaction time needed. In some embodiments, there may be between three and five reactors. As an example, for a two-hour reaction with four reactors having total volume V, the scaled total volume needed would be 4/3 V for three reactors and 2V for two reactors. The same rule applies when increasing the number of reactors. In some embodiments, the size of the reactors 110, 111, 112, and 113 may be reduced using weirs.

[0040] The one or more reactors 110, 111, 112, and 113 may be connected in overflow mode (material overflows from the top of a reactor to the next reactor) or underflow mode (material flows from the bottom of a reactor to the next reactor), or material may be transferred using one or more pumps between the one or more reactors. In some embodiments, the one or more reactors 110, 111, 112, and 113 may be continuously stirred tank reactors (CSTRs), stirred tank reactors, and/or in-line (located in a transfer line) reactors. In some embodiments, the first reactor 110 may be a small, high intensity reactor to thoroughly mix the FGD gypsum feedstock and reagent, followed by two to three (larger, in some embodiments) reactors 111, 112, and/or 113 to hold the mixture long enough for the reaction to reach completion (i.e. 99+% conversion of FGD gypsum feedstock) resulting in a reacted slurry. In the depicted embodiment, the reactor cascade 105 vents ammonia gas from the ammonium carbonate reagent through vent 115a to the scrubber 117. Either water or between 0.01 to 0.1M sulfuric acid may be used in the scrubber 117. The ammonia from the vents 115a-e dissolves in water to yield ammonium hydroxide or, in the case of sulfuric acid, the ammonia reacts to form ammonium sulfate. The ammonium hydroxide or ammonium sulfate from the scrubber 117 may optionally be recycled back into the reagent feed line into reactor 110, in some embodiments.

[0041] After the reaction has reached completion, the reacted slurry is pumped, underflows, or overflows from the reactor cascade 105 into a filter 120 resulting in calcium carbonate residue and ammonium sulfate filtrate liquor. Wash water is pumped through filter 120 in the depicted embodiment Ammonia off-gases from the filter 120 vent through vent 115c to scrubber 117. In some embodiments, filter 120 may be a drum filter or other similar continuous filter. The calcium carbonate residue from filter 120 proceeds to dryer 125 to produce calcium carbonate product. In the depicted embodiment, dryer 125 vents through vent 115c ammonia to scrubber 117. In some embodiments, the calcium carbonate product may be further processed. Further processing options are discussed in the Examples.

[0042] In the depicted embodiment, ammonium sulfate (AS) filtrate liquor proceeds from filter 120 to evaporator 130 where water is evaporated from the ammonium sulfate liquor, and then to crystallizer 135 where ammonium sulfate crystals are produced in ammonium sulfate liquor (also referred to as processed liquor). Centrifuge 140 separates the ammonium sulfate crystals from the ammonium sulfate liquor (processed liquor) resulting in separated ammonium sulfate crystals and saturated ammonium sulfate liquor. Dryer 145 dries the separated ammonium sulfate crystals resulting in ammonium sulfate product. The dryer 145 vents through vent 115e to scrubber 117. In some embodiments, saturated ammonium sulfate liquor is pumped from the centrifuge 140 back into the evaporator 130. Overheads or vapors coming off the top of the evaporator 130, containing excess ammonium carbonate reagent, may optionally proceed through a condenser 150 (evaporator condensate) to be recycled back into the reactor cascade 105 to react with the FGD gypsum feedstock thus reducing reagent demand and reducing waste streams. In the depicted embodiment, water is pumped into the reactor cascade 105 and into the ammonia scrubber 117. In the depicted embodiment, all off-gases, including water vapor and ammonia in some embodiments, vent through vents 115a, 115b, 115c, 115d, 115e to ammonia scrubber 117.

[0043] In some embodiments, the ammonium sulfate may be vacuum evaporated, the salt allowed to crystallize out, and the solid product is then filtered using a solid/liquid separation device. The conditions in the crystallizer 135 may be controlled to produce larger crystals which are more desirable in some markets. The ammonium sulfate product may be greater than or equal to 99% pure. The ammonium sulfate crystallization and the centrifuge separation processes may be continuous.

