The present invention relates to a method according to the preamble of claim 1 for dosing and mixing viscous substan¬ ces accurately and at a desired proportion.

The invention further relates to an apparatus for the ap- plication of the invention.

Accurate dosing and mixing, in a predetermined proportion, of highly viscous liquids and, in particular, such liquids the viscosity of which changes strongly with changing tem- perature, is difficult. In order to obtain an accurate do¬ sing, a flow meter must be placed in the tube line and in order to compensate for changes in temperature, pressure and density, metering devices for determining these factors must be placed therein as well. Thus, there will be four different metering devices the values of which have to be handled in order to determine the correct dosage and mixing proportion. On the basis of the measure results received, the pump or the throttle valve of the tube line is control¬ led to keep the volume flow within desired limits. The de- vices used for measuring the volume and mass flows consti¬ tute the most expensive parts of the system. For measuring the volume flow, meters based, among other things, on the pressure difference caused by the the throttle or on the centrifugal force of the flow, turbine meters and deplace- ment meters are used. Such a system is expensive and comp¬ licated and requires accurate design and careful use in or¬ der to operate with a reasonable accuracy and reliability. In addition, the devices used for measuring the volume flow operate accurately only within a certain viscosity range, wherefore only those products the viscosity of which is within the narrow operating range of the apparatus can be treated reliably with them. This is a severe drawback when

handling liquids the viscosity of which changes strongly with e.g. arying temperature or dry substance content.

When handling highly viscous substances or substances of highly changing viscosity it is of utmost importance that the flow in the piping is continuous, so that the substanc under treatment is not allowed to stiffen in the piping. Such substances are e.g. bitumen mixtures, glues, polymers and a plurality of other chemical products and products containing powdery or particulate substances. For the mea¬ surement and control of amount such devices are to be used which are influenced by the changes in viscosity to an ex¬ tent as small as possible. In such systems where such subs tances are handled, it is by means of a return circulation to be ensured that there always is a sufficient flow of substance in the piping. The realization of the return flo leads, however, to great difficulties in dosing the amount, particularily if substances flowing in different pipings are to be mixed in accurate proportions. When substance is taken from the recirculation piping, the pressure decreases in the piping and the volume flow taken is no longer accu¬ rate. In such a system, systems based on measuring the amount of substance flowing in the piping lead to an expen¬ sive and complicated apparatus structure and to control problems difficult to manage. If the system includes a plu¬ rality of different pipings for different substances, the apparatus easily becomes so complicated that its reliabili¬ ty does not fulfil the requirements to be set for a produc¬ tive process apparatus. Thus, there is a need for a method by means of which substances of changing viscosity can be treated in a more reliable manner than before and, above all, by means of a simple apparatus.

The object of the present invention is to provide a method by means of which substances and mixtures of high viscosity can be dosed with accuracy.

The object of the best mode of the invention is to provide a method by means of which it is possible to produce a mix¬ ture of a plurality of highly viscous substances, with ac¬ curately determined proportions of the different substan- ces.

The essence of the invention is to be seen therein, that a mixture produced in a mixing container is recirculated by means of a constant volume pump within a recirculation pi- ping including a pressure difference valve for setting the pressure of the piping, and a mixture output connection is provided upstream of the valve.

More specifically, the method of the present invention is characterized by what is presented in the characterizing clause of claim 1.

The apparatus of the invention is characterized by what is presented in the characterizing clause of claim 6.

Considerable advantages are obtained by means of the inven¬ tion.

By means of the method, many expensive and often inaccurate measuring and control devices can be replaced. The control method is simple and there is little need for working devi¬ ces and meters, wherefore the system is reliable even in case the number of lines is great. By means of the method, it is possible to provide systems by means of which many difficultly treatable substances can be mixed into such mixtures that a desired amount of the final product is re¬ ceived. The mixing temperatures of the different partial mixtures can be controlled to desired values. Intermixing of the partial mixtures before the final mixing is preven- ted, in spite of the fact that the flow pipings may, be in direct contact to each other. The mixtures flow con- tiunuously in the pipings, whereby no blockings occur and

at the same time the mixtures move within the storage con¬ tainers as well, wherefore the need for mixing within the container is smaller.

When the product to be handled changes, the system can quickly and simply be calibrated for a new mixture, and thus the apparatus of the invention is useful within a con siderably large range of viscosity, contrary to previously known systems. One piping can be provided with a plurality of mixture outputs, as long as care is taken that the out¬ put of the constant volume pump is enough to maintain a pressure of required magnitude within the piping in spite of several outputs being in use. Alternatively, it is, by programming, possible to prevent simultaneous opening of too many outputs. In treating hot liquids, the benefit of the invention is to be seen therein, that the piping forms an expansion system, because the recirculation circuit and the pressure difference valve are open, wherefore heat ex¬ pansion of the liquid does not influence the function. The distance between the mixing point and the storage and mi¬ xing container can be long in case of reasonable mass flows, wherefore line costs are lower than earlier. Thus, the invention enables production of paints and other cor¬ responding products of a large industrial plant or of a large construction site, and the transportation of the pro ducts to the respective locations of use. The mixers, con¬ tainers, and pumps as well as the control apparatus needed in the system can be collected in transportable containers whereby they are easy to transport and the system is rea- dily installable for operation.

