GB2211433A - Mixing method and apparatus - Google Patents

Abstract

A pumping method includes pumping of a liquid mixture of active constituents, together with a volume of air at least equal to the volume of the active constituents, and preferably at least five times the volume of the active constituents. The air/liquid mixture flow is pumped using centrifugal pumps having radially outer vane extensions (3b) beyond the circumference of the rotor disc (4), to accommodate and to impel the airflow. The method may be used in the manufacture of cattle feed. <IMAGE>

Description

'MIXING METHOD AND APPARATUS" The present invention relates to a method of and apparatus for mixing liquid constituents. In particular, the mixing method can be used in the manufacture of cattle feed.

Our EP-A-0163395 discloses a process for manufacturing animal feedstuffs by mixing a fatty acid with a basic oxide such as calcium oxide, and optionally with further nutritional material such as soya bean meal, and adding water to the mixture so that the exothermic reaction of the basic oxide with the water heats the mixture and allows it to dry, when spread out, to form a friable product serving as cattle feed. The disclosure of EP-A-0163395 is incorporated herein by reference.

It is an object of the present invention to improve the quantity and uniformity of throughput of the apparatus disclosed in EP-A-0163395 and to provide a method which is capable of maintaining the containing apparatus free from premature deposits of the drying mixture during operation of the apparatus.

In EP-A-0163395 there is disclosed the use of two self-cleaning pumps, one to feed the mixture of acid oil and the basic oxide, and the other to feed that mixture after combination with additional water direct to a spraying tube.

We have found that the exothermic reaction of the basic oxide with the water starts so quickly that the mixture begins to solidify in the spraying tube, giving a risk of blockage of the spraying tube. Also, there is a tendency for the liquid mixture to thicken during its passage through the pumping stage so that the uniformity of the mixture flow rate is less than adequate and the pumps operate in a "gulping" mode requiring intermittent power surges to expel thickened slugs of the mixture.

Because the pumps are self-cleaning, the same problem does not normally arise there to the same extent.

In accordance with the present invention we provide a method of making an aqueous mixture of a basic oxide, comprising mixing water and the basic oxide to provide a liquid mixture, and pumping a flow comprising the liquid mixture and a volumetric flow rate of air at least equal to the volumetric flow rate of the liquid mixture, to a discharge location.

The present invention also provides mixing apparatus including a feed line for a basic oxide, a feed line for water, and a centrifugal type of mixing pump having a pump rotor including a rotor disc, pumping vanes on one side of said rotor disc, and a radial extension of each pumping vane radially outward beyond the periphery of the rotor disc.

By use of the method and apparatus of the present invention, the previous problem of clogging of the spraying tube and of other apparatus ducts through which the basic oxide/water mixture passes can be avoided.

The liquid mixture may further include an oily constituent, for example an edible fatty acid, in which case the correspondingly more acute clogging problem is likely to lead to interruption of the mixing process unless the method.

of the present invention is employed.

In order that the present invention may more readily be understood the following description is given, merely by way of example, with reference to the accompanying drawings in which: FIGURE 1 is a longitudinal sectional view, taken on the plane I-I of Figure 2, showing a self-cleaning pump used in the apparatus in accordance with the present invention; and FIGURE 2 is an end elevation of the pump rotor, viewed along the direction of arrow II of Figure 1.

As indicated above, the full disclosure of EP-A-0163395 is incorporated herein by reference.

The apparatus shown in Figure 1 of that earlier disclosure includes a mixing pump (3) where calcium oxide and a fatty acid, optionally together with other nutritional material such as soya bean oil, is thoroughly mixed before passing to a self-cleaning pump (5) where the mixture is impelled through a zone where it is combined with water ready for the start of the drying process. A further self-cleaning pump (6) immediately downstream of the water injection conduit (4) provides a pressurized jet of the liquid along a spraying tube onto a conveyor which allows the spread liquid mixture to dry to a friable product during a total dwell time in excess of two and a half hours as the mixture passes along the conveyor which may be a belt conveyor as in EP-A-0163395 or a drag link conveyor as in our subsequently filed UK Patent Application 8705010.