[0044] Filter 120 and centrifuge 135 are both solid/liquid separators and may be substituted by other solid/liquid separators in other embodiments. For example, a belt filter may be used in place of filter 120 and a rotating drum filter may be used in place of the centrifuge 135. In some embodiments, a spray dryer may be used in place of the evaporator 130 and crystallizer 135. The spray dryer evaporates the water and forms small crystals all in one step. Continuous filtration systems other than those depicted in FIG. 1 may be utilized in the process. The equipment used in the process may be sized to fit the desired input/output. Material transfer between processes/equipment may be carried out with the use of pumps, etc.

[0045] Reagents

[0046] In the embodiment depicted in FIG. 1, ammonium carbonate reagent is synthesized using ammonia (NH.sub.3) and carbon dioxide (CO.sub.2) gases in flowing water. In some embodiments, the NH.sub.3 and CO.sub.2 gas are injected in the stoichiometric ratio of 2:1 respectively. The gases may be introduced sequentially using gas nozzles into a flowing water stream in either a batch process or a continuous process. The gases are best fed sequentially with the NH.sub.3 first followed by the CO.sub.2 because NH.sub.3 is more soluble in water than CO.sub.2 and CO.sub.2 is more soluble in ammonium hydroxide than in plain water. This order of gas introduction into the water has been found to reduce the chances of an ammonia gas release. In alternative embodiments, the order of gas introduction into the water may be reversed. Sequential feed of the NH.sub.3 and CO.sub.2 gases reduces chance of clogging in the gas nozzle; however, the NH.sub.3 and CO.sub.2 gases may be premixed, in some embodiments. The NH.sub.3 and CO.sub.2 gases may be mixed with process water using a mixer 108 such as an in-line mixer or a reactor tank with mixer resulting in an ammonium carbonate reagent solution. In some embodiments, the gases may be fed directly into mixer 108.

[0047] The pH may optionally be monitored to ensure carbonate is formed (between pH 8.7-9.0), not bicarbonate. Conductivity and/or the specific gravity may be monitored using an electric conductivity meter and a hydrometer, respectively, to determine the concentration of ammonium carbonate reagent formed. Both conductivity and specific gravity increase as the concentration of the ammonium carbonate formed in solution increases. For example, for a 15% concentration of ammonium carbonate in solution, the conductivity is 80-90 mS/cm (milli-siemens/centimeter).

[0048] The resulting ammonium carbonate reagent may be fed directly into reactor cascade 105. In some embodiments, the ammonium carbonate reagent is added in excess (more than stoichiometric) to ensure the reaction goes to completion (i.e. that all the FGD gypsum feedstock is reacted). In some embodiments, 140% stoichiometric addition of the ammonium carbonate reagent results in the reaction going to completion. If the reaction is not complete, then the calcium carbonate product is contaminated with FGD gypsum feedstock.

[0049] Products

[0050] In some embodiments, to make the products more commercially attractive, the ammonium sulfate and/or the calcium carbonate products may be agglomerated in an agglomerator to larger, more flowable particles to facilitate product application. In some embodiments, the particles are several millimeters in size. In some embodiments the ammonium sulfate and/or calcium carbonate products may be further treated with coating agents, such as stearic acid and stearates, to improve their properties for specific markets, such as to reduce their moisture absorption. In some embodiments, the ammonium sulfate and/or calcium carbonate products may be treated with an additive to reduce the absorption of water.

[0051] Ammonium Sulfate

[0052] The ammonium sulfate product produced by production plant 100 (FIG. 1) may be used as a solution. In some embodiments, the ammonium sulfate product is greater than 99% pure. In some embodiments, the ammonium sulfate solid product is fertilizer grade Ammonium sulfate is primarily used in the global fertilizer industry as a soil amendment to replenish depleted levels of nitrogen and sulfur to the soil. An additional use in the fertilizer industry is as an adjuvant for various insecticides, herbicides, and fungicides Ammonium sulfate may also be used in non-agricultural products and processes such as for flameproofing of select materials, textile dyeing, a cattle feed supplement, and for certain water treatment processes.

