Abstract:
Magnesium hydroxide of high purity, having a fine particle size suitable for use in flame retardants and other high-end uses, is produced by utilizing flue gas desulfurization system process slurry as feedstock; the production process is absent energy-intensive steps as well as high-cost chemical usage.

Description:
FIELD OF THE INVENTION 
     The present invention relates to production of high-purity magnesium hydroxide from process slurry of a flue gas desulfurization operation utilizing magnesium-enhanced lime. 
     BACKGROUND OF THE INVENTION 
     Production of high-purity magnesium hydroxide (Mg(OH) 2 ) for uses such as flame retardants and pharmaceuticals by known processes involves numerous energy-intensive steps as well as high cost chemical usage. Various processes for such production are described in U.S. Pat. No. 5,476,642; No. 5,286,285; No. 5,143,965; No. 4,698,379; and No. 4,695,445. 
     In a flue gas desulfurization process, controlling alkalinity of process slurry prevents the formation of scale which is detrimental to the system. Magnesium-enhanced lime is used to increase alkalinity of the process slurry through the formation of magnesium sulfite (MgSO 3 ). Lime presenting a magnesium oxide (MgO) content is selected for such use to enable the formation of the magnesium sulfite. Such process slurry is a lime slurry containing magnesium ions. A portion of such magnesium sulfite laden slurry is bled from the desulfurization system to maintain slurry chemistry. Its disposal can present high costs and environmental concerns. 
     Such bleed-off slurry provides an economical source of feedstock for the production of high purity magnesium hydroxide. 
     SUMMARY OF THE INVENTION 
     The present invention provides for the production of high-purity magnesium hydroxide from process slurry of a flue gas desulfurization process. Such slurry is prepared using lime having a magnesium oxide content. Such magnesium presence enables formation of magnesium sulfite in the process slurry. Control of slurry chemistry during operation of the desulfurization system facilitates recovery of magnesium sulfite hexahydrate crystals (MgSO 3 .6H 2 O) presenting a low level of impurities. Such crystals are the source material for the high-purity magnesium hydroxide of the invention. 
     In the process of the invention magnesium sulfite hexahydrate crystals are converted to an aqueous solution of magnesium sulfate (MgSO 4 ) by a reaction with water and an oxidizing agent. The resulting aqueous solution is then reacted with a caustic solution to precipitate a magnesium hydroxide solid. Such solid precipitate is then separated from the reaction medium solution by filtering means to obtain a magnesium hydroxide filter cake. Entrained solution in the cake is removed and a drying process is used to eliminate residual water from the high-purity magnesium hydroxide product. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Specific procedures of the invention are described in more detail with reference being made to the accompanying drawing. 
     In the drawing: 
     FIG. 1 is a flow diagram for describing processing steps carried out in a preferred semi-continuous embodiment of the invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Increasing environmental concerns require desulfurization of industrial flue gases. Such desulfurization process can be carried out using flue gas scrubbing means wherein a gaseous stream containing sulfur dioxide is contacted by an aqueous scrubbing medium referred to as a desulfurization process slurry. Referring to FIG. 1, flue gas scrubbing system indicated generally at reference numeral  10 , comprises flue gas desulfurization unit  11  through which sulfur containing gas flows for contact with the aqueous slurry. Such slurry is recirculated through the unit by means of pump  12 . Slurry chemistry, during continuous operation, is maintained by addition of chemical components of the slurry in combination with removal or “bleed-off” of a portion of the slurry as indicated at  13 . Such bleed-off slurry, which in prior practice presented disposal concerns, provides a source of feedstock for production of high-purity magnesium hydroxide. Magnesium, in the form of magnesium sulfite is present in the slurry to control alkalinity and prevent the formation of scale in the desulfurization system. The slurry is prepared using lime having a presence of about 2-6% by weight of magnesium oxide. Such magnesium oxide leads to the formation of the magnesium sulfite in the slurry during system operation. 
     Control of slurry chemistry during continuous operation of the system facilitates recovery of high-purity magnesium sulfite hexahydrate crystals from the slurry bleed-off. Slurry chemistry promoting such high-purity hexahydrate crystal formation include: 
     1. magnesium ion content between about 5,000-12,000 ppm, 
     2. sulfite content of between about 3,000-18,000 ppm, and 
     3. a pH of between 6.0 and 7.0. 
     Those conditions as well as optimum conditions, described below, of a crystallizer of a subsequent step, are found in Canadian Patent No. 2,133,694 which is incorporated herein by reference. 
     In such subsequent step, carried out in crystallizer  14 , bleed-off slurry, which is supersaturated with respect to magnesium sulfite, is treated by lowering the temperature thereof to between about 15 to 40° C. and adjusting the pH to between about 7.0 to 7.5. Such conditions promote production of magnesium sulfite hexahydrate crystals (MgSO 3 .6H 2 O) as opposed to magnesium sulfite trihydrate crystals (MgSO 3 .3H 2 O). Such hexahydrate crystals, after forming, are separated from the remaining aqueous solution at dewatering unit  15 . H 2 O is removed at line  16  and magnesium sulfite hexahydrate crystals are removed at line  17  for further processing. A low level of impurities characterize these crystals and they provide an excellent source for production of high-purity magnesium hydroxide. Additional steps required for purifying starting materials in prior practice processes are eliminated by such source and the process of the present invention. 
     Results of chemical analysis of such hexahydrate crystals for impurities is presented in Table I. 
     
