MXPA99010376A - Improved method for preparing low-concentration polyaluminosilicate microgels - Google Patents

Abstract

An improved method and apparatus for preparing low-concentration polyaluminosilicate microgels from a water soluble silicate and a strong acid in which the silicate and acid are mixed at a rate to produce a Reynolds number of at least 4000, the mixture is aged and then diluted to a silica concentration of not more than 1.0 wt.%. The method achieves reduced silica deposition during the preparation of the microgels.

Description

IMPROVED METHOD FOR THE PREPARATION OF MOLES CROGELS OF LOW CONCENTRATION POLL IALUMI NES • BACKGROUND OF THE INVENTION The present invention relates to an improved method and apparatus for preparing low concentration polysilicate microgels, ie, the aqueous solutions have an active silica concentration generally less than about 1.0 ° by weight, which is formed by the partial gelation of an alkali metal silicate or a polysilicate, such as a sodium polysilicate, having in its most common form a part of Na.O to 3.3 parts of SiO- by weight. The microgels, which are referred to as "active" silica in contrast to commercial colloidal silica, comprise solutions from 1 to 2 nm in diameter attached to silica particles which have a surface area of at least about 1000 μg. The particles are bonded together during the preparation, that is, during partial gelation, to form aggregates which are arranged in three-dimensional networks and chains. The polysilicate microgels can be further modified by the incorporation of aluminum oxide in its REF .: 31747 structure. Such modified aluminum polysilicates are classified as polyaluminosilicate microgels and are easily produced by a modification of the basic method for polysilicate microgels. A critical aspect of the invention is the ability to produce the microgels within a reasonable period of time, i.e. not more than about 15 minutes until the microgel is ready to be used, without the risk of solidification and with minimal formation of undesirable silica deposits inside the processor equipment. In this connection, the incorporation of alumina in the polysilicate microgel has found beneficial in that these increase the speed of microgel formation. The polysilicate microgels produced according to the invention are particularly useful in combinations with cationic polymers soluble in water as a dissipator and the retention that aids in the manufacture of paper. At lower pH values, followed by a pH of 5, these products are more appropriately referred to as polysilicic acid microgels. As the pH value is high, these products may contain mixtures of polysilicic acid and polysilicate microgels; the proportion is pH dependent. For convenient purposes, these products will be referred to later as polysilicate microgels.