[0053] Calcium Carbonate

[0054] The calcium carbonate product produced by production plant 100 (FIG. 1) is insoluble. In some embodiments, the calcium carbonate product may contain small amounts of impurities, such as carbon and iron, which may cause it to have a grey or tan color. In some embodiments, the calcium carbonate is 90-99% pure. In some embodiments, the calcium carbonate product may be further processed to obtain a higher purity white calcium carbonate product which typically has higher market value. Some exemplary calcium carbonate whitening processes are described in the examples under the heading Calcium Carbonate Processing.

[0055] Calcium carbonate has a plethora of uses in many diverse industries including: the oil and gas industry as drilling fluid make-up to increase the fluid density, as an additive to control fluid loss to formation, and in oilfield cementing as a loss circulation material; the building materials and construction industry for roofing shingles, tiles, and cement, brick, and concrete block manufacture; and commercial applications such as industrial filler in the paper, paint, plastics, and rubber industries.

[0056] Environmental Benefits

[0057] The processes described herein are environmentally sound with internal recycles and near zero waste. All parts of the processes where ammonia gas may be released may be exhausted to one or more water (or dilute sulfuric acid) scrubbers where the ammonia is recaptured and recycled to one or more of systems/processes. Coupling to an adjacent Haber process (a process for producing ammonia from nitrogen and hydrogen), in some embodiments, could minimize the amount of ammonia that would need to be stored on site thus reducing the hazards associated with storing large quantities of ammonia. Locating a production plant 100 (FIG. 1) near a source of carbon dioxide, such as a coal power plant in some embodiments, could allow around 10% by volume of the carbon dioxide emissions from the coal power plant to be utilized in the production plant 100 (FIG. 1) using a side stream taken from the exhaust stack. CO.sub.2 gas may be provided from other processes, plants, or sources including naturally occurring or stored CO.sub.2 gas which may be pumped from underground formations. Carbon capture is another potential environmental benefit of the processes described herein as CO.sub.2 gas is converted to a solid carbonate compound. In some embodiments, one or more internal recycles may be incorporated to recover reagents resulting in near-zero waste stream which is of significant environmental benefit.

EXAMPLES

[0058] Preliminary Testing

[0059] The systems and methods disclosed herein were first developed by testing batch reactions under different conditions to arrive at initial operating conditions for a continuous demonstration. The following data was generated in preliminary testing with a particular feedstock and should not be considered limiting. Other operating conditions are anticipated.

[0060] FGD gypsum feedstock from a typical coal power plant was used as the feedstock in preliminary testing. The composition of the FGD gypsum feedstock used in preliminary testing of the FGD conversion process is depicted in FIG. 2 and the particle size analysis of the FGD gypsum feedstock is shown in FIG. 3. Values shown <X are below detection limits, where X is the detection limit of the equipment used in the analysis.

[0061] Batch Process

[0062] In preliminary batch testing, FGD gypsum feedstock samples were slurried in water at 19% by weight solids and reacted with 15% concentration ammonium carbonate reagent solution at ambient temperature and pressure. Higher solids samples can also be used with equivalent increases in the ammonium carbonate reagent. Higher temperatures are not desirable because the ammonium carbonate reagent is less stable at higher temperatures. Kinetic data for varying reagent additions used in preliminary testing of the FGD conversion process, depicted in the chart in FIG. 4, shows that at 140%-150% stoichiometric additions of reagents to reactants the reaction between FGD gypsum feedstock and ammonium carbonate worked well and after one to three hours, at atmospheric pressure and ambient temperature, produced ammonium sulfate >99.9% in the liquor and 93-95% calcium carbonate product. When evaporated to dryness, the purity of the ammonium sulfate was >99.7%. Assays for the crystallized ammonium sulfate product produced in preliminary testing of the FGD conversion process are depicted in FIG. 5. The assay results were 99.7% or 99.9% depending on the assay method. Values shown <X are below detection limits, where X is the detection limit.