       
         
               
               
               
             
           
               
                   
                 TABLE I 
               
               
                   
                   
               
             
             
               
                   
                 Cl −   
                 Less than 100 mg/kg 
               
               
                   
                 NO 3   
                 Less than 46 mg/kg 
               
               
                   
                 Ca ++   
                 Less than 60 mg/kg 
               
               
                   
                 Fe 
                 Less than 7 mg/kg 
               
               
                   
                   
               
             
          
         
       
     
     A preferred embodiment for carrying out the process of the invention using such crystals as feedstock and described with reference to the flow diagram of FIG. 1 is continuous or semi-continuous; however, a batch mode to carry out the process of the invention is not to be ruled out. 
     Chemical analysis of crystals from crystallizer  14  indicates that the crystals are primarily magnesium sulfite hexahydrate crystals. Such crystals are relatively insoluble in H 2 O and are preferably converted to magnesium sulfate (MgSO 4 ) in an oxidation reaction carried out at oxidation unit  18 . In such unit, crystals from dewatering unit  15  are combined with hydrogen peroxide (H 2 O 2 ) from source  19  for oxidation reaction MgSO 3 +H 2 O 2 →MgSO 4 +H 2 O. In batch tests carried out to determine optimum conditions for operation of the oxidation unit it was determined that a mixture of 50 ml. (˜50 gms) of deionized water, 8 gms of magnesium sulfite hexahydrate crystals and 5 ml (˜7 gms) of 30% H 2 O 2  gave the best oxidation results with the least temperature rise. Such results are contained in Table II 
     
       
         
               
               
               
             
               
               
               
             
           
               
                 TABLE II 
               
               
                   
               
               
                 Time (minutes) 
                 Temperature (° C.) 
                 pH 
               
               
                   
               
             
             
               
                   
               
             
          
           
               
                  0 
                 22 
                 8.8 
               
               
                  2 
                 60 
                 6.7 
               
               
                  5 
                 57 
                 6.7 
               
               
                 15 
                 51 
                 6.8 
               
               
                 30 
                 45 
                 7 
               
               
                   
               
             
          
         
       
     
