Abstract:
A process for controlling the concentration of aluminum in the brine of a membrane chlor-alkali cell is disclosed wherein the brine has its calcium carbonate settler solids digested in the calcium chloride flow stream by the addition of an inorganic mineral acid to bring the pH to about 2.0 to about 3.0 to solubilize the aluminum in the brine. The brine has the pH adjusted upwardly to reprecipitate the aluminum onto carrier particles and then removes the reprecipitated aluminum and carrier particles from the brine by filtration or centrifugation.

Description:
BACKGROUND OF THE INVENTION 
     This invention relates generally to a process for controlling aluminum concentrations in membrane chlor-alkali cell brine. More specifically, this invention relates to the system by which aluminum is removed from the brine by concentrating it in the calcium chloride production system. 
     In a membrane cell chlor-alkali plant the membrane usable life is directly related to the quality of the brine employed in the membrane cells. These brines are typically alkali metal halide brines that are concentrated solutions which are prepared by dissolving the alkali metal halide in water or a less concentrated aqueous brine solution. However, this preparation naturally produces impurities in the brine that must be strictly controlled in order to guarantee membrane life. The impurities can vary in both types and concentration depending on the source of the alkali metal halide or salt. Typical brine solutions contain significant concentrations of calcium, magnesium, iron, and silica as impurities. Lower concentrations of complex-forming elements such as aluminum, zinc, tin, and lead are also present. Aluminum has been determined to be an extremely harmful impurity affecting membrane life. The normal aluminum concentration limit established by membrane manufacturers for brine is 60 parts per billion (ppb) in the feed brine. 
     Other sources of aluminum contamination, besides the alkali metal halide salt, include the make-up calcium chloride, the make-up water, soda ash, the primary filter media, the filter aid, hydrochloric acid, and sodium hydroxide. These are all present as components at various stages in the chlor-alkali production cycle. Minimizing the amount of aluminum contaminant that enters the brine system as feed is critical to controlling the overall aluminum contaminant concentration in the brine system. This is especially significant since once aluminum enters the brine system and becomes soluble, it is extremely difficult to remove from the solution. A number of different processes have been used to attempt to control the aluminum concentration in alkali metal brines. One process is described in U.S. Pat. No. 4,450,057, issued May 22, 1984 to P. P. Kelly. This reference discloses the acidification of concentrated alkali metal halide brine to a pH of between 2.0 and 3.0 to convert the aluminum present to the soluble A +3  form. The acidic brine is in contact with a strong macroreticular cationic chelating resin to remove the dissolved aluminum ions at negative hydroxyl sites on the resin. In this process, hydrogen ions from the acid compete with the aluminum ions for the negative hydroxyl sites on the resin. The neutralization of these hydroxyl sites with hydrogen ions makes the resin less effective for aluminum ion removal and requires frequent regeneration of the resin and significant additional costs. 
     Alternate processes which stabilize aluminum as solid particles of an aluminum-silica complex are described in U.S. Pat. Nos. 4,515,665 and 4,618,403 issued to D. L. Fair et al. In these references, the brine is maintained at a pH in the range of 4-12 to prevent aluminum from dissolving. The solid particles of the aluminum complex are removed from the spent brine by suitable processes, such as filtration. 
     However, these prior processes do not address the gradual increase in the concentration of aluminum that occurs within the brine that eventually requires a treatment for removal to avoid an aluminum storehouse effect caused by layers of quickly dissolving aluminum attaching to insoluble particles present in the brine system. 
     These problems are solved in the process of the present invention which removes aluminum from an alkali metal halide brine in a closed-loop membrane cell chlor-alkali brine system in the calcium chloride feed stream. 
     SUMMARY OF THE INVENTION 
     It is an object of the present invention to provide a process for the removal of aluminum from alkali metal halide brine in a closed-loop membrane cell chlor-alkali brine system. 
     It is another object of the present invention to utilize the calcium chloride feed stream flowing from the calcium carbonate settler into the calcium sulfate crystallizer. 
     It is a feature of the present invention that the pH of the calcium chloride feed stream is lowered in the calcium carbonate digester to solubilize the aluminum and then, by raising the pH, to reprecipitate the aluminum onto carrier surfaces and then remove them by filtering or centrifuging the calcium chloride stream. 
     It is an advantage of this system that aluminum entering the brine system is minimized and that aluminum levels in the alkali metal halide brine are kept at acceptable concentrations. 