SUMMARY OF THE INVENTION The present invention is an improved method and apparatus for continuously preparing a low concentration polysilicate microgel which comprises: (a) simultaneously introducing a first stream comprising a water soluble silicate solution and a second stream comprising a strong acid which has a pKa of less than 6 in a mixing zone where the streams converge at an angle of not less than 30 degrees and at a rate sufficient to produce a Reynolds number of at least about 4000 and a resulting silicate / acid mixture having a silica concentration in the range from about 1.0 to 6.0% by weight and a pH in the range from 2 to 10.5; (b) aging the silicate / acid mixture for a period of time sufficient to achieve a desired level of partial gelation (i.e., formation of the microgel); commonly by at least 10 seconds but not more than approximately 15 minutes; and (c) diluting the aged mixture to a silica concentration of no greater than about 2.0% by weight whereby the gelation is stabilized. To produce polyaluminosilicate microgels, a water-soluble aluminum salt is first added to the acid stream prior to mixing with the silicate stream. For best results, the silica concentration of the water-soluble silicate start solution is in the range of 2 to 10 wt.% Silica, and the concentration of the strong acid (e.g., sulfuric acid) is in the range of 1 to 20% by weight of acid while the two streams are introduced into the mixing zone. The preferred conditions in the mixing zone are a number of Reynolds greater than 6000, a silica concentration in the range of 1.5 to 3.5% by weight and a pH in the range of 7 to 10. The most preferred conditions are a Reynolds number greater than 6000, a silica concentration of 2% by weight and a pH of 9. The modified alumina microgel preparation is best conducted by adding a soluble aluminum salt to the acid stream in an amount that is of the range from about 0.1% by weight up to the solubility limit of the aluminum salt. The most used polyaluminosilicate microgels are those prepared with one mole of AI2O3 / SiO2 in proportion to a range of 1: 1500 to 1:25 and, preferably, of 1: 1250 to 1:50. The apparatus according to the invention comprises: (a) a first reservoir for containing a water-soluble silicate solution; (b) a second reservoir for containing a strong acid having a pKa less than 6; (e) a mixing device having a first inlet communicating with said first reservoir, a second inlet arranged at an angle of at least 30 degrees with respect to said first inlet communicating with said second reservoir, and an outlet; (d) a first pump means located between the first tank and the mixing device for pumping a stream of silicate solution from the first tank in the first inlet, and a first control means for controlling the concentration of silica in the tank. silicate solution while the solution is pumped such that the concentration of silica in the outlet solution of the mixing device is in the range of 1 to 6% by weight; (e) a second pumping means located between the second tank and the mixing device for pumping an acid stream from the second tank in the second outlet at a velocity relative to the speed of the first pumping means sufficient to produce a number of Reynolds within the mixing device of at least 4000 in the region where the streams converge by means of the complete mixture of the silicate and the acid; (f) the mixing control means located within the outlet and responding to the flow rate of the acid in the mixing device to control the pH of the silicate / acid mixture in the range from 2 to 10.5; (g) a reception tank; (h) an elongated transfer circuit which communicates with the output of the mixing device and the receiving tank for transferring the mixture therebetween; (i) a dilution medium for diluting the silicate / acid mixture in the receiving tank for a silica concentration of not more than 1.0% by weight; (j) a fourth reservoir for containing a water soluble aluminum salt; (k) a fourth pumping device for introducing the aluminum salt into the acid stream; and (1) a valve \ "~" ~ - control sensitive to the flow of aluminum salt and joined in parallel with the silicate control valve, and located between the fourth pumping device and the point of introduction of the aluminum salt in the acid stream. In an alternate embodiment, the apparatus of the invention includes a deposit of NaOH and means to periodically clean the production system with hot MaOH which has been heated to a temperature from 40 to 60 ° C whereby the silica deposits can be In a further embodiment of the invention, a stream of gas agitating such as a stream of air or nitrogen or other inert gas can be introduced into the described mixing device by means of an additional inlet located at or near the junction of The agitation of gas provides an important industrial benefit in which it allows the low silicate flow rates employed while it is maintained turbulen required and the Reynolds number in the mixing zone. Even in a further embodiment of this invention, the mixing of the acid, the aluminum salt and the water soluble silicate solution can be performed , in an annular mixing device. This device may be a conduit or inner tube which protrudes into and subsequently discharges into a larger conduit or tube. The point of discharge of the inner tube is commonly, but not necessarily, concentrically located in the outer tube. One of the two fluids to be mixed is fed into the inner tube. The second fluid is fed into the outer tube and flows around outside the inner tube. The mixing of the two fluids occurs where the first fluid exits the inner tube and combines with the second fluid in the larger outer tube. Commonly, the acid and aluminum salt solution are premixed prior to feeding into one of the tubes. In order to mix the two liquids, the water-soluble silicate solution and the acid can be fed either into the inner or outer tubes at sufficient velocities such that when the two streams are combined, a Reynolds number greater than 4000 occurs at the mixing zone. A stream of stirred gas can also be optionally employed to assist in the mixing of the two streams. As a further embodiment to this invention, the mixing of the acid-and the water-soluble silicate solution can be carried out in a vessel equipped with mechanical means to create the necessary turbulence, such that the mixing of the two streams is carried out in a number of ways. Reynolds greater than 4000. The vessel can optionally be equipped with deflectors. The acid and the water soluble silicate solution can be but do not have to be fed to the container simultaneously. To produce polyaluminosilicate microgels, a concentrated solution of an aluminum salt, preferably aluminum sulfate, is pumped from an additional reservoir and mixed in the dilute acid stream to a point before the diluted acid and the silicate streams are mixed. and react. By the addition of an aluminum salt to the acid stream, the speed of microgel formation is increased and a polyaluminosilicate microgel is formed having aluminum halves incorporated along the microgel structure. The method and apparatus of the invention are capable of producing stable polysilicate and polyaluminosilicate microgels resulting in reduced silica deposition within a convenient time period of no more than about 15-16 minutes, but commonly within 30 to 90 seconds, without the risk of solidification and with minimal formation of undesirable silica deposits within the processing equipment. The operating temperature is commonly within the range of 0-50 ° C. The deposition of silica in the production apparatus is undesirable because all layers of the internal surfaces of the apparatus can impede the operation of vital moving parts and instrumentation. For example, silica deposition can be elaborated to the point where the valves can no longer function and can restrict the flow of fluid through the conduits and the pipeline. Silica deposition is also undesirable on the pH sensitive electrode while preventing control of the pH process, a critical quality control parameter for the production of silica microgel.

BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 is a schematic diagram of the process _ which includes a deposit of NaOH and means for periodically leveling the production system.

Figure 2 is a schematic diagram of a dual line polysilicate microgel production system _ which is provided for the continuous production of microgel.

Figure 3 is a schematic diagram of the process of the invention for the production of polyaluminosilicate microgels which includes an aluminum salt deposit and means for introducing said salt into the dilute acid stream.

DETAILED DESCRIPTION OF THE INVENTION Active silica is a specific form of microparticulate silica comprising a very small particle diameter of 1-2 nm which are linked together in chains or networks to form the three-dimensional structures known as "microgels". The surface area of the microparticulate active silica, ie, the microgels, is at least about 1000 m2 / g. General methods for preparing polysilicate microgels are described in U.S. Patent 4,954,220, the samples of which are incorporated herein by reference.

Of the methods described therein, the acidification of a dilute aqueous solution of an alkali metal silicate with an inorganic acid or an organic acid, is a strong ag, having a pKa less than 6, is the method which this invention is particularly applicable. The present invention is provided for the reliable and continuous preparation of low concentration polysilicate and polyaluminosilicate microgels to the intended consumption site without the formation of undesirable silica deposits within the processing equipment and very reasonable aging times generally less than 15 minutes, and preferably between 10 to 90 seconds. The method of the invention is effected by simultaneously introducing a stream of a water-soluble silicate solution and a strong acid stream having a pKa of less than 6, together with an aluminum salt, into a mixing zone or a bond of mixed such that the currents converge at an angle of generally not less than 30 degrees, with respect to each other and at a velocity which is sufficient to produce a Reynolds number in the region where the two currents converge to at least 4000 , and preferably in the range of about 6000 and more. The Reynolds number is a dimensional number used in engineering to describe the conditions of liquid flow within a conduit or tube. The numbers above 2000 represent the laminar flow (sparse mixing environment) and the numbers of 4000 up represent the turbulence flow (good mixing environment). As a general rule, the higher the Reynolds number, the better the mixing. The Reynolds number, (Re) for the flow in a pipe or pipe, is determined from the equation Re Q xd D xu Where: Q = - Flow in meters (feet) cubic per second d = Density in kg (pounds) per cubic meter (foot) D = diameter of the tube in meters (feet) u = Viscosity in kg (pounds) per meter (foot) per second The Reynolds number for the impeller agitators is determined from the equation Re = (D2 x N xp) / u Where: D = impeller diameter in cm N = revolving speed in revolutions per second P = fluid density in grams per cm3 - u = viscosity in grams per (second) (centimeter) The concentration of the converged silicate solution and the acid / aluminum salt streams are controlled so that the resulting silicate / acid mixture thus produced has a silica concentration in the range of 1 to 6% by weight and a pH in the range from 2 to 10.5. More preferably the silica concentration is in the range of 1.5 to 3.5% by weight and the pH is in the range of 7 to 10. The most preferred operating conditions are with a Reynolds number greater than 6000, a silica concentration of 2. % by weight and a pH of 9. Aging is generally carried out from 10 to approximately 90 seconds by passing the silicate / acid mixture through an elongated transfer circuit directed to the receiving tank of the final product in which the mixture is diluted immediately thereafter it is maintained at an active silica concentration not greater than 2.0 by weight preferably, not greater than 1.0% by weight. The partial gelation which produces the networks and the three-dimensional aggregate chains of active silica particles of larger surface area are achieved during the period. The dilution of the silicate / acid mixture operates at low concentration to interrupt the gelation process and stabilize the microgel for subsequent consumption. The method of the invention and an apparatus for carrying it out will now be discussed in more detail with reference to the drawings in which Figure 1 is a schematic diagram of the process in its simplest form for preparing polysilicate microgels. The sizes, capacities and speeds described therein can vary over a larger range depending primarily on the amounts of the polysilicate microgel required and the expected rate of consumption. The sizes and capacities described in the reference to the drawings are related to the system for production, that is, generating, polysilicate microgel on a generally continuous basis for consumption as a retention aid and casting in a papermaking process in which the consumption speed ranges from about 10 to 4000 lbs. of microgel per hour. 1 shows a dilution water tank 1_0, an acid tank 2, and a silicate tank 1_4. The tanks, ie the tanks, are conveniently made of polyethylene, with the The tank has a capacity of 1,890 1 (500 i "- gallons), the acid tank has a capacity of 378 1 (100 gallons), and the silicate deposit has a capacity of 1,134 1 (300 gallons). Another container shown in Figure 1 is a tank filled with NaOH 1_6 and a tank receiving final product 1_8. The tank filled with NaOH is made of a non-corrosive material, such as, for example, 316 stainless steel; It has a capacity of 75.5 1 (20 gallons) and is heated with an electrically resistant drum heater that wraps around it (Cole-Palmer, 2000 watts, 115 volts). The final product receiving tank has a capacity of 3,780 1 (1000 gallons) and is made of polyethylene. A critical element of the process is a mixing joint 20 which defines a mixing zone in which an acid stream and a water-soluble silicate stream are introduced along individual paths which converge within the mixing zone. An angle generally not less than 30 degrees. A "T" or "Y" joint mix is suitable for practicing the invention and can be easily constructed of an appropriately sized 316 stainless steel "S agelok" coupling compression adapted with a stainless steel pipe. A "T" joint is generally preferred. The speeds at which the two streams enter, i.e. are pumped into, the mixing zone are selected to produce a Reynolds number therein of about 4000 and preferably up to 6000 or greater which is practically instantaneous and a complete mixing of the acid and the silicate such that the resulting mixture has a silica concentration in the range of from 1.5 to 3.5% by weight and a pH from 7 to 10. Any commercially available water soluble silicate source can be employed, such as, for example, , "PQ (N)" sodium silicate (41 Baume, SiO2: Na20 = 3.22: 1 by weight, 28.7% by weight of SiO2) marketed by the PQ corporation. The commercial silicate will be kept undiluted in tank 14, commonly in a concentration of 24 to 36% by weight to be supplied by the manufacturer, until needed. The blended mixture 20 is supplied by means of an appropriate pipe 22 (316 SS, 0.64 cm (1/4 inch) OD) by means of a low flow rate apparatus or micropump 24 (e.g., Micropump).

