Patent Publication Number: US-4147657-A

Title: Mixing reactive liquids and preparing coagulant aids

Description:
BACKGROUND OF THE INVENTION 
     This invention relates to mixing reactive liquids and to the preparation of sodium metallo silicates that are useful as coagulant aids in water treatment. The particular characteristics of the apparatus and process are such that said active silicate dispersions can be prepared without the use of high shear in a mixing block having no moving parts. 
     Preparation of a sodium alumino silicate dispersion useful as a coagulant aid is disclosed in U.S. Pat. No. 3,963,640 to Smith. This patent teaches that the complex can only be made by subjecting sodium silicate, alum, and water to high shear mixing. Smith further teaches that gels and not dispersions are formed when the reactants are mixed with less shear than that developed by his carefully defined apparatus. 
     It is an object of the present invention to provide sodium metallo silicate dispersions without the need for high shear mixing by using a mixing block with no moving parts. It is a further object of this invention to provide apparatus and a process for mixing reactive liquids at low shear in a device with no moving parts. 
    
    
     BRIEF DESCRIPTION OF THE DRAWING 
     The drawing is a plan view of the mixing block showing how the reactant streams enter the block and are sequentially mixed to form the dispersion which exits the block to the water treatment facility. The drawing shows a single embodiment of the mixing means of our invention showing only one partition and two separate chambers. Additional partitions and inlets can also be employed to enable the addition, dilution and mixing of more than two reactive ingredients. 
    