     These and other objects, features and advantages are obtained in an alkali metal halide brine system wherein aluminum is removed in the calcium chloride feed stream by recycling brine from the calcium carbonate settler and digesting the solids recycled from the calcium carbonate settler at a lower pH to solubilize the aluminum, followed by a raising of the pH to reprecipitate the aluminum onto carrier particles to facilitate easier removal. The calcium carbonate settler solids and the calcium sulfate serve as surfaces upon which the aluminum may deposit and the pH adjustment serves as the driving force to remove the aluminum from solution. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The preceding objects, features and advantages of the present invention will become apparent upon consideration of the following detailed disclosure of the invention, especially when it is taken in conjunction with the accompanying drawings wherein: 
     FIG. 1 is a diagrammatic illustration of a membrane cell brine system wherein the calcium carbonate settler bottoms or solids are first digested with an acid and then are given a pH adjustment to remove the solids by settling or through purging; and 
     FIG. 2 is a diagrammatic illustration of a membrane cell brine system wherein the calcium carbonate settler bottoms are first filtered, then digested and pH adjusted to between 5-7. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     The following is a detailed description of the preferred or alternative embodiments of the brine system employing the aluminum removal process of the present invention. 
     Concentrated alkali metal halide brines treated by the instant process of this invention include aqueous solutions of halides of the elements of Group I of the periodic table, and particularly sodium, potassium, and lithium. For commercial reasons, sodium and potassium are the preferred alkali metals. The term halide is intended to include chloride and bromide brines. 
     Chloride brines are the preferred. It is desired to have brines containing aluminum concentrations of about 60 parts per billion or less after treatment. Typical sodium chloride brines contain from about from 100 to about 2500 parts per billion of aluminum. It is theorized that aluminum goes from the acidic side in the anolyte through the membrane to the alkaline side in the catholyte by forming aluminum complexes with silica hydroxide chains. 
     In the above 60 parts per billion concentration, the aluminum complexes break the ion exchange membrane polymer chains down at transfer sites. The membrane can then no longer pass sodium ions to the catholyte chamber. Caustic then migrates back through the membrane into the anolyte because of anolyte to catholyte side transport. This totally breaks down the efficient operation of membrane chlor-alkali cells, as well as doing the aforementioned permanent damage to the membranes by breaking down the ion exchange polymer chains. 
     The preferred sodium chloride brine is resaturated with NaCl in a saturator and at a pH of between about 5 and about 8. This is best seen in FIGS. 1 and 2. Sulfate is next removed in a sulfate crystallizer in the form of hydrated calcium sulfate, with the pH being adjusted to be between about 8 and about 9. Subsequently, sodium carbonate and sodium hydroxide are added to the brine to substantially reduce the concentration of alkaline earth metals, such as calcium and magnesium, as well as heavy metals, such as iron. This occurs in the carbonate removal step. 
     At this state of the brine treatment system, the pH of the brine is above 10, for example, in the range of about 10.5 to about 11.5. Filtering subsequently occurs to the brine through the use of primary filtering devices. 
     At the site of the carbonate removal via the calcium carbonate settler, the precipitated alkaline earth metals, such as calcium and magnesium, along with the other settler bottoms, can be processed in one of two ways as a part of supplying the calcium chloride feed stream back into the sulfate crystallizer. Both methods pinpoint the location of the aluminum removal by providing an aluminum carrying surface and a driving force in the form of a narrow band of pH adjustment to effect removal of the aluminum carrying particles in the calcium chloride feed stream. This feed stream is produced by recycling and digesting the calcium carbonate settler solids previously referred to, thereby allowing the aluminum to build to extremely high levels in this flow loop. 
     FIG. 1 gives the variation wherein all of the calcium carbonate settler bottoms or solids are digested by the addition of an inorganic mineral acid, such as hydrochloric acid to reduce the pH to about 3. This dissolves or solubilizes the aluminum. The pH is then adjusted to between about 5.0 and about 8.0, and preferably about 5.0 and about 7.0, by adding sodium hydroxide to form aluminum hydroxide complexes which precipitate out and are removed via mud removal, or purging by a settling tank, or centrifuge. This pH adjustment causes the solubilized aluminum to become a gel which is then easily attachable to other insoluble materials. 
     An alternative variation is shown in FIG. 2, the calcium carbonate settler bottoms or solids are filtered first and then followed by digestion of the solids at a pH of about 2.0 to about 3.0 by the addition of the appropriate inorganic mineral acid. The pH is then subsequently adjusted by the addition of sodium hydroxide to about 5.0 to about 7.5, and more preferably between about 5.5 and 7.5. A purging of the aluminum hydroxide complex occurs by either filtration or centrifugation of the calcium chloride feed stream. 
     The instant process for the removal of aluminum from alkali metal halide brines is particularly suitable for electrolysis occurring in membrane cells at high current densities where the presence of aluminum and other cation impurities is more detrimental to the efficient operation of the cell. 
     Using the novel aluminum removal process of the present invention, high purity alkali metal halide brines may be produced for use in electrolytic membrane cells which have concentrations of aluminum reduced to a level that will not cause damage to a cation exchange membrane or affect the membrane&#39;s ability to transport cations or water molecules. 