     In such batch test sample all the reactants stayed in solution when the sample was cooled and ion chromatography analysis indicated 0 ppm of SO 3  and 66569 ppm of SO 4 . 
     Operation of the oxidation unit, in light of the results of such batch test, is carried out with proportions of the reactants substantially matching that of the batch test above. 
     It has been observed that magnesium sulfite hexahydrate crystals from the crystallizer, if not immediately processed in the oxidation step, form a MgSO 4  crust on their exposed surface areas. Such oxidation (MgSO 3 →MgSO 4 ) is not a detriment to the process as such oxidation reaction is similar to that which takes place in the just described oxidation unit. Therefore, magnesium sulfite hexahydrate crystals can be generated independent of the remaining processing steps and stored under ambient conditions until needed. 
     The aqueous magnesium sulfate solution from oxidation unit  18  reports at  20  to precipitation reactor  21  in which magnesium hydroxide is precipitated by a reaction with a strong base, such as sodium hydroxide (NaOH), indicated by the reaction MgSO 4 +2NaOH→Mg(OH) 2 ↓+Na 2 SO 4 . Sodium hydroxide is the preferred reagent for ease of use and high product yield. However, the use of other strong bases such as ammonia, ammonium hydroxide or potassium hydroxide is not to be ruled out by the invention. In such precipitation reactor, control of pH between about 9.5 and 11.0 or preferably between about 10.5 and 10.7, and reaction conditions characterized by 1) gradual addition of reactants, 2) continuous mixing, and 3) absence of over mixing, promotes precipitation of fine (2-5 micron) sized magnesium hydroxide particles. Such particles are of high purity and suitable for use as flame retardants and other high end uses. Magnesium hydroxide precipitate of such fine particle size, in the preferred embodiment of the process, is separated from the sodium sulfate (Na 2 SO 4 ) by a plate and frame filter press or similar device as indicated in FIG. 1 at  22 . Following removal of the sodium sulfate, a resultant filter cake is thoroughly rinsed with a high purity (deionized) rinse water,  23 , to remove entrained sodium sulfate. The filter cake is then dried using forced air,  24 , to remove residual moisture. The resultant high-purity magnesium hydroxide product is removed from the filter press as indicated at  25 . 
     Rinsate and filtrate from such filter press separating step, indicated at  26  and  27  respectively, are utilized within the continuous or semi-continuous system. The rinsate solution which contains traces of Na 2 SO 4  and Mg(OH) 2  reports to the oxidation unit and subsequently to the precipitation reactor, as indicated at  28 , for re-use of the chemicals in the oxidation and precipitation reactions. The filtrate, Na 2 SO 4 , reports as indicated at  29  to a sodium hydroxide re-generator  30 . In such re-generator the Na 2 SO 4  is converted to NaOH for use in precipitation reactor  21 . Such re-generation is indicated by the reaction Na 2 SO 4 +Ca(OH) 2 →2NaOH+CaSO 4 . Ca(OH) 2 . is present in the re-generator through the introduction of a lime slurry  31 , indicated at  32 . 
     Products of the re-generation reactor are separated by a belt filter,  33 , or similar device, to effect liquid and solid separation. The liquid NaOH filtrate  34 , regenerated in the regeneration step, reports to precipitation reactor  21 , as indicated at  35 , for use in precipitating Mg(OH) 2 . Solid phase gypsum, indicated at  36 , is a by-product of the process and is indicated at  37 . 
     To balance the continuous or semi-continuous system chemically and prevent an overload of liquid phase sodium in the system, Na 2 SO 4  is discharged by means of bleed-off  38  in the filtrate circuit of the filter press. To compensate for that loss of liquid in the system, make-up water  39  is added to the rinsate circuit of the filter press at  40 . Such rinsate and make-up water report to oxidation unit  18  as indicated at  28 . 
     As indicated above, control of pH within precipitation reactor  21 , to a level between about 9.5 and 11.0, facilitates precipitation of magnesium hydrate particles presenting a desirable particle size of about 2-5 microns. Such pH level is maintained by addition of NaOH,  41 , to the filtrate circuit of the belt filter at  42  for entry into the precipitation reactor. In the preferred embodiment such NaOH addition is carried out using a 50% NaOH solution for ease of handling. 
     While specific materials and process steps have been set forth for purposes of describing the preferred embodiment of the invention, various modifications can be resorted to, in light of the above teachings, without departing from applicant&#39;s novel contributions, therefore, in determining the scope of the present invention, reference shall be made to the appended claims.