WO2014068185A1

WO2014068185A1 - A method of preventing scaling on surfaces of an injection mixing apparatus and an injection mixing apparatus

Info

Publication number: WO2014068185A1
Application number: PCT/FI2013/051022
Authority: WO
Inventors: Jouni Matula; Jussi MATULA; Jari RÄSÄNEN; Karri TAHKOLA
Original assignee: Wetend Technologies Oy
Priority date: 2012-10-30
Filing date: 2013-10-29
Publication date: 2014-05-08
Also published as: FI20126128A

Abstract

The present invention relates to a method of preventing scaling on surfaces of an injection mixing apparatus and an injection mixing apparatus (40, 70, 140). The injection mixing apparatus of the invention is especially applicable in pulp and paper industry, i.e. in wood processing industry. The injection mixing apparatus (40, 70, 140) of the present invention is provided with electrical means (130) for preventing scaling on its surfaces.

Description

A METHOD OF PREVENTING SCALING ON SURFACES OF AN INJECTION MIXING APPARATUS AND AN INJECTION MIXING APPARATUS

Technical field

[0001 ] The present invention relates to a method of preventing scaling on surfaces of an injection mixing apparatus and an injection mixing apparatus. The injection mixing apparatus of the present invention may be applied in any industry where there is a risk of formation of scaling on the surfaces of the injection mixing apparatus. The injection mixing apparatus of the invention is especially applicable in pulp and paper industry, i.e. in wood processing industry.

Background art

[0002] Injection mixing apparatuses are used in various applications in different industries. A part of the applications is, in a way, simple as the liquid or gas to be injected neither itself nor with any other liquid or gas it gets into contact with is able to form any chemical compounds or other particles that would tend to adhere to the surfaces of the mixing apparatus. Another part of the applications is such that the gas or liquid to be injected forms scaling or other particles either alone or together with another injected gas, liquid or injection or mixing liquid, the scaling or particles adhering to the surfaces of the mixing apparatus and cause different kinds of problems. As examples of the various ways the scaling may be formed the following manners are briefly discussed. Firstly, polymers when reacting rapidly with the anionic trash (pitch, for example) of the injection flow may form scaling that sticks to the surfaces of the nozzle part of the injection mixer. Secondly, a thin bio- or bacterial material film may be grown on the surfaces of the injection mixing apparatus. Thirdly, lignin dissolved from the wood as well as some chemicals may also adhere to the surfaces of the injection mixer or any other surface/s nearby. Fourthly, when injecting milk of lime a hard shelllike calcium carbonate precipitation is formed on the surfaces of in-line PCC production devices. In a similar manner there may be other multi-component chemicals that form scaling when reacting in or in the nearhood of an injection mixing apparatus. And fifthly, resin has a strong tendency of forming scaling, especially in case of temperature changes.

[0003] The scaling has a number of negative effects on the mixing, i.e. effects relating to the runnability of the process, increased disruptions in the process and quality of the end product. Firstly, the scaling may have an influence on the flow characteristics of the mixing apparatus, i.e. the scaling may reduce the cross sectional flow area of the injection mixing apparatus, which may lead to reduction in the amount of chemical injected by the mixing apparatus, and result in changes in the properties of the end product. Such reduction in the flow area may also have other effects on the mixing, i.e. making the mixing unequal, uneven or inhomogeneous At worst the scaling may block the injection flow entirely, or at least one or more injection openings at the nozzle part of the injection mixer. Secondly, the reduction in the cross sectional flow area results in higher pressure difference, which means increased energy required for pumping the desired injection flow. Thirdly, in some cases the scaling may reduce the cross sectional flow area of the actual process pipe resulting in the reduction of volume flow in the process pipe, or causing at least possibly negative changes in the flow characteristics, i.e. turbulence, fluctuation etc. or increasing the energy needed for pumping the process liquid. Such changes in the flow area may also have an effect on the mixing, i.e. making the mixing unequal, uneven or inhomogeneous. And fourthly, the scaling that has formed on the surfaces of the injection mixing apparatus may from time to time loosen as substantially large particles from the surfaces, and proceed along with the flow up to the end product breaking the end product, reducing the quality of the end product, or at least increasing the reject material of the process if the particles are captured by the possible screens arranged in the process piping.

[0004] The injection mixing apparatus discussed in CA 2541528 may be used as an example of injection mixing apparatus the present invention is concerned about. In principle the above discussed injection mixing apparatus is composed of a feed conduit for a chemical, a first casing at least partially surrounding the chemical feed conduit, and a second casing at least partially surrounding the first casing and by means of which the injection mixing apparatus is attached to the wall of the process liquid flow pipe or to any wall of a chamber or cavity inside which the process liquid is contained. The first casing is used for receiving mixing liquid and the second casing for receiving the feed/injection liquid. The first casing comprises an outside wall, an open interior, and an inlet opening in communication with a first inlet duct for a so called mixing liquid, an opening via which the chemical feed conduit extends into the first casing, and one or more openings at an end opposite the opening for the chemical feed conduit. The second casing comprises an outside wall, an open interior and three openings through the outside wall, i.e. an outlet opening that is, when the mixing apparatus is attached to the wall of the process liquid flow pipe, in flow communication with an opening in the wall of the process liquid flow pipe, an inlet opening in communication with a second inlet duct used normally for introducing so called injection or feed liquid into the mixing apparatus, and an opening, opposite to the outlet opening, via which the first casing extends into the second casing. A prior art injection mixing apparatus has one opening at an end of the first casing for allowing the chemical feed conduit to pass the opening and extend outside the first casing for feeding a chemical into the process liquid chamber, cavity or flow pipe. The injection mixing apparatus is, when in operation, attached to a wall of a chamber, a cavity or a flow pipe containing process liquid by means of a sleeve, the first end of which conforms to the outer surface of the process liquid chamber, cavity or flow pipe, and the second end having a flange or other communicating means communicating with a flange or other communicating means at an end of the second casing. Naturally, it would also be possible to attach the injection mixing apparatus directly from its second casing to the wall of the process liquid chamber, cavity or flow duct, but such is considered a less attractive option.