    
     SUMMARY OF THE INVENTION 
     We have invented a process and apparatus whereby active sodium metallo silicate dispersions, such as sodium alumino silicate, can be prepared in a mixing block that has no moving parts and does not develop high shear. The process does not require the instantaneous mixing of all components. The process comprises the steps of: 
     (A) PROVIDING A MIXING BLOCK, COMPRISING: 
     (1) a chamber or tube formed by boring a hole in a solid block, the initial portion of which is partitioned to form separate chambers through which fluids can flow; 
     (2) an inlet for introducing fluid into the chamber so that the fluid flows through said mixing block and said separate chambers; 
     (3) inlets formed by boring holes in said block intersecting said separate chambers; 
     (4) a mixing area formed by that portion of the chamber subsequent to the partitioned area having a constriction of said chamber; and 
     (5) an outlet from the block; 
     (B) PROVIDING A FLOW OF WATER TO SAID MIXING BLOCK SO THAT TWO STREAMS OF WATER ARE FORMED, FLOWING IN SAID SEPARATE CHAMBERS; 
     (C) INJECTING SODIUM SILICATE SOLUTION INTO ONE OF SAID INLETS, THEREBY FORMING A FLOWING STREAM OF DILUTE SILICATE IN ONE OF SAID SEPARATE CHAMBERS; 
     (D) INJECTING OF A METAL SALT SOLUTION INTO ANOTHER OF SAID INLETS, THEREBY FORMING A FLOWING STREAM OF DILUTE METAL SALT SOLUTION IN THE OTHER SEPARATE CHAMBER; 
     (E) MIXING THE STREAMS OF DILUTE METAL SALT SOLUTION AND DILUTE SODIUM SILICATE SOLUTION BY ALLOWING THEM TO FLOW INTO SAID MIXING AREA, AND 
     (F) RECOVERING THE SODIUM METALLO SILICATE DISPERSION FROM THE OUTLET. 
     The apparatus and process of this invention can be used to mix and react numerous materials in addition to those discussed in this specification. The process and apparatus is especially well suited to those materials that react quickly or are subjected to locallized gelling without proper dilution prior to mixing. 
     In addition, the drawing and specification are largely drawn to an apparatus and process for mixing two reactive ingredients; it will be recognized that this apparatus and process is equally well suited to mixing of reactive ingredients in excess of two, by simply adding more partitions and thereby forming additional separate chambers. 
     THE INVENTION 
     The apparatus and process of our invention is effective in mixing any liquid materials that react readily and require thorough dilution to achieve the desired results. The preparation of uniform silica gels by the controlled sequential dilution and mixing of a silicate solution and an acid is easily achieved. The preparation of varioles organic polymers which set in some finite time after mixing is also facilitated by our apparatus and process. 
     Although sodium, potassium or lithium silicate solutions can be used in the main embodiment of our invention, we prefer to use sodium silicates. In general, any sodium silicate can be used, but we prefer silicates having 2.5 to 4.5 moles of SiO 2  per mole of Na 2  O and silicates with 3.0 to 4.0 moles of SiO 2  per mole of Na 2  O are most preferred. The solutions should contain 25 to 45% silicate solids before dilution. 
     Any salt that is a source of ferric or aluminum ions is useful in the process of our invention. Such salts include among others, ferric chloride, ferric sulfate, ferric nitrate, aluminum chloride, aluminum sulfate (alum) and aluminum nitrate. Alum is preferred as a source of aluminum ions and is supplied at the mixing block inlet as a solution containing 30 to 60% Al 2  (SO 4 ) 3  · 14 H 2  O. We prefer to use 45 to 55% solution. 
     The remainder of this disclosure and the examples are limited to the use of alum and the production of sodium alumino silicate. This limitation is imposed strictly to simplify the discussion and no limitation of our process or apparatus other than those defined in our claims is implied. 
     The components, water, alum, solution and sodium silicate solution are combined in the mixing means shown in the drawing. The mixer can be constructed of any material that is compatible with the raw materials and the final product; we prefer to form the device by machining or molding a block of plastic. The outline of the block is not shown in the drawing. The mixer consists of chamber 1 formed by creating a cylindrical hole in the block, most commonly accomplished simply by drilling the hole. Water enters the block through inlet 2 and is split into 2 flowing streams by partition 3 which separates the initial portion of chamber 1 into chambers 10 and 10a. Inlets 4 and 5 are used to introduce alum solution and sodium silicate solution into the separate chambers thereby forming flowing streams of dilute alum and sodium silicate solutions. These streams flow separately into the mixing area 7 which begins at the end of the partition. Turbulent flow and complete mixing are achieved when the streams combine and flow through constriction 8 and into area 9 which has the same diameter as chamber 1. The product formed is removed by outlet 12. 
     The sodium alumino silicate complex formed is completely dispersible in HCl and any gel that might be formed on the interior of the mixing block can be removed by flushing with hydrochloric acid. The acid can be interjected through inlet 6. The acid can also be injected through inlet 4 or 5 while eliminating inlet 6. The inlet used to inject alum is used for acid addition. 
     The diameter of the bore forming the mixing device can be varied widely and depends on the flow of water required to carry the raw materials and product through the mixer especially if it is mounted vertically with the flows directed upwards. Flows of 0.2 to 7.65 gallons of water per minute require bore diameters of 0.250 to 1.50 inches. 
     Partition 3 must be of sufficient length to ensure uniform and complete mixing of the alum solution and silicate solution with the water stream. The length of the partition should be 50 to 80% of the mixing block which should be between 10 and 24 inches. Therefore, the partition can be between 5 and 21 inches long. Some situations may require a longer mixing block. 
     The mixing area 7 constitutes the remainder of the mixing block or about 2 to 12 inches. The diameter of the constriction 8 must be sufficiently smaller than that of the remainder of the mixing chamber to provide turbulent flow. The diameter of the constriction should be about 0.025 to 0.75 inches or about 10 to 50% of the diameter of chamber 1. The partition 3 can form chambers 10 and 10a so that they have equal volume, thereby maintaining equivalent water flows in both chambers. The partition can also create chambers of unequal volume so that either chamber can have 30 to 70% of the flow of water through the block. 
     The alum and sodium silicate solutions are delivered to the mixing block by appropriate pumps and are selected so that the sodium alumino silicate product contains 0.5 to 3% silica and a sufficient amount of Al 2  (SO 4 ) 3  · 14 H 2  O to provide a pH of about 4.5 to 6.0. A preferred concentraction range for the product is 0.5 to 2.0% silica and Al 2  (SO 4 ) 3  · 14 H 2  O while our most preferred range is 0.5 to 1% SiO 2  and Al 2  (SO 4 ) 3  · 14 H 2  O. 
     The product is useful as a coagulant aid, interacting with primary coagulants such as alum or ferric chloride to precipitate suspended solids. 
     EXAMPLES 
     The following examples illustrate preferred embodiments of our invention. The examples are not considered to restrict the scope of our invention to the conditions shown therein. The scope of our invention is fully defined in the claims and specifications. 
     EXAMPLE 1 
     A laboratory simulation of a commercial installation of the mixing block of our invention was carried out. The block used was 15&#34; in length, the bore or chamber diameter was 1&#34; and the diameter of the constricted portions of the chamber was 1/4&#34;. The silicate and alum feed lines were 1/2&#34; which constricted to 1/4&#34; before entering the mixing block. The partition extended 8&#34; into the block. The sodium silicate had a concentration of 37.5% by weight of silicate solids and a mole ratio of 3.3 SiO 2  /Na 2  O. The alum had a concentration of 50% Al 2  SO 4  · 14 H 2  O. The silicate was pumped into the block at a rate of 6.5 gallons per hour (gph) while the rate for the alum was 3.75 gph. The flow of the dilution water through the block was 4.2 gallons per minute. These rates provided about 1% SiO 2  in the final product as determined by chemical analysis. The block was flushed with 350 ml/min of commercial concentrated HCl for 1.5 minutes every 2 hours. 
     The mixing block worked well and formed the desired alumino-silicate complex. There was no notable gel build-up, especially when the acid flush was accomplished. 
     The activity of the resultant coagulant aid was tested as follows. Two samples of the reaction product were taken at 3 hour intervals and tested using the jar test method. This procedure is described in TAPPI Monograph #18, Chapter V. The primary coagulant, alum, was added at 25 ppm while the coagulant aids were added at 3 ppm. The test water was a suspension of fine kaoline clay. The jar test conditions were: fast mixing (100 rpm) during addition and for 3 minutes thereafter, followed by 5 minutes slow stirring (20 rpm) and 1 minute settling before samples for turbidity measurements were taken. The samples were taken 1&#34; below the surface. 
     A Klett-Summerson Photoelectric Colorimeter was used to measure turbidities using a 40 mm cel depth, a blue filter and deionized water as a standard with a reading of zero. The Klett-Summerson values were converted to Jackson Turbidity Units (JTU) using a correlation curve based on the Formazin Turbidity Standard. 
     A prior art coagulant aid containing sodium silicate and ammonium sulfate was used as a comparison in this test. The results are summarized in the following table: 
     
