KR100582139B1 - Improved Continuous Process for Preparing Microgels - Google Patents

Abstract

Silica deposits formed during the continuous process of polysilicate microgel preparation elastically deform the vessel walls to remove them from the process and purged to reduce clogging.

Silica deposits, polysilicate microgels, effluent flow resistance, mechanical strength, aging

Description

Improved Continuous Process for Preparing Microgels < RTI ID = 0.0 >

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to an improved continuous manufacturing process for producing polysilicate microgels by reducing or eliminating silica deposits.

Polysilicate microgels (i.e., aqueous solutions formed by partial gelation of alkali metal silicates) are known in the art. Partial gelation is usually achieved by mixing the alkali metal silicate with a gel initiator, aging the mixture for a short time, and then diluting the mixture to stop the gelation. Inorganic acids and alum are the most commonly used gel initiators. The resulting microgels have commercial utility in drainage and retention aids in paper manufacturing, as coagulants in drinking water purification systems, and in similar fields.

Polysilicate microgels are good coagulants and are environmentally advantageous, but various practical factors currently limit the commercial use of polysilicate microgels. For example, microgel solutions must be diluted, which can not carry large volumes over long distances. Microgels also tend to gel to form silicate deposits in the apparatus used to make the product. These problems can be overcome by people trained in the device design and manufacturing environment, but there are greater difficulties in areas where it is relatively easy to operate and maintain the device.

The batch process for preparing microgels ages the microgels in large scale mixing and storage tanks, which is not only cost prohibitive, but also causes problems of product non-uniformity and process control inherent in the batch process. A continuous process of preparing and aging microgels is more desirable for consistency of product quality. Unfortunately, silica deposits may be better tolerated in batch holding tanks than in continuous process piping, where the deposits clog the device causing frequent shut-downs for maintenance.

U.S. Patent No. 5,279,807; 5,503,820; And 5,658, 055 disclose an improved continuous process for preparing polysilicate microgels that significantly reduce silica deposits by mixing a water soluble silicate solution with a gel initiator under certain conditions. Although the designs taught in these patents have further improved performance and have been found to be of commercial utility, it is still often the case that the pipe and microgelerator generator is clogged with silica deposits. It is therefore still necessary to periodically stop the process and to remove the deposit by dissolving it in a base, such as sodium hydroxide. Also, an emergency generator is still needed when continuous manufacturing is required.

Therefore, there is a need for a further improved continuous process for preparing polysilicate microgels.

SUMMARY OF THE INVENTION [

The present invention

(a) mixing an aqueous solution of a water soluble silicate and a gel initiator in a mixing vessel to produce an aqueous mixture having a silica concentration of about 0.5% to 15% by weight; And

(b) aging the mixture in a long aging vessel to partially gel the mixture,

Wherein the mixing vessel and / or the aging vessel is elastically deformable to temporarily deform during the process to desorb the deposit formed on the vessel wall, thereby removing deposits from the vessel. Which is a continuous process.

In a preferred embodiment, step (a) mixing is performed under the conditions described in U.S. Patent Nos. 5,279,807, 5,503,820, and 5,658,055 to minimize silica deposit formation. Aluminum ions may also be present during step (a) mixing to form a modified polysaccharide microgel that requires less aging time in step (b).

Flexible hoses are typically used as mixing vessels and / or aging vessels. The hose is easily deformed by increasing or decreasing the pressure in the hose or by easily deforming it by mechanical means, for example by passing a hose through the nip of a pair of rollers moving along the length of the hose. Ultrasonic waves or sound waves may be used in the semi-rigid or polymeric container (s).

DESCRIPTION OF THE PREFERRED EMBODIMENTS [

The polysilicate microgel is an aqueous solution formed by partial gelation of alkali metal silicate or polysilicate, for example sodium polysilicate, having 1 part by weight of Na ₂ O per 3.3 parts by weight of SiO ₂ in the most common form. Microgels usually consist of water, and bonded silica particles having a diameter of 1 to 5 mm and a surface area of 500 m < 2 > / g or more. Particles combine with each other during manufacturing (during partial gelation) to form agglomerates with three-dimensional networks and chains. Microgel solutions have a lower S-value than commercial colloidal silica. The S-value defined by Iler and Dalton (see J. Phys. Chem. 60 (1956), p. 955-957) is the weight percent silica in the dispersed phase, . The S-value of the colloidal silica is usually in the range of 80% to 90% by the Aylar and Dalton definitions. The S-value of the silica microgel solution is usually less than 50%, preferably less than 40%.