[0005] The above discussed injection mixing apparatus functions well as long as any combination of the injection liquid, mixing liquid, the chemical and the process liquid are not able to form any solid compounds that are prone to attach on the surfaces of the process equipment including the injection mixing apparatus. However, as the injection liquid, mixing liquid, and/or the process liquid (sometimes the process liquid is used as the injection liquid and/or the mixing liquid) very often contain solids or such chemicals that either together or together with the chemical/s to be injected into the process liquid are able to form scaling, there is a great risk that the scaling adheres to the injection mixing apparatus resulting in the problems discussed already above.

[0006] WO-A2-201 1 1 10745 discusses a method and a reactor for mixing one or more chemicals into a process liquid flow. Since the one or more chemicals are often such that tend to form precipitations or scaling on the surface of the reactor or flow pipe in which the process liquid flows, the reactor is provided with electrical means for keeping the surface of the reactor and possibly the structures therein clean. The means is a first electrode arranged centrally in the flow pipe, a second electrode arranged in connection with the surface of the flow pipe, or the surface of the flow pipe itself acting as the second electrode, and an electrical circuit with a voltage source and a control unit arranged between and in connection with the electrodes. When the electrodes are connected to the voltage source a potential difference is created between the electrodes and one electrode starts acting as an anode and the other as a cathode resulting in the collection of oppositely charged ions on the surfaces of the electrodes. As a further result some compounds start adhering to the electrode surface. However, the control unit is programmed to switch the polarity of the electrodes from time to time whereby both the direction of the movement of the ions and the pH at the surface of the electrodes changes resulting in dissolving of the possibly created scaling on the surface of an electrode.

[0007] However, it has been learned that the above discussed arrangement is able to keep both the central electrode surface and the reactor surface clean, but the arrangement is not able to keep the surfaces of the injection mixer clean. In the performed tests it has been learned that the speed of formation of the scaling on the tip surfaces of the injection mixer is so much faster than on the other surfaces of the reactor that the scaling once formed on the tip surfaces cannot be entirely dissolved any more but the openings or holes at the surfaces are gradually clogged or at least severely throttled by the scaling.

Brief summary of the Invention

[0008] Thus, an object of the present invention is to develop a method of preventing scaling on surfaces of an injection mixing apparatus that overcome at least some of the above discussed problems.

[0009] Another object of the present invention is to develop an electrical arrangement for preventing scaling in connection with an injection mixing apparatus such that no such additional means in the mixing apparatus is needed that could influence on the flow of the fluids in the mixing apparatus.

[0010] Thus a further object of the invention is to utilize existing components of the injection mixing apparatus as the electrodes of an electric circuit so that the injection mixing apparatus needs to be provided with only the necessary electrical insulations and connections to a voltage source, i.e. components outside of the actual flow channels within the injection mixing apparatus.

[001 1 ] At least one object of the present invention is fulfilled by a method of preventing scaling on surfaces of an injection mixing apparatus, the injection mixing apparatus comprising a conduit for a first fluid and at least one casing, the conduit for the first fluid being positioned centrally within the at least one casing and having a first end extending outside the at least one casing and the conduit for the first fluid being connected to a fluid source, the injection mixing apparatus, when in use, being attached to a wall of a process liquid flow pipe, the method comprising providing the injection mixing apparatus with means for preventing scaling on surfaces of the injection mixing apparatus.

[0012] A further feature of the present invention is the steps of electrically insulating the conduit for the first fluid from the at least one casing and the source of the first fluid, electrically insulating at least the at least one casing from the fluid source and the process liquid flow pipe, coupling the conduit for the first fluid to a terminal of an electrical circuit, coupling the at least one casing to another terminal of an electrical circuit, and changing the polarity of the electrical circuit from time to time for changing the pH at surfaces of the conduit for a first fluid and the at least one casing.

[0013] At least one object of the present invention is fulfilled by an injection mixing apparatus comprising a conduit for a first fluid and at least one casing, the conduit for the first fluid being positioned centrally within the at least one casing and having a first end extending through the at least one casing and the conduit for the first fluid being connected to a fluid source, the injection mixing apparatus, when in use, being attached to a wall of a process liquid flow pipe, wherein the injection mixing apparatus is provided with means for preventing scaling on surfaces of the injection mixing apparatus.

[0014] A further feature of the present invention is an electrical circuit coupled in electrical communication with such surfaces of the injection mixing apparatus that tend to collect scaling, wherein such surfaces are electrically insulated from one another.