[0063] Test conditions and results of preliminary testing of the FGD conversion process are depicted in FIG. 6. Calculated final product generated in preliminary testing of the FGD conversion process is depicted in FIG. 7. Based on these tests, the optimum stoichiometry for the FGD conversion process was 140% to 150% and the FGD conversion reaction was complete after one to three hours. From 140% to 100% stoichiometry the reaction slows down as excess reagent is decreased. Stoichiometry lower than 100% resulted in less than 99% conversion of FGD gypsum feedstock, while higher than 150% stoichiometric resulted in wasted reagent. Variations in the composition of the feedstock may produce different results.

[0064] Continuous Process

[0065] As discussed herein, the FGD conversion process may be operated in a continuous mode. Continuous mode was demonstrated in a pilot production plant 800, depicted in FIG. 8, operated at an FGD gypsum feedstock feed rate of 1 kg/hr. Ammonium carbonate reagent was mixed by mixer 802 with water in vessel 805 to produce a 15% concentration ammonium carbonate solution that was pumped by pump 807 into the first reactor 810, operating in an overflow mode to three other reactors 811, 812, and 813, to provide sufficient reaction time for the conversion to go to completion. In some embodiments, material may be transferred between the reactors 810, 811, 812, and 813 using underflow, overflow, or a pump. The FGD gypsum feedstock was fed as a powder from bin 820 using a screw feeder 825 to the first reactor in the reactor cascade 822, comprising reactors 810, 811, 812, and 813, where it was mixed with the ammonium carbonate solution. The slurry is then kept in suspension by mixers 831, 832, and 833 in each reactor 811, 812, and 813 to allow sufficient time for the reaction to take place. The slurry overflowed from reactor 813 into a continuous filter 840 (alternating between two pan filters) to remove the solid calcium carbonate product (which was then washed) and the resulting filtrate, ammonium sulfate liquor, was collected in tank 845. The wash liquid was collected in tank 846.

[0066] The pilot production plant 800 depicted in FIG. 8 was operated at a constant 20 C.3 C. and a pH ranging between 7.5 and 8.5 for 110 hours (over the course of five days) at the following conditions: [0067] Condition 1A: 150% of the stoichiometric quantity of reactants, Day 1-2 [0068] Condition 2: 125% of the stoichiometric quantity of reactants, Day 2 [0069] Condition 1A: 150% of the stoichiometric quantity of reactants, Day 3 [0070] Condition 1B: 150% of the stoichiometric quantity of reactants+catalyst, Day 3 [0071] Condition 3: 140% of the stoichiometric quantity of reactants, Day 4 [0072] Condition 4: 150%, of the stoichiometric quantity of reactants and at double the feed rates (2 kg/hr), Day 4

[0073] FIG. 9 depicts calculated gypsum conversion with changing conditions in pilot production plant 800 (FIG. 8). These tests showed that: [0074] 140%-150% stoichiometric addition of reagent with respect to the quantity of reactants was sufficient for quantitative conversion. [0075] The catalyst addition reduced the reaction time. [0076] Doubling the feed rates of FGD gypsum feedstock reduced the reaction time.

[0077] FIG. 10 depicts discharge sulfur assays from the pilot production plant 800 (FIG. 8). Referencing FIG. 8, the majority of the conversion took place within the first two reactors 810, 811 (<1.5 hours for Conditions 1A and 3; and <0.75 hours for Conditions 1B and 4). The third and fourth reactors 812, 813 provided extra time to complete the reaction for the remaining gypsum.

[0078] The purity of the ammonium sulfate product produced in preliminary testing of the FGD conversion process was 99.9% (FIG. 7). The purity of the calcium carbonate produced in preliminary testing of the FGD conversion process was 93-95% (FIG. 7) with an average D50 particle size of 44 m. While the calcium carbonate product was of good purity, the small amounts of impurities tinted the product a grey to tan color. The impurities causing the color were carbon and iron which are dependent on the impurities in the FGD gypsum feedstock. FIG. 11 depicts ammonium sulfate and calcium carbonate products generated by the pilot production plant 800 (FIG. 8). FIG. 12 depicts the composition of the ammonium sulfate crystal product produced in the pilot production plant 800 (FIG. 8). Variations in FGD gypsum feedstock may produce different results.