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Coagulant Aid &amp;                                                           
             Coagulant &amp;            Turbidity                             
Dosage (ppm of SiO.sub.2)                                                 
             Dosage (ppm)                                                 
                         Floc Size  (JTU)                                 
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None         Alum 25     Fine to small                                    
                                    17                                    
Prior Art 3  Alum 25     Small to   5                                     
                         medium                                           
Invention 3  Alum 25     Medium     3.5                                   
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     These results show the effectiveness and consistent activity of the coagulant aid made in the block of our invention. 
     EXAMPLE 2 
     A trial of our mixing block and process was carried out at a plant having a low daily throughput, but a high and difficult to flocculate solids content. The suspended solids included paper and mineral fibers, expanded inorganics, clays and starch. These solids are difficult to remove because of the protective colloid activity of the starch. The mixing block and reagents were the same as in Example 1. 
     The feed rates to the unit were: 
     
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Dilution Water      2 gallons/minute                                      
Sodium Silicate    74.4 gallons/day                                       
Alum               41.7 gallons/day                                       
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     These rates produced a product containing 1% SiO 2  as determined by analysis. This product and alum were dosed to the effluent water separately at 250 ppm SiO 2  and 700 ppm respectively. Improved flocculations were obvious in the clarifier, and after an appropriate time, samples of reclaim water were noticeably improved. The reclaim water is taken from the clarifier at a point 6 to 7&#34; below the normal water level. 
     Jar tests were also carried out as described previously using the heavily laden effluent water. 
     
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Treatment (Dosages ppm)                                                   
Alum      SiO.sub.2    Results                                            
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None      None         No flocculation                                    
600                    No flocculation, turbid                            
                       supernatant                                        
600       200          Good flocculation, clear                           
                       supernatant                                        
700       300          Good flocculation, very                            
                       clear supernatant                                  
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