[0015] Other characteristic features of the method and the injection mixing apparatus of the present invention are disclosed in the appended patent claims. [0016] The most remarkable advantage gained by the present invention is the fact that the risk of scaling on the surfaces of an injection mixing apparatus is eliminated. More detailed advantages gained by the use of the present invention are, for instance,

• Since existing components of the injection mixer are used as the electrodes there is no need for bringing in additional electrode/s that could have a negative effect on the flow/s within the injection mixer,

• More accurate chemical proportioning into the process liquid,

• Better controllability of the flow conditions in the nearhood of the injection mixing apparatus, i.e. better runnability of the process

• Better predictability of the flow conditions downstream of the injection mixing apparatus

• Better quality of the end product.

Brief Description of Drawing

[0017] In the following, the method and the apparatus of the present invention are discussed in more detail with reference to the appended figures, in which

Figure 1 illustrates a prior art injection mixing apparatus;

Figure 2 illustrates an injection mixing apparatus in accordance with a first preferred embodiment of the present invention;

Figure 3 illustrates an injection mixing apparatus in accordance with a second preferred embodiment of the present invention,

Figure 4 illustrates an injection mixing apparatus in accordance with a third preferred embodiment of the present invention,

Figure 5 illustrates an injection mixing apparatus in accordance with a fourth preferred embodiment of the present invention, and

Figure 6 illustrates the injection mixing apparatus of Figure 3 coupled to an electrical circuit for maintaining the surfaces of the injection mixing apparatus clean. Detailed Description of Drawings

[0018] Figure 1 illustrates a prior art injection mixing apparatus 10. The injection mixing apparatus 10 comprises a conduit 6 for feeding a chemical into the injection mixing apparatus 10, a first casing 2, and a second casing 4. The first and the second casings are fastened to each other by means of, for instance, bolts 8. The injection mixing apparatus 10 is fastened to the wall 12 of the process liquid flow pipe 14 by its first casing 2, for instance, by means of an intermediate sleeve 16. The first casing 2 has an inlet duct 18 for a second fluid, i.e. an injection or feeding liquid, which may be either clean water or almost any non-clean liquid, or the process liquid itself that may be introduced into the process liquid, and at its first end 20 an outlet 22 via which the injection liquid is introduced (first into the sleeve 16, and then) into the process liquid flow pipe 14. The opposite end, i.e. the second end 24 of the first casing 2 is provided with a preferably round opening 26 through which the second casing 4 extends into the first casing 2. The first or inner end 28 of the second casing 4 extends out of the first end 20 of the first casing 2 inside the sleeve 16 such that the flow path (functions as an extension of the outlet 22) of the second fluid or the injection liquid between the sleeve 16 and the first end 28 of the second casing 4 is preferably annular.

[0019] The second or outer end 30 of the second casing 4 is provided with an inlet 32 for a so called third fluid, so called mixing liquid, and a preferably axial opening 34 for the conduit 6 for a first fluid, most often a chemical. The chemical conduit 6 extends preferably axially though the second casing 4 leaving an annular flow passage between the conduit 6 and the second casing 4 for the second fluid, i.e. the mixing liquid. The first end 36 of the chemical conduit 6 is located within the first end 28 of the second casing 4. The chemical conduit 6 is connected at its second end 38 farther away from the process liquid flow pipe 14 to a source of the chemical. The first or inner end 28 of the second casing 4 is located at about the level of the wall 12 of the process liquid flow pipe 14. However, the position of the second casing 4 may be adjusted by means of its fastening bolts 8 in its longitudinal direction so that the first or inner end 28 thereof may be left within the first casing 2, within the sleeve 16 or extend inside the process liquid flow pipe 14.

[0020] The problem related to the formation of scale or some other solids on the surfaces of the injection mixing apparatus may be most easily seen in the parts being in communication with both the process liquid and the chemical to be injected in the process liquid flow pipe. Thus the parts being in contact with the chemical and the process liquid need special attention. Now that the structure of the injection mixing apparatus is such that the chemical is brought down to the first or inner end of the second casing such that it has no contact with the liquids present in the mixing apparatus, it is the tip part or first end of the second casing that needs to be provided with means for preventing the scale from forming thereon. After extensive testing of various alternatives, an electrical means for preventing the formation of scale was considered the most practical means for solving the present problem. However, it should be understood that a coating of a certain material may also be used to prevent the scaling. In other words, the coating material is selected such that the substances or chemicals forming the scaling are not able to adhere to the coating. The coating may be, for instance, rubber, Teflon or a nano surface coating.