Below pH 5, polysilicate microgels are sometimes referred to as polysaccharide microgels. As the pH value increases, these products may contain mixtures of polysilicic acid and polysilicate microgels, the ratio of which depends on the pH. The term " polysilicate microgel "as used herein includes such polysaccharide and polysaccharide microgels.

Polysilicate microgels often incorporate aluminate ions into their structures to modify them. Aluminum may be present at all stages throughout the polysilicate aggregate, or only on their surface, depending on which aluminum source is added to the process. Aluminum can be added to increase the rate of microgel formation and thus reduce the aging time. Aluminum also allows microgels to maintain charge under low pH conditions. The term " polysilicate microgel "as used herein includes aluminum-containing polysiloxane microgels, sometimes referred to in the art as polyaluminosilicate microgels.

mix

Conventional water soluble silicate solutions and gel initiators can be selected to form a mixture in step (a) in which the silica concentration ranges from 0.5 wt% to 15 wt%, preferably from 1 wt% to 10 wt%. In practical use, microgels will generally form too slowly at concentrations below 0.5%. With more than 15% silica, the gelation rate is too fast to be effectively controlled.

Suitable gel initiators are well known in the art and include acid-exchange resins, inorganic acids, organic acids, acid salts, acid gases, alkali metal salts of amphoteric metal acids (e.g. sodium aluminate) For example, certain anhydrides, amides, esters, lactones, nitriles, and sulfones. Inorganic acids, typically sulfuric acid and alum, are conventional gel initiators. The gel initiator may be added as an aqueous concentrate to a solubility limit, or as a dilute solution to facilitate mixing.

When the gel initiator is an inorganic acid, the resulting mixture has a pH in the range of 2 to 10, and the flow rate of the acid (or the ratio to silica) is usually controlled by a pH controller. If an organic acid such as carbonic acid or carbon dioxide is chosen as the gel initiator, the flow rate (s) of the gel initiator and / or silicate solution can be controlled by volumetric measurement (in the pH range 2 to 10) due to the buffering effect of the carbonate produced have. Volumetric control has the advantage of avoiding pH sensors that may require frequent cleaning, calibration and replacement.

If the selected gel initiator is a solution of alkaline, e.g. sodium bicarbonate or sodium aluminate, the flow rate (s) of the gel initiator and / or silicate solution can be easily controlled by volume measurement since both streams are alkaline Do. The pH range of the resulting mixture is pH 7-13.

If desired, the aluminum salt may conveniently be added as a water-soluble component to the gel initiator or sodium silicate solution, or added to the mixture as a separation stream. The superior polyaluminosilicate microgels are prepared by mixing an aluminum salt with an acidic gel generator stream in a molar ratio of Al ₂ O ₃ / SiO ₂ of 1: 1,500 to 1:25, preferably 1: 1,250 to 1:50 . Alternatively, the polysilicate microgel can be prepared by directly reacting an alkali metal aluminate with a silicate using a polyaluminosilicate solution to form a silicate having an Al ₂ O ₃ / SiO ₂ molar ratio of about 1: 1 or less.

Although any mixing state can be used to carry out the invention, it is particularly advantageous to use the mixing conditions described in U.S. Patent Nos. 5,279,807, 5,503,820 and 5,658,055, which are incorporated herein by reference. The turbulent mixing conditions described herein significantly reduce gel and silica deposit formation during aging of the microgels, and either (i) concentrate the silicate solution and the gel initiator stream at an angle of 30 ° or more, or (ii) It has been found that this is accomplished using an annular mixing device that discharges one stream from the pipe to the second stream stream flowing through the outer annular pipe to converge the two streams. Although turbulent mixing conditions are not required in the present invention, it is preferred that the Reynolds number in the mixed region is above 1,000, preferably above 6,000.

ferment

Subsequently, the mixture is aged for a sufficient time to reach the desired level of partial gelation, usually for a time period of from 10 seconds to 15 minutes. Partial gelation produces a three-dimensional aggregate network and high surface area silica particle chains known in the art as polysilicate microgels.