Corp., model 140, maximum flow 1.7 gp). Non-corrosive construction materials, for example, 316 stainless steel, are preferred to avoid any risk of corrosion and subsequent contamination. The The silicate supply line also includes a flow control valve 26 (Whitey, 316 SS, needle 0. 64 cm (1/4 inch)), magnetic flow meter 28 (Fisher Porter, 316 SS, size 0.25 cm (1/10 of an inch)) and a check valve 86 (Whitey, 316 SS, 0.64 cm (1 / 4 inch) in diameter) to control and verify the amount and direction of the silicate flow. In operation, the dilution water is introduced into the silicate supply line 22 in an upstream stream conveniently located from the silicate / acid binding mixture to bind the silica concentration to a value in the range from 2 to 10% in weigh. To ensure complete mixing of the silica and water in a 32-line static mixer (Cole-Palmer, 316 SS, 1/2 inch pipe, 15 elements) is provided followed by a check valve 30 ( Whitey, 316 SS, 1.27 cm (1/2 inch) in diameter). The dilution water is supplied by means of a line 34 (1.27 cm (1/2 inch) OD, 316 SS) by a centrifugal pump 36 (Eastern Pump, 1HP, maximum flow 54 gpm), and a rotar etro 38 (Brooks , Brass Ball, 3.06 gpm max.). Control valve 40 (Whitey, 316 SS, 1/2-inch needle NE) and check valve 42 (Whitey, 316 SS, 1.27 cm (1/2 inch) in diameter) can be used for control and direction of the flow rate. Although a wide range of acidic materials, such as, for example, mineral acids, organic acids, acid salts and gases, ion exchange resins and salts of strong acids with weak bases, have been described for use in the preparation of active silica, simple means and more convenient acidification with a strong acid have a pKa less than 6. The preferred acid is sulfuric acid. Commercial grades manufactured by DuPont and others are generally appropriate. In operation, a base acid solution is maintained at a concentration in the range from 5 to 100% by weight in the acid tank 1_2. The acid is pumped using a similar device or microscope 44 (for example, an icrobomba model 040, 1 / 4HP, maximum flow 0.83 gpm) to the junction mixer 20 through line 46 (316 SS, 0.64 cm (1 / 4 inch) OD) and check valve 88 (Whitey, 316 SS, 0.64 cm (1/4 inch) in diameter).