[0021 ] Figure 2 illustrates a novel injection mixing apparatus 40, which differs from the prior art injection mixing apparatus at the tip part or the first end 44 of the second casing 42. The parts of the injection mixing apparatus that are the same as in the prior art mixing apparatus of Fig. 1 have maintained the same reference numerals, whereas the parts that have changed have been renumbered. Also from now on mainly different fluids have been discussed, as the structure of the injection mixing apparatus is not dependent on the type of fluid but the effect of the different fluids to one another. However, examples of the type of fluid have been given. Thus, the second casing 42 of the injection mixing apparatus 40 has a first or inner end 44 provided with a cap 46 having a central opening 48 for a conduit 50 for the first fluid, for instance the first chemical. In other words, the first end 52 of the conduit 50 for the first fluid extends through the cap 46 into the process liquid flow pipe 14. The first end 52 may just extend through the cap 46, i.e. open immediately to the level of the cap, or the first end may extend deeper in the process liquid flow pipe, as shown in Figures 3 and 4. The cap 46 is provided with holes 54 for injecting third fluid, for instance a mixing liquid and/or a second chemical or their mixture in more or less radial direction (in view of the second casing 42) into the annular flow path between the sleeve 16 and the second casing 42, in other words into an annular or fan-shaped jet of second fluid, i.e. injection liquid entering the flow pipe 14 from the annular flow path between the second casing 42 and the sleeve 16.

[0022] As an example of scaling the formation of calcium carbonate precipitation on the mixing apparatus surfaces may be discussed. However, it has to be understood that the scaling of any chemical or a compound of two or more chemicals takes place in a substantially similar manner. The practice has shown that an injection mixing apparatus used for injecting milk of lime as the first chemical into the process liquid, very quickly collects a considerable amount of calcium carbonate on the outside surfaces of both the cap 46 and the first end 52 of the conduit 50 for the first fluid. The reason is that the process liquid either itself contains carbon dioxide or the C0₂ has been added in the process liquid earlier or by means of the injection mixing apparatus as the second chemical, for instance in connection with the mixing liquid. Now that the concentration of at least one chemical at the tip area of the injection mixer is high, the chemical environment changes significantly such that the speed of formation of the calcium carbonate is high. And as the calcium carbonate has a tendency of adhering to all kinds of solids, including solid surfaces, there is a high risk that the injection holes 54 at the cap and corresponding hole/s at the first end 52 of the conduit 50 are gradually clogged or at least throttled by the scaling. Such would result in the reduction of the added chemical and the reduction in the quality of the end product.

[0023] To prevent such a scaling the injection mixing apparatus 40 is provided with means (discussed in more detail in connection with Figure 6) changing the polarity of an electrical circuit between the second casing 42 and the conduit 50 for the first fluid. For such a purpose, the central opening 48 in the cap is provided with an electrical insulation 56. Also, the opening 34 at the opposite or second end 30 of the second casing 42 is provided with an electrical insulation 58, so that the conduit 50 for the first fluid and the second casing 42 may be connected to opposite (plus and minus) terminals of an electrical cleaning system. For completing the electrical insulation the opening 26 at the second end 24 of the first casing 2 is provided with an electrical insulation 60 so that the first casing 2, the sleeve 16 and the process liquid flow pipe remain non-electrical components of the system. Naturally, all the fluid connections of the electrical components (here the second casing and the conduit for the first fluid) of the mixing apparatus have to be insulated from their respective fluid sources. In other words, also the conduit 50 for the first fluid has to be insulated from the source of the first fluid and the conduit 32 for the third fluid from the source of third fluid. And finally the attachment connecting the first casing 2 with the second casing 42 has to be electrically insulated or made of non-conducting materials. For instance the bolts 8 may be manufactured of non-conductive material.

[0024] Thus the conduit 50 for the first fluid and the second casing 42 may be connected to opposite (plus and minus) terminals of an electrical cleaning system, which prevents the chemicals or the reaction products from adhering on the surfaces of the injection mixing apparatus electrochemically. In this first preferred embodiment of the present invention the conduit 50 for the first fluid functions as one electrode and the second casing 42 as another electrode. When the electrodes are connected to an electrical circuit, one electrode forms a cathode and the other anode. The existence of electric current changes the pH in the liquid (water) layer on the electrode surfaces such that the pH at the anode is lowered and, correspondingly, the pH at the cathode is increased. In practice this means that when the surface of the cap 46 is the anode, and the electric current strong enough, the pH at the surface of the cap is so low that, for instance, calcium carbonate is not able to precipitate on the cap surface, and the calcium carbonate already present on the surface dissolves to the liquid phase. However, the conduit 50 for the first fluid being the cathode attracts the carbonate crystals, whereby the surface of the first end 52 of the conduit 50 for the first fluid is gradually covered with calcium carbonate crystals. For that purpose the electrical circuit is arranged to change the polarity such that, for a certain period of time, the conduit 50 for the first fluid acts as the anode, and the cap 46, i.e. the first end 44 of the second casing 42 as the cathode. Now the pH in the liquid layer close to the surface of the conduit 50 for the first fluid gets lower, whereby the calcium carbonate dissolves from the electrode surface to the liquid phase, and the electrode surface gets clean. Simultaneously, naturally, the anode, i.e. the cap surface collects the calcium carbonate crystals. For dissolving the CaC0₃ crystals from the cap surface the polarity of the electrical circuit may again be changed.