In order to avoid a practically-encountered problem of at least partial clogging of the spraying tube, we now propose to pump the liquid mixture, initially comprising the basic oxide and fatty acid and optionally the nutritional material, and subsequently comprising additional water, in the presence of a considerable quantity of air, at least in a 1:1 ratio of air to liquid, through the self-cleaning pumps and the spray tube.

The air takes no chemical part in the reaction but its presence has been found beneficial in assisting the self-cleaning action of the pumps and in avoiding build-up of a skin or other deposit of the drying mixture particularly downstream of the water introduction point.

Whereas it might be expected that the presence of high proportions of air in the pumped medium would give rise to cavitation, destroying the efficiency of the pumps, we have found quite the reverse and that the presence of the air increases both the quantity and uniformity of the throughput of the mixture and lengthens the intervals between servicing operations on the apparatus. Furthermore, despite the fact that air is being pumped together with the reacting mixture, the power requirements for the pumps are reduced as compared with the prior art.

Figure 1 of the present application shows a sectional view of the type of self-cleaning pump preferred for use just upstream and just downstream of the water introduction location. As can be seen from Figure 1, the pump rotor includes a rotor vane extension radially outwardly of the rotor disc, and an axial extension guiding the liquid into the discharge passage. The presence of this rotor vane extension both accommodates and impels a large volume of air pumped through the apparatus, and also scours the walls of the pump stator to avoid undesirable build-up mixture, particularly in the final self-cleaning pump where water is already present and where the violence of the exothermic reaction of the water with the calcium oxide or other basic oxide is most likely to affect the stability of the process.

Each of these self-cleaning pumps is a centrifugal pump having a central inlet 1 and a localized circumferential outlet 2 on the opposite side of the pump.

In addition to the customary radially extending vane portion 3a to each vane 3, there is an axial extension 3b outboard of the rotor disc 4 and having a bevelled end edge 5 positioned to the left of the disc 4 (i.e. on the opposite side from that where the inlet 1 is disposed).

The pump stator 6 comprises a first housing portion 7 defining the curved annular passage deflecting the centrifugally moving flow of pumped liquid and air from the radial direction into the axial direction, and a rear housing portion 8 which both encloses the back of the rotor disc 4 and its axial extension 4a, and defines a closed wall 8a along which the pumped liquid is guided circumferentially towards the outlet 2.

The rotor disc 4 is held in place against a shoulder 9a on a shaft 9 by virtue of a nut 10 on a threaded extension 11 of the shaft.

As shown in Figure 2, the rotor disc 4 has four of the rotor vanes 3 equiangularly spaced around a central opening 12 to receive a stepped-down portion 9b of the pump shaft.

In use of the apparatus of EP-A-0163395 incorporating the pumps of the type shown in Figures 1 and 2, there will be a considerable volume of air pumped through the apparatus in addition to the reaction liquid. We prefer that the air:liquid mixture ratio should be at least 1.5:1, and have found it more preferable to be from 5:1 to 8:1.

One particularly preferred ratio is 7.75:1, for example in a pump having a rating of 7,000 gallons per hour the throughflow of liquid mixture is 800 gallons and the throughflow of air is 6,200 gallons.

The high air:liquid mixture ratio has various beneficial effects, despite the fact that the air does not play any chemical part in the reaction to produce the eventual friable product cattle feed.

Firstly, the presence of high proportions of air have been found beneficial in scouring the apparatus and allowing high liquid volumetric flow rates to be used in mixture conduits which might otherwise choke. These volumetric flow rates of air, enabling the aerated liquid to scour the conduit walls, avoid the mixture building up by solidification during its passage along the conduits. In the second self-cleaning pump 6 of EP-A-0163395, the already commencing reaction of the water and the calcium oxide is sufficiently violent to begin to boil the water off as steam which augments the airflow. This in turn adds to the cleaning effect in the reaction tube through which the mixture passes to be sprayed onto the drying conveyor.

Secondly, the effects of any loss of pumping efficiency during the life of the pumps will te more likely to show up as a reduction in the air proportion, rather than loss of pumped mixture, with the result that the flow rate of the important liquid constituents through the worn pump will be much less reduced than would otherwise be the case, although of course after such wear the air:liquid mixture volumetric ratio becomes nearer to unity.