A single circuit controller 90 (Moore, Model 352E) combined with a pH 48 transmitter (Great Lakes Instruments, Model 672P3FICON) and a pH 48A Tester (Great Lakes Instruments, Type 6028PO) to regulate eT flow of acid to the blender 20 by means of the "« SvO.50 automatic flow control valve (Research Control, K Trim, 0.64 cm (1/4 inch) OD, 316 SS) with respect to the pH of the silicate / acid mixture measurement at the junction of the blender. An automatic three-way valve 52 (Whitey, 316 SS, 1.27 cm (1/2 inch) in diameter) is also used within the control system to allow the possibility of diverting especially the silicate / acid mixture to the drain . The dilution water of the water reservoir 1_0 is provided via line 54 (316 SS, 1/2 inch OD) to dilute the acid supplied upstream of the supply of the junction mixer 20 to a predetermined concentration in the range from 1 to 20% by weight.

A static mixer 56 (Cole-Palmer, 316 SS, 1.27 cm (1/2 inch) in diameter, 15 turns) is provided downstream from the point where the dilution water is introduced into the acid supply line to ensure complete mixing and dilution of the acid. A rotameter 58 (Brooks, Brass Ball, 1.09 gpm maximum), control valve 60 (Whitey, 316 SS, needle 1.27 crn) (1/2 inch)) and check valve 62 (Whitey, 316 SS, 1.27 cm (1/2 inch) in diameter) are used to control the flow rate and flow direction of the dilution water. ? The silicate / acid mixture which leaves the binding mixture 20 preferably has a concentration of SiO? in the range from 1.5 to 3.5% by weight and a pH in the range from 7 to 10. More preferably the silica concentration is maintained at 2% by weight and the pH at 9. The mixture is passed through a line of elongated transfer 64 (1.27 cm (1/2 inch) of tube 40 of PVC programmed, 22.50 m (75 feet) in length) en route to the receiving tank of final product 18. The length of the transfer line is selected to ensure that the transfer will take at least 10 seconds, but preferably from about 30 seconds to 90 seconds, during which the "aging" or partial gelation time of the mixture takes place. The transfer time can be as long as 15-16 minutes at very low flow rates and still produce satisfactory results. The dilution water from reservoir 1_0 is added via line 66 (316 SS, 1.27 cm (1/2 inch) OD) to the joint mixture prior to its entry into the final product receiving tank 1_8 or to any Another convenient location as large as the silicate / acid mixture is diluted to a Si02 concentration of less than 1.0% by weight which stabilizes the gelation process. The dilution water is supplied with the centrifugal pump 68 (Eastern, 316 SS, 1 HP, 54 gpm maximum), and flow control is performed at a predetermined speed with control valve 70 (Whitey, 316 SS, 1/2 inch needle) and a Rotameter 72 (Brooks, SS Ball, 12.46 gpm maximum). The final product receiving tank 18 is provided with a level control system 74 (Sensall, Model 502) which is operated in conjunction with an automatic three-way valve 76 (Whitey, 316 SS, 1.27 cm (1/2 inch) in diameter) to divert the flow of the silicate / acid mixture to the drain if the level of the final product becomes too high. After a period of continuous operation, which depends on the amount of active silica produced, it may be desirable to suspend the generation of the active silica and to rinse the binding mixture 20 and the portion of the system which is downstream, i.e. , tubing, valves, transfer lines, etc., which have been in contact with the silicate / acid mixture, with water and hot NaOH. The wash system removes any undesirable silica deposits that may have accumulated in parts of the apparatus where turbulent flow conditions are required that could not have been maintained due to design restrictions, such as in the pH measurement region. The washing procedure helps to keep the system free of silica deposition and begins by first completely closing the dilution pump 68, the acid pump 44 and the silicate pump 24. The dilution water of the pump 36 then circulates through of the portion downstream of the system for approximately 5 minutes, after the pump 36 is completely closed, and the dilution water tank is isolated by closing the valves 40, 60 and 70. The automatic three-way valves 52 and 76, and manual valves 78, 80 and 82 (all of Whitey, 316 SS, 1.27 cm (1/2 inch) OD) are then activated together with the circulating centrifugal pump 84 (Eastern, 316 SS, 1.5HP, .15 gpm maximum) to allow NaOH, to be maintained at a concentration of 20% by weight and a temperature in the range of from 40 to 60 ° C, to circulate through the portion of the downstream