[0025] Figure 3 illustrates a second preferred embodiment of the present invention. Here an injection mixing apparatus similar to that discussed in Figure 2 is used as the starting point. Therefore, the same reference numerals are used for the parts or components having the same structure and function. Also the types of fluid introduced into the mixing apparatus are the same. The main difference between the embodiments of Figures 2 and 3 can be seen in the way the electrical insulations have been arranged. The main difference is that in the embodiment of Figure 2 the first casing 2 was electrically insulated from the second casing 42, whereas in the embodiment of Figure 3 both the first casing 2 and the second casing 42 form one electrode and the conduit 50 for the first fluid another electrode. For that purpose the conduit 50 for the first fluid is insulated from the second casing 42 by means of electrical insulators 56 and 58, just like Figure 2 teaches. But since the first and the second casings are not insulated from one another, the first casing 2 has to be electrically insulated from the wall 12 of the process liquid flow pipe 14. Such has, for instance, been performed by manufacturing the sleeve 62 used for connecting the injection mixing apparatus 40 to the wall 12 of the process liquid flow pipe 14 of an electrically non-conductive material. Naturally, fluid sources, i.e. the sources of the chemical/s, the mixing liquid and the feed or injection liquid are electrically insulated from the injection mixing apparatus 40. As yet another difference, which was already discussed in connection with Figure 2, is the length of the conduit 50 for the first chemical. Now here the first end 52' of the conduit extends deeper in the process liquid flow pipe 14.

[0026] Figure 4 illustrates an injection mixing apparatus in accordance with a third preferred embodiment of the present invention. The injection mixing apparatus 70 of Figure 4 comprises a conduit 50 for a first fluid, for instance a first chemical, a third casing 72 with an inlet 74 for a fourth fluid, for instance a third chemical or a mixing liquid or their mixture, a second casing 76 with an inlet 78 for a third fluid, for instance a mixing liquid or a second chemical or their mixture, and a first casing 80 with an inlet 82 for a second fluid, for instance a feed or injection liquid. The first, second and third casings are fastened to each other by means of, for instance, bolts 85. The injection mixing apparatus 70 is fastened to the wall 12 of the process liquid flow pipe 14 by its first casing 80, for instance, by means of an intermediate sleeve 16. The first casing 80 has, in addition to the inlet 82 for the second fluid, which may be either clean water or almost any non-clean liquid that may be introduced into the process liquid, including the process liquid itself, an outlet 84 via which the second fluid is introduced into the sleeve 16 and further to the process liquid flow pipe 14. The opposite end, i.e. the second end 86 of the first casing 80 is provided with a preferably round opening 88 via which the second casing 76 extends into the first casing 80. The first or inner end 90 of the second casing 76 extends through the first casing 80 down to the sleeve 16 such that the outlet 84 and the flow path of the second fluid between the sleeve 16 and the first end 90 of the second casing 76 is preferably annular. The inner or first end 90 of the second casing 76 terminates preferably within the sleeve 16 though it may extend up to the level wall 12 of the flow pipe 14 or sometimes even further. In the earlier embodiments illustrated in Figures 2 and 3 the longitudinal position of the second casing 42 was, optionally, adjustable within the first casing 2. The same applies here, too. Thus the position of the first end 90 of the second casing 76 may be longitudinally adjusted, if desired, between the conical part or the first end 92 of the first casing 80 and the inside of the process liquid flow pipe 14. Additionally, the longitudinal, i.e. axial position of the third casing 72 is, preferably but not necessarily, adjustable, too. It means that, for instance, the position of the third casing 72 within the second casing 76 may be adjusted.

[0027] The second or outer end 94 of the second casing 76 is provided with an inlet 78 for the second fluid (the mixing liquid or the second chemical or their mixture), and a preferably axial opening 96 for the third casing 72. The third casing 72 comprises a casing body 98 and a conduit 100 for the fourth fluid (the third chemical or the mixing liquid or their mixture). The casing body 98 has an inlet 74 for the fourth fluid (the third chemical or the mixing liquid or their mixture), an outlet opening 102 communicating with the conduit 100 for the fourth fluid, and an opening 104 for a conduit 50 for the first fluid, for instance the first chemical. The conduit 100 for the fourth fluid extends preferably axially though the second casing 76 leaving an annular flow passage for the third fluid between the conduit 100 for the fourth fluid and the second casing 76. The first or inner end 90 of the second casing 76 and the conduit 100 for the fourth fluid leave an annular outlet opening 106 via which the third fluid is introduced into the flow of the second fluid. The conduit 100 for the fourth fluid terminates to a cap 108, which is provided with a central opening 1 10 via which the conduit 50 for the first fluid passes into the flow pipe 14. The cap 108 is provided with holes 1 12 for injecting the fourth fluid into the annular or fan-shaped jet of the third fluid and the second fluid entering the flow pipe 14 along the annular passage between the conduit 100 for the fourth fluid and the sleeve 16.

[0028] This kind of an injection mixing apparatus may also be provided with electrical insulations to facilitate the cleaning of the surfaces of the cap 108 and the first end 1 14 of the conduit 50 for the first fluid. Here an insulation 1 16 is arranged in the opening 1 10 between the conduit 50 and the cap 108 at the first end 1 18 of the conduit 100 for the fourth fluid, an insulation 120 in the opening 96 between the conduit 100 for the fourth fluid and the second casing 76, and an insulation 122 in the opening 104 between the third casing 72 and the conduit 50 for the first fluid. Just like in the earlier embodiments, also here the various fluid conduits have to be insulated from their fluid sources, i.e. the conduit 50 for the first fluid has to be insulated from its source, for instance from the chemical source, and the conduit 74 for the fourth fluid from its source. Also the attachment connecting the third casing 72 with the second casing 76 has to be electrically insulated or made of non-conducting materials. For instance the bolts 85 may be manufactured of non-conductive material.