In the following, the invention is described in more de¬ tail, with reference to the accompanying drawings.

Figure l illustrates a mixing apparatus usable in connecti on with the invention.

Figure 2 is a schematic illustration of one embodiment of the invention.

Figure 3 is a schematic illustration of another embodiment of the invention.

The apparatus shown in Figure 1 is, except for the features relating to the present invention, presented in the Finnish Patent Application No. 940882. The main parts of the appa- ratus are a mixing container 1 with a side mixer 3 in its lower portion. The container 1 is supported by weight sen¬ sors 2. To the container 1 is connected a mixing means 4 by a feed line 15 entering the upper portion of the con¬ tainer, and a return line 16 starts from the bottom portion of the container back to the mixing means 4. To the mixing means 4 is connected a powder feed device 5 connected to a large sack 14 connected via a weight sensor 2 to a lifting device 6 and a powder suck line 17 for sucking powder from small sacks 18. The weight sensors 2 of the large sack 14 are used when there is a need for emptying dosage of the sack. In addition, the lower portion of the container 1 comprises heating resistances or coils 10 and at its bottom there is for the final product a discharge line 19 running through a filter 8 and a pump 7 to, e.g., a packing appara- tus, or as in the present case, to a recirculation line 21. The upper portion of the container 1 comprises a liquid feed piping 9 with accessories, for feeding liquids into the container 1. The apparatus further comprises a control center with weighing and control units 11, 12 and stairs and service platforms 13 and necessary washing connections.

The mixing means 4 is in this embodiment the mixer disclo¬ sed in the International Patent Application WO 92/21436. This mixer comprises a mixing chamber with a liquid feed connection and a connection for feeding substances to be mixed, respectively. In the mixing chamber is positioned a rotor rotated by means of an electric motor. The powder-

like substances are fed to a location at the center axis o the rotor and the rotor brings the liquid in the mixing chamber into rotation at a high speed, whereat a vacuum is established at the feed point. The powder is quickly and i uniform distribution sucked into the liquid and the mixture is quickly transferred to the outer periphery of the rotor and out through a discharge connection in the side of the mixing chamber. Mixing is fast and effective in this devi¬ ce, because the liquid and the powder collide with each other quickly and the mixing speed in the mixing chamber is very high. This device is described in more detail in the publication mentioned above.

The exemplifying apparatus shown in the figure works in the following way. Into the container 1 is at first dosed the basic raw material of the mixture, e.g., water, a suitable solvent or some other liquid. The mixing device 4 is star¬ ted and liquid is fed therein from the container 1 through the return line. Thereafter powder can be mixed into the liquid, from the large sack 14 or, by means of the sucking line 17, from the small sacks 18. The powders mix into the liquid in the mixing device 4 wherefrom the mixture is transported along the feed line 15 back to the container 1. The liquid fed into the container 1 circulates continuously through the mixing device 4 where different additives are introduced until all additives have been mixed into the basic raw material. During the intermixing, the mixture in the container can be mixed by means of the side mixer 3 in order to keep it flowing and ^" , if necessary, the mixture is heated by means of the heater 10. When the additives have been mixed in, basic raw material is added into the con¬ tainer until the correct mixture proportion and amount of the product is obtained. At the same time, the temperature of the final product is regulated to the one desired for packing. The completed final product can be disharged from the container for instant use, or can be packed for delive¬ ry to customers.

The present invention relates to the further distribution of the mixed substance to sites of use, or to the mixing of several pumpable mixtures into a final product ready for use or for packing. In Figure 2 is shown a detail of a sys- tern where the intermediate products circulate in circulati¬ on lines 21 starting from the mixing system. The system has four lines and at the operation point shown in Figure 2, each line has a branching closed by a shut-off valve 27. The branchings are connected to a mixer valve 28. A mixture prepared by means of the mixing device or some other re¬ quired constituent is fed into each line by means of the constant volume pump 7. The pressure in the line is cont¬ rolled by means of a pressure difference valve 26 arranged after the branchings to such a magnitude that a considera- bly lower pressure is sufficient for feeding a partial mix¬ ture through the branching out of the line. The volume flow in each line is determined according to the desired mixture proportion of the final product in such a way that the pro¬ portion of the volume flows of the lines corresponds to the mixture proportion of the final product. The control is carried out by changing the frequency of the drive motor of the constant volume pump 7 and at the same time care is taken, by controlling the pressure difference valve or by automatics, that the pressure in the line sets to the one desired. When the volume flow and the pressure of each line are at set value, the shut-off valves 27 can be opened, whereat the pressure difference valves 26 close or remain in their respective desired control positions and the par¬ tial mixtures flow to the mixer valve 28. Now a desired amount of the final product can be taken from the mixer valve 28. When the mixer valve 28 is closed, the shut-off valves 27 can be closed, whereat the pressure difference valves 26 open and the circulation of the partial mixtures in the circulation line 21 continues. The output of each line 21 further comprises back valves 31 in order to ensure the flow direction.