system, generally not greater than about 20-30 minutes. The NaOH circulating in the pump 84 and the flow tank 1_6 are then isolated from the system by re-activating the three-way valves 80 and 82, and the dilution water flows back through the downstream system and releases it to the drain. Having completed the cleaning / washing process, the production of active silica can be resumed. With reference now to Figure 2, which shows 'v ~~ a schematic diagram of a dual inline production system for active silica, whereby one line can be operational all the time while the other line is being washed or kept in a permanent condition. The parts of the component are numbered according to Figure 1. A commercial system of agreement, either of Figures 1 or 2, will generally be constructed of stainless steel or polyvinylchloride pipe of generally 2.54 cm (one inch) in diameter or less. , depending on the requirement for active silica. When the stainless steel pipe is used, the connections of the various instruments, adjusters, valves, and sections can conveniently be made with "S agelok" compression joints. Figure 3 is a schematic diagram showing a modification of the basic apparatus of Figure 1 suitable for the production of polyaluminosilicate microgels. From tank 100, a concentrated solution of an aluminum salt, preferably aluminum sulfate, can be pumped through the pipe (0.64 cm (1/4 inch) diameter 316 stainless steel) by means of a pump Diaphragm 102 (Pulsatron Model LPR 2-rMAPTCI, a .pack filled with polypropylene, diaphragm Teflon, maximum flow 12.5 ml / min). The counting pump 102 can be electronically linked to the controller 90 and can move in parallel with the silicate treatment. After passing through the check valve 104 (Whitey, 316SS, 0.64 cm (1/4 inch) in diameter), the aluminum salt solution can be introduced into the line of acid diluted to point 106 by means of a union "T" 316 SS. Completing the mixing of the aluminum salt with the diluted acid can be completed by the mixture in line 56 before the reaction with the silicate, to produce polyaluminosilicate microgels, it occurs in a "T" junction 20. An aluminum salt solution preferred for use in the method is a commercial solution of aluminum sulfate such as liquid aluminum solution Al2 (S04), 14H20 containing 8.3 wt% of A1203 supplied by the American Cyanamid Company. Periodically, it is necessary to wash the free polyaluminosilicate apparatus from the silica deposits by means of a hot caustic soda solution as described above. It should be understood that an in-line dual apparatus for the continuous production of polyaluminosilicate microgels can be constructed by appropriate codings of the dual line apparatus of Figure 2.

Example 1 - Demonstrate the effect of turbulence in the silica reducing deposition. A laboratory generator for producing polysilicate microgels was constructed according to the principles described in figure 1. The silicate and the sulfuric acid feeds, before dilution and mixing, contained 15% by weight of silica and 20% by weight. in acid weight respectively. The critical joint mixer was constructed in 0.64 cm (1/4 inch), 316 stainless steel T compression fitting "Swagelok" adapted with 6 inch arms with 1/4 inch pipe ) OD 316 SS. The internal diameter of the adapter was 0.409 cm. For tests in which a gas was introduced into the joint mix similar to a "Swagelok" X compression coupler, it was used with the fourth arm of X as an inert gas. An in-line filter compressor of 2.54 cm (1 inch) diameter 60 mesh stainless steel screen was placed approximately 30.48 cm (12 inches) from the acid / silicate bond to trap the particulate silica. The screen was weighed at the beginning of each test and again at the end of each test, after washing and drying, to give a measure of the silica deposition. All tests were run to maintain conditions of 2% by weight of silica and pH 9 at the point of silicate acidification and each test was run for a sufficient time to produce a total amount of 1590 gms of polysilicate microgel. The. Test results are given in Table 1 below. The liquid flow represents the total liquid flow, which is the flow of the combined silicate / acid mixture in the outlet pipe. In tests where a gas is introduced to improve liquid flow and turbulence, the Reynolds number was calculated on the basis of the increased flow velocity of the single liquid portion, assuming that the liquid density and viscosity did not change appreciably. This method of calculation was adopted because there is no formula prepared to calculate the Reynolds number of the liquid / gas mixtures.