[0029] The above disclosed injection mixing apparatus for two (or more) chemicals may, as an alternative to the one shown in Figure 4, be provided with a similar electrical insulation as discussed in connection with Figure 3. In other words, the insulation 120 between the second casing 76 and the conduit 100 for the fourth fluid may be, in a way, transferred to the sleeve 16, i.e. the sleeve may be manufactured of a non-conductive material. By doing this the main parts in view of cleaning the mixing apparatus, i.e. the conduit 50 for the first fluid and the conduit 100 for the fourth fluid may still be coupled to opposite (plus and minus) terminals of the electrical circuit, and the parts kept clean. This kind of an insulation naturally precludes that all fluid conduits or ducts introducing a chemical, a mixing liquid and a feeding/injection liquid into the injection mixing apparatus have to be also electrically insulated from their sources.

[0030] Figure 5 illustrates a fourth preferred embodiment of the present invention. Here the injection mixing apparatus 140 is of somewhat simpler construction than those of the earlier embodiments, i.e. the injection mixing apparatus 140 of Figure 5 has a conduit 142 for the first fluid and a (first) casing 144 for the second fluid, whereby the second casing utilized in the earlier embodiments is missing. Thus the injection mixing apparatus 140 is fastened from its first casing 144 to the wall 12 of the process liquid flow pipe. In spite of the structural differences, the types of fluid introduced into the mixing apparatus are the same. In the embodiment of Figure 5 the conduit 142 for the first fluid forms one electrode and the first casing 144 for the second fluid another electrode. For that purpose the conduit 142 for the first fluid is insulated from the casing 144 by means of electrical insulator 146. The casing 144 has been insulated from the wall 12 of the process liquid flow pipe by means of an insulator sleeve 148, manufactured of an electrically non-conductive material and by means of which the mixing apparatus 140 is actually fastened to the wall 12 of the process liquid flow pipe. Naturally, fluid sources, i.e. the sources of the chemical/s, and the feed or injection liquid (conduit 142 introducing the feed/injection liquid into the casing 144) are electrically insulated from the injection mixing apparatus 140. Also, if the mixing apparatus is provided with the adjusting means, shown in an exemplary manner as a bolt 150, the adjusting means is either made of electrically non-conductive material, or its electrical insulation is carried out by some other appropriate means. [0031 ] Figure 6 discusses in more detail the electrical circuit and the operation of the system discussed in connection with the earlier Figures 2 - 5. The electrical means for preventing scaling on the surfaces of an injection mixing apparatus shown, as an example of the four earlier discussed embodiments, in Figure 3 comprise a control unit (CU), which is coupled by means of its plus and minus terminals to the conduit 50 for the first fluid, normally for the chemical, and, in this case, to the second casing 42, though also the first casing 2 is an option since the first and second casings are not insulated from one another in this particular embodiment. When a certain electric current is switched on in the electric circuit 130 comprising the control unit CU, the wiring 132 coupling the conduit 50 for the first fluid and the second casing 42 to the control unit CU, and the conduit 50 for the first fluid and the second casing 42, a certain pH (acidic or alkaline) is formed in the nearhood of the surface of the conduit 50 for the first fluid and a corresponding opposite pH (alkaline or acidic) in the nearhood of the surface of the second casing 42. Thus, the pH value attracts some substances to precipitate on the surface of the first end 52 of the conduit 50 for the first fluid and some other substances on the surface of the first end 44 of the second casing 42, and simultaneously the pH is able to make some other substances dissolve and thus loosen from those surfaces. For instance, if it is a question of calcium carbonate crystals, they cannot even contact the surface as long as the liquid close to the surface is at low pH. However, at a surface where the liquid is at high pH, the crystals adhere to the surface. However, as soon as the polarity is changed, the pH changes and the crystals dissolve in the liquid phase at a low pH. The disadvantages arising from said precipitation tendency are easy to eliminate by programming the control unit CU to change the polarity of the electrical circuit from time to time, whereby the carbonate or any other substance previously precipitated on the surface acting as the cathode is quickly dissolved in the acidic liquid formed near the electrode now acting as the anode. The simplest control method is to program the control unit to change the polarity of the electrical circuit by means of a timer at certain intervals (from fractions of a second to seconds, minutes or hours) for keeping both electrodes clean. Another way to control the polarity changes is to use a control impulse from the process. It is, for example, possible to monitor the voltage difference between the cathode and the anode (illustrated by means of a voltage meter V), whereby a certain increase in the voltage in practice means a precipitation layer of a certain thickness (the layer acting as isolation). Thus the control unit may be calibrated to change the polarity of the electrical circuit at a certain potential difference. Correspondingly, when said potential difference has been reduced back to its original level or when the potential difference no more changes, the control system switches the polarity back to the original state.