Thirdly, the action of the air passing through the apparatus assists in entraining the entering flow of water so that the induction of the mixture is achieved purely by virtue of the action of the self-cleaning mixing pumps. In the absence of entrained air in the pumped flow, it has been found necessary to provide some pumping effort to urge water into the mixture in the apparatus of EP-A-0 163 395.

Fourthly, the uniformity of flow of the mixture is enhanced and as a result, the quality and uniformity of the product are improved in that in the absence of a generous quantity of air in the pumped mixture the pumps have been observed to operate in a "gulping" mode in that they appear to be intermittently starved of feed mixture and then expel a surge of the mixture requiring a corresponding surge in the power input to the pumps and entailing a higher overall power consumption for the pumping operation and additional wear of the pumps over prolonged periods of time. The uniformity of flow is considerably enhanced when a deliberate generous air constituent is present in the pumped mixture.

Fifthly, as a possible consequence of the third and fourth advantages mentioned above, the overall power consumption of the apparatus is reduced when provision is made for an air constituent in the pumped mixture.

Sixthly, it has been found that the presence of an air flow induced through the mixing pumps more positively controls the flow of the finely divided basic oxide constituent. For example, when the preferred ranges of air component are used there is efficient induction of the basic oxide, for example calcium oxide, into the apparatus whereas it the absence of such air components there has been observed a tendency for airborne powder, for example airborne lime, to appear in the atmosphere and this presents a considerable environmental hazard.

It will, of course, be appreciated that there must be some limitation of the quantity of air pumped because of the need to avoid excessive cooling of the reaction mixture due to the air present. The rate of drying of the reaction mixture on the conveyor is dependent upon the heat generated in the exothermic reaction taking place both in the pumping stage and subsequently in the conveying stage, and excess quantities of air would tend to remove some of this useful heat in the pumping stage.

We therefore prefer to have an upper limit of 8:1 for the ratio of air to liquid in the pumped mixture, but it has been found that a particularly convenient result can be obtained in terms of the quality and uniformity of product when the ratio is 7.75:1.

Claims (14)

1. A method of making an aqueous mixture of a basic oxide, comprising mixing water and the basic oxide to provide a liquid mixture, and pumping a flow comprising the liquid mixture and a volumetric flow rate of air at least equal to the volumetric flow rate of the liquid mixture, to a discharge location.

2. A method according to claim 1, wherein the ratio of air to liquid in the pumped flow is at least 1.5:1.

3. A method according to claim 2, wherein the ratio of air to liquid in the pumped flow is from 5:1 to 8:1.

4. A method according to claim 2, wherein the ratio of air to liquid in the pumped flow is 7.75:1.

5. A method according to claim 1, 2, 3 or 4, wherein the liquid mixture includes further constituents in addition to the water and the basic oxide.

6. A method according to claim 5, wherein the further constituents include a fatty acid.

7. A method according to any one of claims 1 to 6, wherein the basic oxide is calcium oxide.

8. A method according to any one of the preceding claims, wherein said flow of said liquid mixture and air is pumped using at least one centrifugal pump having a pump rotor equipped with a rotor disc and rotor vanes which extend generally radially of the rotor disc and terminate in radially outer vane extensions at a greater radius than that of the periphery of the rotor disc.

9. A method according to claim 8, wherein the vanes and the vane extensions closely sweep internal walls of a pump stator housing, and wherein the radial extension of each rotor vane includes an axial extension on the opposite side of the rotor disc from the pump inlet.

10. A method of mixing, substantially as hereinbefore described with reference to the accompanying drawings.

11. Mixing apparatus including a feed line for a basic oxide, a feed line for water, and a centrifugal type of mixing pump having a pump rotor including a rotor disc, pumping vanes on one side of said rotor disc, and a radial extension of each pumping vane radially outward beyond the periphery of the rotor disc.

12. Apparatus according to claim 11, wherein the radially outer vane extension includes an axial vane extension extending from said one side of the rotor disc to the other side thereof.

13. Apparatus according to either of claims 9 and 10, wherein the rotor disc, the pumping vanes, and the radially outer vane extensions, all co-operate snugly with an enclosing stator housing which is swept clean by the rotating disc, pumping vanes and radially outer vane extensions.

14. Mixing apparatus substantially as hereinbefore described with reference to, and as illustrated in, the accompanying drawings.