The extent of the desired partial gelation will vary depending on the components selected and the application, but is generally achieved within 10% to 90% of the time the complete gelation takes place. Thus, one skilled in the art can easily measure the gel time and adjust the selected aging time by varying the flow rate through the aging vessel. For example, the length and / or diameter of the aging vessel and the flow pressure can be optimized in a particular application.

In a continuous process, aging occurs when the mixture passes through an elongated container, and is essentially completed when the mixture reaches the container outlet. The elongated vessel usually has a constant diameter (i.e., a pipe) selected to provide the residence time necessary to "mature" the mixture into the desired range of diameters and lengths. Conventional aging vessels will range from 0.5 to 25 cm (1/4 to 10 inches) in diameter and from 60 to 150 m (2 to 500 feet) in length to provide a retention time of 10 seconds to 15 minutes. Generally, it is not advantageous to set the residence time to 15 minutes or longer.

According to the present invention, the mixture is formed (or aged) in an elastically deformable elongated vessel (e.g., a pipe or tube) that is sometimes temporarily deformed to desorb the deposit formed on the vessel wall. The desorbed deposit is removed from the vessel by aging mixing as it passes continuously through the vessel. The deposit is comprised of silica and in many cases it is not necessary to separate and remove them from the mixture exiting the vessel. The advantages of the present invention are particularly evident in the mixing and early maturing stages, in which deposits tend to form.

The container may accommodate ancillary devices, such as valves, mixers, and process equipment. The container is comprised of a material having surface properties such that (i) it is more resilient than the silica deposit, (ii) the deformation force of the container overcomes the adhesive force between the container and the deposit, do. The material selected will vary depending on the method chosen to temporarily deform the container wall.

In one embodiment, the container wall is temporarily deformed by increasing or decreasing the container internal pressure to inflate or shrink the wall. Such pressure changes can be achieved by any method known in the art, for example (i) a method of periodically varying the pressure of the feed stream, (ii) a method of using a feed pump having discontinuous feed characteristics such as a piston pump, (iii) a method of periodically changing the outflow resistance to a programmed control valve, (iv) a method of periodically introducing a liquid or gas which does not unduly affect the aging process, or a combination thereof have. Examples of materials suitable for the container structure in this embodiment are polymeric materials that resist cyclic elastic deformation without breakage by cracking and that are chemically resistant to the aging mixture, such as vinyl plastic, "biotone" co- "Teflon" polytetrafluoroethylene, silicone rubber, neoprene rubber and other rubbers or elastomers.

In a second embodiment, the container wall is temporarily deformed by a mechanical force against the container wall. Mechanical forces include, but are not limited to, any method known in the art including, but not limited to, squeezing, bending, and relaxing the wall by a roller, press, or other mechanical device, . The mechanical force can be applied by stretching the container longitudinally to reduce its diameter and then relax. A process of moving the pair of rollers along the longitudinal axis of the take-out vessel is particularly preferable. Similar materials will be selected for embodiments such as those described above in which the internal air pressure is varied.

In a third embodiment, the vessel walls are temporarily deformed by vibrational forces, for example vibrations transmitted from the surrounding liquid with the flooded vibrator to the vessel. Typically, an ultrasonic vibrator will be selected for this purpose. Alternatively, the mixture contained in the container may vibrate and transmit vibrations to cause elastic deformation of the container. In this embodiment, the container is typically comprised of a semi-rigid material, such as steel or stainless steel, that is more resilient than a silica deposit or polymeric material such as those described above.

Industrial purpose

Polysilicate microgels will generally be treated to inhibit or minimize further gel formation. The treatment may be either a simple dilution step, or a pH adjustment step, or a combination of dilution and pH adjustment, which reduces the silica concentration to less than about 10% by weight, preferably less than 5% by weight, Everything can be achieved. Other techniques known in the art can also be selected to inhibit gel formation.