[0032] In view of the above, it has to be understood that, since the scaling-related problem is limited to such surfaces of the injection mixing apparatus that are in communication with at least one of the chemicals injected by the mixing apparatus and either one of the fluids being injected or the process liquid or both, the surfaces are at a close proximity of the process liquid flow duct, possibly within the process liquid flow duct. Also, it has to be understood that the injection mixing apparatus itself should have those two different surfaces on which the scaling may precipitate. As long as such is the case the insulation of the two different surfaces is quite simple, as shown by the above examples. If the other surface, on which the scaling tends to collect, were the wall of the process liquid flow duct, the wall should form one electrode of the electrical circuit. Since such is not desired, unless a certain length of the process liquid flow duct were to be insulated electrically from the rest of the flow duct, the two different surfaces should be found from the injection mixing apparatus.

[0033] Figures 2 - 5 illustrate a few exemplary embodiments of the structures of the applicable injection mixing apparatus. Thus, the basic principle of the present invention is that the two elements of the injection mixing apparatus extending closest to the process liquid flow pipe should be electrically insulated from one another, and also from the fluid sources including sources for the injection liquid and the mixing liquid (if used), and possibly one or more chemical sources, and from the process liquid flow pipe. In principle, it does not matter whether the radially outer element is directly or indirectly insulated. In other words, Figure 2 teaches a direct insulation 60 of the second casing 42 from the first casing 2 surrounding it and Figure 3 an indirect insulation 62 of the second casing 42 from the process liquid flow pipe 14.

[0034] If it were a question of the embodiment of Figure 4, the terminals would be coupled to the chemical conduit 50 and to the first casing 42. Likewise in the embodiment of Figure 5, the terminals would be coupled to the conduit 142 and the (first) casing 144.

[0035] Figure 6 illustrates as a preferred feature of the present invention a reference electrode 134, which is arranged in a close proximity of the injection mixer 40. Thus, the reference electrode may be attached to the injection mixer itself, preferably to such a part that is located within the reactor or flow pipe, or to a wall of the flow pipe or reactor in the nearhood of the injection mixer. The purpose of the reference electrode 134 is to sense the surface charge at its position, i.e. the polarity of its surface, or, in other words, the chemical environment at the tip area of the injection mixer. The information it gives may be used either manually when adjusting the operating parameters (for instance, the voltage and the cycle time) of the control unit and the circuit it is connected to or automatically for giving instructions to the control unit to change its operating parameters if the chemical environment at the tip area of the injection mixer appears to be changing. Naturally, the reference electrode 134 may be used in connection with any one of the embodiments discussed above as well as all the variations of the present invention covered by the appended claims.

[0036] As may be seen from the above, a novel method of preventing scaling on surfaces of an injection mixing apparatus and an injection mixing apparatus has been developed. It should be noted that although the above description discusses two different types of injection mixing apparatuses the present invention is not limited to the discussed embodiments but only to the methods and structures disclosed in the attached claims. Also it has to be understood that the use of the above discussed means for preventing the formation of calcium carbonate scaling on surface/s of the injection mixing apparatus is only an example. The present invention has to be understood more broadly, whereby the prevention of all kinds of scaling and other attachment of solid material on the surfaces of the injection mixing apparatus are within the scope of the present invention irrespective of the nature of the chemical/s, mixing liquid, injection liquid or the process liquid. Thus, the invention may be applied anywhere chemicals need to be fed and mixed into a process medium flow evenly and in precise amounts. Thus, the field of application and the scope of protection of the invention are defined by the appended patent claims, only. Also, it should be realized that the word "chemical" is understood in a broad sense, i.e. it covers each and every additive, treatment agent, filler, pigment etc. being introduced in the process liquid for treating the process liquid or for changing its properties or the properties of an intermediate or end product.

Claims

1 . A method of preventing scaling on surfaces of an injection mixing apparatus, the injection mixing apparatus (40; 70; 140) comprising a conduit (50, 142) for a first fluid and at least one casing (2; 42; 80; 76; 98; 144), the conduit (50, 142) for the first fluid being positioned centrally within the at least one casing (2; 42; 80; 76; 98; 144) and having a first end (52; 52'; 1 14) extending through the at least one casing (2; 42; 80; 76; 98; 144), the at least one casing (2; 42; 80; 76; 98; 144) and the conduit (50, 142) for the first fluid being connected to a fluid source, the injection mixing apparatus (40, 70, 140), when in use, being attached to a wall (12) of a process liquid flow pipe (14), characterized by providing the injection mixing apparatus with means for preventing scaling on surfaces of the injection mixing apparatus.

2. The method as recited in claim 1 , characterized by steps of

• Electrically insulating the conduit (50; 142) for the first fluid from the at least one casing (2; 42; 80; 76; 98; 144) and the source of the first fluid, · Electrically insulating at least the at least one casing (2; 42; 80; 76; 98;

144) from the fluid sources and the process liquid flow pipe (14),

• Coupling the conduit (50; 142) for the first fluid to a terminal of an electrical circuit (130),

• Coupling the at least one casing (2; 42; 80; 76; 98; 144) to another terminal of the electrical circuit (130), and

• Changing the polarity of the electrical circuit (130) from time to time for changing the pH at surfaces of the conduit (50; 142) for a first fluid and the at least one casing (2; 42; 80; 76; 98; 144).

3. The method as recited in claim 2, characterized by controlling the operation of the electrical circuit (130) by means of a control unit (CU).

4. The method as recited in claim 3, characterized by changing the polarity of the electrical circuit (130) by means of a timer arranged in the control unit (CU).