[0079] Variations in Feedstock

[0080] FGD Gypsum Feedstock Mixed with Ash

[0081] In some embodiments, where the FGD gypsum feedstock is mixed with coal ash, the FGD conversion process can produce a high purity ammonium sulfate and a second product that is comprised of calcium carbonate and ash. This product can be marketed as such, particularly to building material applications, or further processed in other separation schemes. The processing system and methods for FGD gypsum feedstock that is mixed with ash is the same as that depicted in FIG. 1; however, the calcium carbonate product may be lower purity than that generated from an FGD gypsum feedstock that is not mixed with ash. The amount of ash in FGD gypsum feedstock that is mixed with ash affects the purity of the calcium carbonate product when FGD gypsum feedstock mixed with ash is used in the FGD gypsum conversion process.

[0082] Removal of Chloride from Flue Gas Desulfurization Gypsum Feedstock

[0083] Some FGD gypsum feedstock contains levels of chloride that are too high for certain applications. The excess chloride is removed from FGD gypsum feedstock through a process of water leaching, in some embodiments. Water leaching may be carried out at any temperature between room temperature (20 C.) and boiling (100 C.).

[0084] An example chloride removal process used in testing is described below. The following process could be scaled according to processing requirements. Testing was carried out at 75 C. with two water leaches. [0085] 1) First add 1000 g of hot 75 C. deionized water in a reactor. Add 250 g of FGD gypsum feedstock sample. The mixture results in a slurry. Equip reactor with lid and impellor. The reactor and/or lid may be glass in some embodiments. [0086] 2) Agitate the slurry for half an hour. [0087] 3) After half hour slurry time, filter the leached FGD gypsum feedstock solids and collect the filtrate. Record filtration properties. [0088] 4) Add 1000 g of hot 75 C. water to the reactor along with the solids from step 3. Agitate for half an hour. [0089] 5) After the half hour agitation time, filter out the leached solids from step 4 and collect the filtrate, record filtration properties. [0090] 6) Combine 25 mL of filtrate 1 (step 3) with 25 mL of filtrate 2 (step 5) and submit for assay. [0091] 7) Dry the leached FGD gypsum feedstock at 95 C. or lower until the weight does not change. [0092] 8) Submit samples for assay by inductively coupled plasma-mass spectrometry (ICP-MS) and Chloride analysis.

[0093] The results obtained on an FGD gypsum feedstock sample that contained around 0.5% by weight chloride, showed that >99% of the chloride can be leached out in the chloride removal process. The concentration of chloride in the wash water was 1033 ppm. The cations associated with the chloride were calcium at 894 ppm and magnesium at 166 ppm. The chloride level in the washed FGD gypsum feedstock was reduced to around 100 ppm.

[0094] There are several techniques to remove impurities from the filtrate after the water leach before discharge including ion exchange columns, reverse osmosis, and other similar deionization techniques known in the art.

[0095] A test was run to determine where the chloride in FGD gypsum feedstock winds up when processed through the FGD gypsum conversion process. In the test, FGD gypsum feedstock containing 0.5% by weight chloride was processed by reacting with ammonium carbonate to convert the calcium sulfate to calcium carbonate and ammonium sulfate. That test showed that the CaCO.sub.3 product had 16 ppm chloride and the ammonium sulfate had chloride at 434 ppm. The filtrate from the ammonium sulfate crystallization had 672 ppm chloride. On a weight percentage basis, the filtrate from the ammonium sulfate crystallization contains most of the chloride at 94.2%, the ammonium sulfate contained 5.2% and the calcium carbonate 0.6%. These results showed that water leaching to remove chlorides in the FGD gypsum feedstock prior to FGD conversion processing greatly enhances the qualities of the ammonium sulfate and calcium carbonate products by reducing the chloride impurity from 0.5% by weight to 100 ppm.

[0096] If the washed FGD gypsum feedstock was processed through the FGD gypsum conversion process depicted in FIG. 1, for example, negative impacts are not expected on the product quality due to chloride since 98% of the chlorides may be removed by washing.