Thus, the microgel can be stored or consumed for the intended use. Alternatively, dilution or pH adjustment of the microgel would not be necessary if the microgel would be consumed immediately, or if the additional gelation falls within the limits allowed for the intended application. If desired, the aged microgel can be filtered to remove many unacceptable silica deposits that are desorbed during the practice of the present invention.

The polysilicate microgel prepared according to the present invention can be easily produced in situ, and thus can be used not only in ordinary applications where microgels are consumed but also in new applications to be tested. For example, microgels can be used as coagulants to remove solids from aqueous suspensions, or as paper retention aids with other polymers and / or chemicals often used for this purpose.

BEST MODE FOR CARRYING OUT THE INVENTION In describing the present invention, the following examples will be described, but the present invention is not limited thereto.

 ≪ Example 1 >

A polyaluminosilicate microgel solution was prepared by reacting a dilute sodium silicate solution with a dilute sodium aluminate solution in a cyclic adiabatic mixer. A sodium silicate solution containing 2 wt% SiO ₂ was fed into the annular mixing zone through a Swagelok 1.27 cm (1/2 inch) T fitting at 1.9 gpm (7.2 L / min). The T-joint was purchased from a reinforced Tygon (a B44-4K type of vinyl tube, 1.27 cm (1/2 inch) in diameter, purchased from Norton Performance Plastics Corp., Wayne, NJ ) At 30 feet (100 feet). A sodium aluminate solution containing 2 wt% Al ₂ O ₃ at about 20 cm (8 ") downstream of the silicate is passed through a 1/2 inch T-joint through a 0.635 cm (1/4 inch) diameter stainless steel tube The aluminate was fed to the mixing zone at a rate to maintain a weight ratio of Al ₂ O ₃ / SiO ₂ of 1/6. The silicate and aluminate were mixed in a Tigon tube.

Silica deposits in the Tigon tube were readily apparent after about an hour of operation. Removal of deposits was accomplished by passing the tube through two plastic rollers and slightly crimping the tire tube. There was no silicate deposit in the hose after passing the roller once. A total of about 1514 liters (400 gallons) of polyaluminosilicate microgel solution was prepared using the described apparatus. The hoses were deformed with plastic rollers to remove all silicate deposits from the apparatus. It has been found that polyaluminosilicate solutions have excellent utility as paper holding and drainage aids.

≪ Example 2 >

This example illustrates how ultrasonic vibration is used to elastically deform the vessel of a process to prevent the formation of silica deposits when producing polysilicate microgel solutions. a solution containing the pH 8.7, and an average flow rate of 49 ℓ / min (13 gpm) SiO ₂ 3.2 wt% by mixing in a dilute sodium silicate and a stainless steel T-shaped fitting connected to the mixture of sulfuric acid in the ratio 3.2 was prepared. After exiting the T-joint tube connection mixer, the solution was introduced into an enhanced tidal tube section with an inside diameter of 0.95 cm (3/8 inch) and a length of 2.1 m (7 ft). The tie-tube was then connected to a stainless steel tube section having an outer diameter of 1.27 cm (1/2 inch) and a length of 20 cm (8 inches), which was then passed through a neoprene-lined elastic hose section Lt; / RTI > A Lakewood Instruments Model 72 pH electrode body downstream from the neon-fren-lined hose was installed in the line to adjust the pH of the silicate / acid mixture. A portion of the neon-fren-lined hose, stainless steel tube and the tigon tube was submerged in a Branson Model 3200 ultrasonic bath operating at 47 kHz. After 6.5 hours of operation no silica deposits were visible in the tubes submerged in the ultrasonic bath. A clear coating of the silica deposit was observed on the pH electrode body which was not immersed in the ultrasonic bath. A pH electrode body is immersed in warm sodium hydroxide solution so that the silica deposit is removed from the pH electrode body. Analysis of the sodium hydroxide solution showed that 0.77 g of SiO ₂ was deposited on the pH electrode body.