5. The method as recited in claim 2, characterized by changing the polarity of the electrical circuit (130) by means of monitoring the voltage (V) difference in the electrical circuit (130) by the control unit (CU).

6. The method as recited in claim 2, characterized by providing the injection mixing apparatus (40; 70; 140) or the wall (12) of the process liquid flow pipe (14) in the nearhood of the injection mixing apparatus (40; 70; 140) with a reference electrode (134).

7. An injection mixing apparatus, which comprises a conduit (50; 142) for a first fluid and at least one casing (2; 42; 80; 76; 98; 144), the conduit (50; 142) for the first fluid being positioned centrally within the at least one casing (2; 42; 80; 76; 98; 144) and having a first end (52; 1 14) extending through the at least one casing (2; 42; 80; 76; 98; 144), the at least one casing (2; 42; 80; 76; 98; 144) and the conduit (50; 142) for the first fluid being connected to a fluid source, the injection mixing apparatus (40, 70; 140), when in use, being attached to a wall (12) of a process liquid flow pipe (14), characterized in that the injection mixing apparatus is provided with means for preventing scaling on surfaces of the injection mixing apparatus.

8. The injection mixing apparatus as recited in claim 7, characterized in that the conduit (50; 142) for the first fluid and the at least one casing (2; 42; 80; 76; 98; 144) are coupled to the terminals (132) of an electrical circuit (130) such that the conduit (50; 142) for the first fluid and the at least one casing (2; 42; 80; 76; 98; 144) form the anode and the cathode of the electrical circuit (130).

9. The injection mixing apparatus as recited in claim 7, characterized in that the conduit (50; 142) for the first fluid and the at least one casing (2; 42; 80; 76; 98; 144) are electrically insulated from one another by means of an electrical insulation (56, 58; 1 16, 122; 146) therebetween.

10. The injection mixing apparatus as recited in claim 7, characterized in that the mixing apparatus (140) comprises a conduit (142) for the first fluid and a first casing (144) for a second fluid, the conduit (142) and the first casing (144) being electrically insulated from one another by means of insulation (146) and that the first casing (144) is electrically insulated from the wall (12) of the process liquid flow pipe by means of an insulation (148).

1 1 . The injection mixing apparatus as recited in claim 7, characterized in that the mixing apparatus (40) comprises a conduit (50) for the first fluid, a first casing (2) for a second fluid, a second casing (42) for a third fluid, the second casing (42) being positioned centrally round the conduit (50) and the first casing (2) being positioned centrally round the second casing (42), the conduit (50) and the second casing (42) being electrically insulated from one another by means of insulation (56, 58) and the second casing (42) being electrically insulated from the first casing (2) by means of an insulation (60).

12. The injection mixing apparatus as recited in claim 7, characterized in that the mixing apparatus (40) comprises a conduit (50) for the first fluid, a first casing (2) for a second fluid, a second casing (42) for a third fluid, the second casing (42) being positioned centrally round the conduit (50) and the first casing (2) being positioned centrally round the second casing (42), the conduit (50) and the second casing (42) being electrically insulated from one another by means of insulation (56, 58) and the first casing (2) being electrically insulated from the wall (12) of the process liquid flow pipe by means of an insulation (62).

13. The injection mixing apparatus as recited in claim 7, characterized in that the mixing apparatus (40) comprises a conduit (50) for the first fluid, a first casing (80) for a second fluid, a second casing (76) for a third fluid, a third casing (98) for a fourth fluid, the third casing (98) being positioned centrally round the conduit (50), the second casing (76) being centrally round the third casing (98) and the first casing (80) being positioned centrally round the second casing (76), the conduit (50) and the third casing (98) being electrically insulated from one another by means of insulation (1 10, 122) and the second casing (42) being electrically insulated from the third casing (2) by means of an insulation (120).

14. The injection mixing apparatus as recited in any one of claims 8 - 10 or 12, characterized in that the injection mixing apparatus (40; 70; 140) is electrically insulated from the process liquid flow pipe (14) by means of an electrically non- conductive sleeve (62; 148).

15. The injection mixing apparatus as recited in any one of the preceding claims 8 - 14, characterized in that the conduit (50; 142) for the first fluid is connected to the source of the first fluid, the first casing (2; 80; 144) to a source of a second fluid, and that the injection mixing apparatus (40; 70; 140) is electrically insulated from at least the sources for the first fluid and the second fluid.8 - 14, characterized in that the conduit (50) for the first fluid is connected to the source of the first fluid, the first casing (2; 80) to a source of a second fluid and the second casing (4, 76) to a source of a third fluid, and that the injection mixing apparatus (40; 70) is electrically insulated from at least the sources for the first fluid and the third fluid.

16. The injection mixing apparatus as recited in any one of the preceding claims 8

- 16, characterized in that the conduit (50; 142) and one of the casings (2, 42, 76, 80, 98, 142, 144) is connected to terminals (132) of an electrical circuit comprising a control unit (CU).

17. The injection mixing apparatus as recited in any one of the preceding claims 8

- 16, characterized in that the injection mixing apparatus (40; 70; 140) or the wall (12) of the process liquid flow pipe (14) in the nearhood of the injection mixing apparatus (40; 70; 140) is provided with a reference electrode (134).