     To further illustrate the novel process of the present invention, the following example is presented: 
     EXAMPLE 
     A concentrated sodium chloride brine containing between about 295 and about 305 grams per liter of NaC1 at a temperature of between about 55-60 degrees Celsius was formed by resaturation of the dechlorinated brine coming from a membrane cell with NaCl at a pH of between 5 and about 8. The resaturated brine was passed through a sulfate crystalizer to remove hydrated calcium sulfate. The resulting brine had a pH of between about 8 and about 9. Concentrated brine was treated with sodium carbonate and sodium hydroxide and fed to a carbonate settler. The calcium carbonate settler bottoms were then filtered to separate the solids and the solids were digested at a pH of about 2 to about 3. The calcium chloride feed stream was then pH adjusted, by the addition of sodium hydroxide, to about 5.5 to about 7.5, and filtration or centrifugation of the stream occurred to remove the solids. Analysis of the brine after the re-entry of the calcium chloride feed stream from the carbonate removal site seen in FIGS. 1 and 2 resulted in aluminum concentrations uniformly being beneath 60 parts per billion for the 45 days analyzed, as seen in Table I under the &#34;After Settler&#34; column. The aluminum concentrations listed under the &#34;After Saturator&#34; column correspond to samplings taken subsequent to the saturator site seen in FIGS. 1 and 2. 
     
         __________________________________________________________________________DAILY BRINE ANALYSISAFTER SETTLER          AFTER SATURATOR(daily)                (daily)    Ca  Al Mg  Si  Sr  Ca Al  Mg  Si   SrDOL ppb ppb      ppb ppb ppb ppm                     ppb ppb ppb  ppb__________________________________________________________________________136 15467   30 247 3188              149 443                     844 2723                              5339                                  2286137  7541   34 795 3129               65 400                     283 3564                              3045                                  2138138  8675    8 660 3727               75 327                     822 3443                              8187                                  2223139 11544   -2 462 3366               57 482                     1028140  4272   11 489 1903               29 546                     1419                         2500                              2171                                  2691141 25979   18 943 4085              203 572                     105  900                              7011                                  2698142  8475   10 545 3696               76 471                     130  940                              5465                                  2654143144145  4176   16 834 32132               44 616                     860 1711                              7510                                  3464146 20106   12 850 8999              243 556                     1317                         4026                             14989                                  3318147 31213   11 1169          8143              450 544                     1017                         4943                             24985                                  4176148 44831   11 1503          5718              605 575                     311 1096                              9748                                  3304149 47017   11 2138          3051              1109                  576                     538150 41323    9 1252          1796              542151 23115    8 1321          3056              348152 25477    8 1432          3126              409153 28807   14 1350          5604              397154 28848    6 1223          4516              440155 36677    7 1055          7627              333 237     550                             129801                                  1247156 59172   12 1551          37502              397 247                     444 31993                             231768                                  33419157 31089    9 1175          10788              233 338                     356  974                             88734                                   908158 30354    7 1395          6519              263 365                     157  393                             83746                                   810159 19515    9 1451          4542              241 280                     176 2285                             30255                                  1694160 13660    8 1144          4247              160 426                     203 5244                             49433                                  1770161 31268    8 1037          5923              735 421                     195 4974                             87765                                  2077162 21164   27 303 3060              155 446                     154 1518     2804163 12857   19 274 4602              125 429                     182 3151                             39415                                  2368164 23862   21 663 6284              331 523                     209 8157                             60648                                  3098165 11193   15 856 3606              133 517                     134 1668     2130166 17503   12 1299          3543              217 460                     116 1924                              7512                                  2921167 25709   35 727 4446              317 451                     170 15309                              5563                                  3817168 13813   43 456 4746              136 386                     183 3186                              4854                                  2825169 30231   15 1304          5156              325 389                     134 11927                             34911                                  2526170 18745   15 797 5308              287 357                     146 2125                             47267                                  2530171 13261   11 1078          3705              222 362                     154 3938     2723172 15405   13 998 4041              171 460                     144 3736     2917173  8352   21 831 5304              148 437                     198 2479                             49314                                  3250174  8329   33 714 8896              154 375                     208 3301                             27873                                  2935175 13506   28 747 5568              173 367                     216 2058                             47351                                  2651176 16994   10 402 3660              174 365                     202 1428                             31152                                  2900177 29528   26 546 4026              288 361                     133 6285                             49247                                  6697178 33634    6 894 5259              359 388                     159 3108                             34937                                  2627179 28223    6 905 4931              299 375                     203 5355                             24965                                  2909180 27278   21 957 4375              271 375                     170 1971                             44509                                  2608__________________________________________________________________________ NOTE: NEGATIVE INDICATES LESS THAN (BELOW DETECTION LIMIT) 
    
     While a preferred process in which the principles of the present invention have been incorporated is shown and described above, it is to be understood that the invention is not to be limited to the particular details thus presented, but, in fact, widely different means and steps may be employed in the practice of the broader aspects of this invention. The scope of the appended claims is intended to encompass all obvious changes in details, materials, and arrangement of parts which will occur to one of skill in the art upon the reading of this disclosure.