≪ Example 3 >

This example describes a method of using an ultrasonic vibrator to elastically deform process vessels to prevent the formation of aluminosilicate deposits when producing polyaluminosilicate microgel solutions. 3.2 parts of dilute sodium silicate containing ₂ % by weight of SiO ₂ 100 ml / min was mixed with 20 ml / min of a sodium aluminate solution containing ₂ % by weight of Al ₂ O ₃ in a T-joint pipe connection mixer. The mixed solution was pumped through a 1/4 inch outer diameter stainless steel tube connected to a Nalgene 180 clear plastic grade tube (VI grade, 5/32 inch inner diameter). Some of the Netsen tubes were submerged in a Branson model 3200 ultrasonic bath operating at 47 kHz. All stainless steel tubes were submerged in an ultrasonic bath except for approximately 2.54 cm (1 inch) at each end. The aluminosilicate deposits are evident in the Yegen tube outside the ultrasound bath after about 1 hour of operation. The deposits in the outside-of-the-willen tube were easily removed by deforming the tube by stretching, bending or squeezing.

After 4 hours of preparation of the polyaluminosilicate microgels, no deposits were found in the hygene tubes submerged in an ultrasonic bath. The absence of any deposits in the stainless steel tube indicates that the ultrasonic bath caused the elastic deformation of the stainless steel tube immersed in the bath and the tube a little distance from the outside of the bath.

<Example 4>

This example demonstrates that a glass processing vessel having a low rate of elastic deformation when stressed is sufficiently burnt to produce a stress that is greater than the bond strength of the aluminosilicate deposit formed when the polyaluminosilicate microgel is made even when exposed to ultrasonic vibration. Explain that they are not sexually deformed. 3.2 parts of dilute sodium silicate containing ₂ % by weight of SiO ₂ 100 ml / min was mixed with 20 ml / min of a sodium aluminate solution containing ₂ % by weight of Al ₂ O ₃ in a T-joint pipe connection mixer. The mixed solution was pumped into a 15 cm (6 inch) length glass tube with an inner diameter of 0.40 cm (5/32 inch). Approximately 7.5 cm (3 inches) of the glass tube was submerged in a Branson model 3200 ultrasonic bath operating at 47 kHz. The glass tube was connected to a transparent vinyl tube with an inner diameter of 1/4 inch (0.635 cm), and some of them were also submerged in an ultrasonic bath. After about one hour of operation, the aluminosilicate deposit is visually apparent at the entire length of the glass tube (inside and outside the ultrasonic bath) and at the site of the moisture tube outside the ultrasonic bath. After 4 hours of operation, the entire length of the glass tube was coated with an aluminosilicate deposit. We could not visually observe deposits in the Nesen tubes that had been submerged in the bath.

Claims (10)

(a) mixing a water-soluble silicate aqueous solution and a gel initiator in a mixing vessel to prepare an aqueous mixture having a silica concentration of 0.5 wt% to 15 wt%; And (b) aging the mixture in a long aging vessel to partially gel the mixture, Wherein the mixing vessel and / or the aging vessel is elastically deformable to temporarily deform during the process to desorb the deposit formed on the vessel wall to remove deposits from the vessel. &Lt; RTI ID = 0.0 &gt;Gt; The method according to claim 1, wherein the container is temporarily deformed by increasing the internal pressure of the container. The method of claim 1, wherein the container is temporarily deformed by reducing the internal pressure of the container. The method of claim 1, wherein the outlet flow resistance is periodically varied to temporarily deform the vessel. 2. The method of claim 1, wherein a mechanical force is applied to the container to temporarily deform the container. 6. The method according to claim 5, wherein the mechanical force is applied by a roller, press or external fluid. 7. The method of claim 6, wherein a mechanical force is applied by one or more rollers moving along the longitudinal axis of the container. The method according to claim 1, wherein the container is exposed to vibration to be temporarily deformed. 9. The method of claim 8, wherein the vibration is transmitted from the surrounding liquid with the vibrating vibrator to the vessel. 9. The method of claim 8, wherein the mixture aged in the vessel delivers vibration to the vessel.