- Table 1 Deposition of Silica as a Function of Reynols Number Test No. of Liquid Flow Time Flow of Siliceous Gas No. Reynolds run ml / m ml / m deposited minutes. ^ sgms 1 1, 036 330 250 None 0.339 2 2, 072 165 499 None .0135 3 4, 144 83 999 None 0.009 4 6,217 55 1, 498 None 0.007 10, 362 33 2, 497 None 0.002 6 12, 433 27 2, 996 None 0.008 7 T2, 2 60 120 694 Air, 2, 260 0.008 8 9, 064 120 694 Air, 1, 490 0.005 9 5, 375 120 694 Air, 601 0. 004 1 0 5, 375 120 694 N2, 61 0. 014 A comparison of the results of Tests 1 & "2 with the results of Tests 3-10 clearly demonstrates the beneficial effect of turbulent liquid flow (Reynolds number above 4,000) in reducing the amount of silica deposition observed Under the turbulent flow conditions of the present invention, the average silica deposition of 0.007 g represents only 0.0004% of the total "of the amount of silica processed. When the Reynolds number was below the minimum of 4,000 required by the present invention, it is undesirable that the silica deposition is at least about 15-parts increased. Once the minimum of the number of the process of this invention is the Reynolds number above 4,000, for example from 4,144 to 6,217 to 10,362, etc. the deposition of additional silica is not appreciably reduced.

Example 2 - Apparatus A commercial apparatus classified to prepare active silica microgels was assembled according to the schematic design shown in Figure 1 and installed in a commercial wastebasket. The apparatus, except for the tanks that supply the raw material, was rigidly mounted on a steel structure in two runners, each measuring approximately six by 2.40 m (eight feet). On the slide 1, the inputs for connection to a commercial supply of sodium silicate and sulfuric acid and an inlet for drinking water are used, which is used for dilution purposes. Also on the slide 1 is mounted the flow control and dilution means, the acid / silicate binding mixture, pH measurement and pH controller, sodium hydroxide flow tank, and pumps, valves and electrical controls required. On the slide 2 the aging circuit was mounted, a deposit of the final product, a ..controller level and pumps and valves required. The total height of each slide was approximately 2. 10 (seven feet). The manufacturers supply the containers that were used as silicate and sulfuric acid tanks and these were connected directly to the appropriate entries on the slide 1. The apparatus was operated continuously for six (6) days during which 0.5% by weight Active silica was produced at a rate which varied between 3 and 18 1 (4.8 gallons) per minute. At a production rate of 3 gpm, a Reynolds number of 4250 was calculated for the mixing zone employed. No silica deposition was observed within the blender 20, although some silica deposition was observed in the vicinity of the pH test located immediately downstream from the junction of the blender after 12 continuous hours of operation. To alleviate this situation, a water / NaOH / water flow sequence was conducted, which took less than 30 minutes, and the system was then returned to normal production. Over the total period of six days, the apparatus was operated without failures and active silica of excellent quality was produced which was used by the factory for the production of a variety of papers with weights of. different base.