[0097] Sulfite to Sulfate Conversion

[0098] Coal combustion products (CCP) are comprised of fly ash (fine particulates from the combustion process collected in filters), a lime or limestone absorption bed to clean out sulfur dioxide (SO.sub.2) gases, and bottom ash remaining behind after coal combustion. The absorption bed is converted to calcium sulfate after absorption of SOx and oxidation of calcium sulfite to calcium sulfate. The calcium sulfate is the FGD gypsum feedstock.

[0099] In some cases, the FGD gypsum feedstock may be in the form of a calcium sulfite slurry. In such embodiments, an oxidation step may be required to convert calcium sulfite to calcium sulfate. While there are several well-established methods to oxidize calcium sulfite to calcium sulfate, none have been coupled to a more comprehensive conversion process. The conversion of calcium sulfite to calcium sulfate (gypsum) is a well-developed technology, which is widely practiced and generally understood. There are a number of oxidation methods that may be coupled to the FGD conversion process depicted in FIG. 1. FIG. 13 depicts a modified production plant 100 (FIG. 1) with the addition of an oxidation step 1300 for calcium sulfite to calcium sulfate conversion prior to feeding into the FGD gypsum conversion process.

[0100] Forced Air Oxidation:

[0101] There are conventional sparger oxidation bubble towers which are expensive to build, can be up to 60 feet in height, and require 200% excess air to achieve complete conversion of calcium sulfite to calcium sulfite. A newer and less expensive approach uses air turbine oxidizer systems. These could be sited remotely and greatly reduce the conventional air oxidation retrofit. This process is also accomplished in an acidic environment. The calcium sulfite is extremely soluble in an acid medium and the sulfite ion in solution oxidizes very quickly in an agitated solution to a sulfate ion. Once the calcium sulfate forms, it precipitates to a gypsum slurry very rapidly. Other approaches use mechanical agitation for froth flotation with added air oxidation.

[0102] Air Oxidation Over Time:

[0103] Calcium sulfite will eventually convert to calcium sulfate when exposed to air and in the presence of water or in a slurry. The reaction is very slow and does not meet normal process requirements. However, inventories that have been stored outdoors for a long period of time may have mostly converted to calcium sulfate and can be used directly in the FGD gypsum conversion processes described herein. The mere fact that calcium sulfite is recognized as a mineral suggests that the sulfite to sulfate conversion kinetics are extremely slow.

[0104] Oxidation with Oxygen:

[0105] The oxidation of calcium sulfite to calcium sulfate can be accelerated by using oxygen in place of air. Oxygen concentrations as low as 5% by volume may be effective. In another embodiment, a low concentration of a metal ion is added as a catalyst to the reaction. An example would be 5 to 10 ppm ferric ion, manganese(II), or cobalt(II).

[0106] Hydrogen Peroxide Oxidation:

[0107] Sulfur dioxide, and/or its aqueous byproduct sulfite, can be oxidized to sulfate with hydrogen peroxide. The reaction occurs over a wide pH range but is faster at lower pHs. This is conducted in an aqueous medium and involves the oxidation of dissolved sulfite ion with peroxide to convert to the more insoluble sulfate. Calcium peroxide may be used in place of hydrogen peroxide.

[0108] Products

[0109] Calcium Carbonate Processing

[0110] Acid Dissolution

[0111] In some embodiments, the calcium carbonate product produced by the FGD gypsum conversion process may comprise contaminants such as iron, carbon, and silicates. When such contaminants are present, the calcium carbonate may proceed through further processing to remove such contaminants resulting in a purer product. In some embodiments, such as the acid dissolution calcium whitening system and process 1400 depicted in FIG. 14, the calcium carbonate product may be dissolved in dissolver 1402 in dilute acid (such as hydrochloric acid (HCl), nitric acid (HNO.sub.3), or another acid forming a soluble calcium salt). The basic reaction is shown in equation 2:

CaCO.sub.3(insoluble)+2HCl.fwdarw.CO.sub.2+Ca(Cl).sub.2 (soluble)+H.sub.2O(2)

[0112] The carbon dioxide generated by equation 2 in dissolver 1402, in the depicted embodiment, may proceed to scrubber 1405 containing sodium hydroxide to form sodium carbonate.