The volume flow circulating in the circulation line 21 is determined according to the revolution number of the cons¬ tant volume pump 7. The revolution number is preferably controlled by means of an electric motor controlled by a frequency transducer. For each pumpaple substance the revo lution number and the setting of the pressure difference valve can be determined in such a way that the volume flow and the pressure in the line are set to desired values. Be cause the volume flow provided by the constant volume pump 7 changes linearily with the revolution number, it is pre¬ ferable to use a linear valve as pressure difference valve 26 whereat, for the control of the line, is sufficient to determine the set value of the valve 26 corresponding to one volume flow, whereafter the set values of the other volume flows can be obtained on the basis thereof. These values always have to be redeter ined when there is a chan¬ ge in the viscosity of the pumpable substance. The deter¬ mination can be made upon installing the system, whereat the values are determined during the test running, or, mos preferably, one can utilize a calibration unit by means o which the control values can be determined whenever the pumpable substance is changing and at other times as well, if required.

One preferable calibration unit is shown in connection wit the mixing apparatus of Figure l. It has, separate from th mixing container 1, a calibration container 24 to which, from the constant volume pump 7, leads a line closable by means of a shut-off valve 22. In the line, between the shut-off valve 22 and the calibration container 24, posi¬ tioned a constant pressure difference valve 23. The cali¬ bration container 24, separated from the weighing system o the mixing container 1, is positioned above the mixing con¬ tainer and therefrom a line 25 closed by a shut-off valve leads to the mixing container l. When the properties of the pumpable mixture change or calibration of the system is required for some other reason, the shut-off valve 22 is

opened whereat pumpable mixture flows into the calibration container 24 through the constant pressure difference valve 23. The mixture is pumped into the calibration container 24 for a predermined period of time, or the drive motor of the pump 7 is fed at a predetermined frequency, whereat the amount pumped over the constant pressure difference at a certain revolution number is obtained. The amount is easily obtained, by means of the weighing system of the mixing container, as the mass flow discharged from the container, or, alternatively, the calibration container 24 can be weighed. After calibration the calibration container 24 is emptied through the line 25 into the mixing container 1.

If the mass flow provided by the constant volume pump is 100 kg/h and the control range of the frequency transducer is 100 Hz, a change of one Herz corresponds to a change of 1 kg/h in the mass flow. If such pumps are installed in five circulation lines, the following exemplifying products are obtained.

Table 1.

Tl T2 T3 T4 T5 T6 T7

LI 10 20 100 45

L2 40 17 40 10

L3 25 13 100 50

L4 15 21 40 50

L5 10 20 20 45

100 100 100 100 100 100

As is seen from the table above, as few as five mixing li¬ nes provide numerous different final products. If such a

line system is used, where each line circulates via a plu¬ rality of operation points, there is provided a paint mi¬ xing system suitable for large construction sites, paint for different purposes being obtainable at operation points by mixing material flowing in the different lines at the operation point, according to the need.

In the example of Figure 3, circulation lines are not used, but to the operation point is arranged ,from four constant volume pumps 7, a feed line 21 wherefrom return lines 29 lead through pressure difference valves 26 back to the mi¬ xing container. The feed lines 21 are connected to a static mixer 30 and a valve 28 is positioned in the line leading out of the mixer 30. As a static mixer is most preferably used a plate mixer with half the plates being heatable and half being coolable, thereby enabling the temperature and the energy content of the final product to be set as desi¬ red.

In addition to the embodiments hereto presented, the pre¬ sent invention includes other embodiments as well. The pre¬ sent method is suitable for all such substances that are to be circulated in a distribution pipe assembly or piping in order to avoid stiffening and blocking. As long as the re- turn flow and the circulation are provided for, the form of the net can be freely designed and, for example, the same container can be provided with a plurality of output lines. As such it is also possible to branch the line coming from the constant volume pump, but in order to more easily ob- tain the control, it is more preferable to use lines with one pump and one pressure difference valve. The number of outputs need not be restricted. The function of the pressu¬ re difference valve can be controlled automatically. When mixture is taken out of the line, the pressure difference valve may close fully or partly according to how one wishes to control the system and how large a volume flow is desi¬ rable to circulate in the line. The method can be applied

as a part of any mixing system and the dosable mixture can, in addition to a storage or mixing container, be taken from another source as well, e.g., from a production line or some other source.