Example 3 - Preparation of Polyaluminosilicate Microgel A commercial apparatus classified for the preparation of the polyaluminosilicate microgel solution was assembled according to the principles shown in Figure 3. The apparatus, except for the tanks supplying the raw material, it was mounted rigidly on the steel structure on two runners, each measuring approximately eight by 2.40 m (eight feet). On the slide 1 the entrances were mounted for the connection to the supply of sodium silicate, sulfuric acid, sodium hydroxide and paper made of aluminum and an inlet for drinking water that is used for dilution purposes. Also mounted on slide 1, the pumps required for each chemical and a reservoir for the content of the final polyaluminosilicate microgel solution. Flux control valves for sodium silicate, acid, and dilution water, acid / silicate binding mixing, pH measurement and pH controller media, an aging circuit, were mounted on slide 2; a deposit of sodium hydroxide flow. The flow of the paper made of aluminum was controlled by a diaphragm pump at the speed proportional to the silicate flow. The paper made of aluminum was introduced into the dilute acid stream prior to acid / silicate binding mixing. The resulting solution of polyaluminosilicate microgel had a molar ratio of Al203 / SiO2 of about 1/1250. The apparatus was used to produce 22,680 1 (6000 gallons) of 0.5% by weight of polyaluminosilicate microgel solution at a rate of 75.5 1 (20 gallons) per minute. A Reynolds number of 22,700 was calculated for the mixing zone. Only the minor silica deposition was recorded on the pH electrode after 5 hours of operation. To remove the silica deposits, a flow of NaOH was conducted, which took less than 30 minutes, and the system was then returned to normal production. The polyaluminosilicate microgel solution was used by a paper mill for the production of a panel for packaging liquids with excellent results.

It is noted that in relation to this date, the best method known to the applicant to carry out the aforementioned invention, is the conventional one for the manufacture of the objects or products to which it refers. Having described the invention as above, property is claimed as contained in the following:

Claims (6)

1. In a process for the preparation of a polysilicate microgel characterized in that (a) a first stream containing a water-soluble silica solution and a second stream containing a strong acid having a pKa of less than 6, are introduced into an area of mixing wherein the streams converge to produce a Reynolds number of at least 4000 and the converged streams have a silica concentration in the range of 1% to 6% and a pH in the range of 2 to 10.5, (b) the resulting mixture is aged to achieve a partial gelation within 15 minutes, and (c) the aged mixture is diluted; the improvement wherein an aluminum salt is present in the second stream of step (a) by which a polyaluminosilicate microgel solution is formed in step (b) which maintains a significant anionic charge at low pHs in the step (c) in the absence of the aluminum salt.

2. The method according to claim 1, characterized in that the concentration of silica in the resulting salt / acid / silica mixture is from 1.5 to 3.5% by weight and the pH is from 7 to 10.

3. The method according to claim 1, characterized in that the pH is from 2 to 7.

4. The method according to claim 1, characterized in that said silica concentration is not greater than 1.0% by weight.

5. An apparatus for continuously producing a stable aqueous polymalumosilicate microgel, characterized in that it comprises: (a) a first reservoir for containing a water-soluble silicate solution; (b) a second tank to contain an acid that. it has a pKa less than 6 and an aluminum salt; (c) a mixing device having a first *. entrance which communicates with the first r-deposit, a second entrance arranged at an angle of at least 30 degrees with respect to the first entrance which communicates with the second deposit, and an exit; (d) a first pumping means located between the first tank and mixing device, for pumping a stream of silicate solution from the first tank in the first inlet "and, a first control means for controlling the concentration of silica within the range of 1 to 6% by weight in the resulting salt / acid / silicate mixture while the solution is pumped; (e) a second pumping means located between the second tank and the mixing device for pumping an aluminum salt and acid stream from the second tank in the second inlet at a speed relative to the speed of the first sufficient pump means to produce a Reynolds number within the mixing device of at least 4000 in the region where the streams converge whereby the silicate, the aluminum salt and the acid are mixed thoroughly; % íf) a mixture of control means located within V the output and responding to the flow rate of the aluminum salt and acid in the mixing device to control the pH of the silicate / acid / salt mixture in the range from 2 to 10.5; (g) a reception tank; (h) an elongated transfer circuit which communicates with the output of the mixing device and said receiving tank for transferring the mixture therebetween; and (i) a dilution means for diluting the salt / acid / silicate mixture in the receiving tank to a silica concentration of not more than 2.0% by weight.

6. The apparatus according to __ claim 5, characterized in that the. The first inlet and the second inlet are arranged at an angle of 90 degrees with respect to each other, said acid is sulfuric acid and said aluminum salt is aluminum sulfate.