[0113] The mixture resulting from equation 2 may then be filtered by filter 1410 with solid impurities proceeding to dryer 1415 and liquids proceeding to reactor 1420. The dried solids may comprise carbon and silicates, in some embodiments. If an iron contaminant is present in the calcium carbonate product produced by the FGD conversion process, hydrogen peroxide (H.sub.2O.sub.2) may be added to reactor 1420 to oxidize ferrous ion to ferric iron. An amount of base such as calcium hydroxide (in depicted embodiment), sodium hydroxide, and/or sodium carbonate may also be added to reactor 1420 to raise the pH in the reactor to 3 or higher to precipitate ferric hydroxide. The advantage of using calcium hydroxide is that the amount of high purity precipitated calcium carbonate produced is increased by the amount of calcium neutralizing agent used, thus improving process economics. The amount of base added is the amount that is necessary to reach the desired pH value. This reaction with sodium hydroxide is shown in equation 3, below:

Fe.sup.+++H.sub.2O.sub.2+NaOH.fwdarw.Fe(III)(OH).sub.3 (insoluble)+Na.sup.+(3)

[0114] The slurry resulting from equation 3 in reactor 1420 may be filtered with filter 1425 to remove ferric hydroxide solids. In some embodiments, some carbon impurity may also filter out with the ferric hydroxide. In some embodiments, the ferric hydroxide is transferred to calciner 1430 resulting in a ferric oxide product. The filtrate from filter 1425 comprises a purified calcium chloride solution, or a mixed calcium and sodium chloride solution depending on the base used, which may then be combined with sodium carbonate, carbon dioxide, or another soluble carbonate, in reactor 1435 to produce precipitated calcium carbonate. The mixture may proceed through filter 1440 to separate solids and liquids. The solids may proceed through dryer 1445 to produce a white and high purity (>98%) precipitated calcium carbonate product. The precipitation reaction with sodium carbonate is shown in equation 4.

Ca(Cl).sub.2+Na.sub.2CO.sub.3.fwdarw.2NaCl+CaCO.sub.3(insoluble)(4)

[0115] The filtrate from filter 1440 may proceed through dryer 1455 to produce sodium chloride.

[0116] In some embodiments wherein HCl was used in the acid dissolution calcium carbonate whitening process, the economics of this purification of calcium carbonate can be significantly improved if the resultant NaCl filtrate is regenerated back to NaOH and HCl using a chlor-alkali cell process.

[0117] FIG. 15 depicts a whitened calcium carbonate product generated by the calcium whitening process depicted in FIG. 14.

[0118] Catalyst

[0119] In some embodiments, a catalyst to delay the formation of calcium carbonate may be added to the reactor cascade 105 (FIG. 1) so that impurities (or impurities plus ash, in some embodiments) may be filtered out before the precipitate is formed. The addition of a catalyst results in a fine white and high purity (>98%) precipitated calcium carbonate product.

[0120] FGD gypsum feedstock may comprise contaminants including carbon and/or fly ash, in some embodiments. An example embodiment of a process for using a catalyst to separate impurities from calcium carbonate is depicted in FIG. 16. In some embodiments, a quantity of a catalyst (0.5-7% by weight, in some embodiments) may be added to an FGD gypsum slurry mixture in a reactor 1610 wherein the FGD gypsum slurry mixture comprises a suspension in the range of 1% to 25% (4%, in some embodiments) weight by mass of FGD gypsum feedstock in water. The catalyst is allowed to mix, by means of a stirring mechanism in some embodiments, with the slurry for several minutes (5-30 minutes, in some embodiments). After mixing, an ammonium hydroxide solution may be added to the reactor vessel 1610 resulting in 1:1 ammonium hydroxide to slurry volumetric ratio. This addition of the ammonium hydroxide is immediately followed by the introduction of carbon dioxide gas at a rate of 4 L/minute1 L/minute, in some embodiments. The concentration of the ammonium hydroxide solution is chosen to be a concentration that will stoichiometrically react with all of the sulfate in the FGD gypsum slurry to form ammonium sulfate according to equation 5:

2NH.sub.4OH+CaSO.sub.4.2H.sub.2O+CO.sub.2-.fwdarw.[NH.sub.4].sub.2SO.sub.4+CaCO.sub.3+3 H.sub.2O(5)

[0121] The progress of the reaction can be followed by monitoring the pH which starts out at approximately 11.6 and with time drops to pH 7. At pH 7 all hydroxide has reacted and the solution is filtered (immediately, in some embodiments) through a 0.45 to 0.7 micron filter 1630. Filtration of the reacted FGD gypsum solution results in the separation of tramp fly ash and carbon from the resulting liquid comprising dissolved calcium carbonate and ammonium sulfate. The calcium carbonate in solution will separate from the ammonium sulfate solution in delay holding tank 1635 and can be collected by an additional filtration step 1640 using a 0.45 to 0.7 micron. In some embodiments, one or more of the filtration steps may be carried out using a filter composed of glass fibers.

[0122] The precipitation of calcium carbonate may be aided by seeding the solution with the desired crystalline form of calcium carbonate. In some embodiments, a small amount of product slurry may be recycled back to the reactor cascade 105 (FIG. 1). The seeds may be calcite. In some embodiments, the CaCO.sub.3 precipitate may be so fine it is nano-sized. In some embodiments, the solution containing the CaCO.sub.3 may be heated to cause the CaCO.sub.3 precipitate to coagulate to improve filtration. This process also allows a wider range of feedstocks such as FGD gypsum feedstock mixed with ash. The solution passing filtration step 1640 contains the ammonium sulfate which can be harvested by various crystallization methods known in the art. In some embodiments, a catalyst is used to slow down the precipitation of calcium carbonate in order to allow the solution to be filtered. Some of the catalyst may remain in the ammonium sulfate solution and/or the crystallized product. The catalyst does not react with the reactants therefore it may be recaptured and/or recycled, in some embodiments.

[0123] In some embodiments, the filtered ammonium sulfate solution may be returned to the beginning of the process to make up the FGD gypsum feedstock slurry. In some embodiments, the appropriate concentration of catalyst may remain in the recycled solution such that no further addition of the catalyst is necessary. In some embodiments, makeup catalyst may be added to the solution as needed.

[0124] The calcium carbonate whitening process with catalyst can also be performed in the production plant embodiment shown in FIG. 1 with some modifications. For instance, referring to FIG. 1, the calcium carbonate whitening process with catalyst may plug in in the place of filter 120. Reacted slurry from the reactor cascade 105 would proceed into reactor 1610 (FIG. 16) through the process depicted in FIG. 16 with the liquor from filter 1640 (FIG. 16) proceeding to evaporator 130 and the whitened calcium carbonate optionally proceeding through dryer 125. In some embodiments, the catalyst may be added to the reactor cascade directly and the reacted slurry with catalyst may proceed from the reactor cascade 105 to filter 1630 (FIG. 16) (i.e. reactor cascade 105 from FIG. 1 replaces reactor 1610 in FIG. 16).

[0125] To facilitate the understanding of the embodiments described herein, a number of terms are defined below. The terms defined herein have meanings as commonly understood by a person of ordinary skill in the relevant art. Terms such as a, an, and the are not intended to refer to only a singular entity, but rather include the general class of which a specific example may be used for illustration. The terminology herein is used to describe specific embodiments, but their usage does not delimit the disclosure, except as set forth in the claims.

[0126] Batch Process: A batch process operates in separate discrete operations that are connected in a stepwise fashion with the materials processed being fed in batches.

[0127] Catalyst: A catalyst is an agent that can either accelerate or decelerate a chemical reaction without reacting with the reactants or products.

[0128] Continuous Process: A continuous process is designed to operate without interruptions. The materials being processed, either bulk dry or fluids, are continuously in motion undergoing chemical reactions or subject to mechanical or heat treatment.

[0129] Having described and illustrated the principles of the systems, methods, processes, and/or apparatuses disclosed herein in a preferred embodiment thereof, it should be apparent that the systems, methods, processes, and/or apparatuses may be modified in arrangement and detail without departing from such principles. Claim is made to all modifications and variation coming within the spirit and scope of the following claims.