GB2262936A - Apparatus and process for production of aqueous dispersion of calcium hydroxide - Google Patents

Abstract

An apparatus for the production of an aqueous dispersion of calcium hydroxide from calcium oxide and water comprises first and second vessels (1a and 1b) which may be used interchangeably as slaking and storage vessels. Screw conveyors (7a) and (7b), or other suitable means, are operable selectively for supplying calcium oxide and water to either vessel (1a) or (1b) respectively. Means, such as planetary stirrers (2a) and (2b) are operable selectively for effecting mixing of calcium oxide and water contained in the vessels (1a) and (1b) respectively to provide the calcium hydroxide product. The product may be transferred from one vessel (1a) to the other vessel (1b), or vice versa, by way of pump and valve combinations, in such apparatus, either one of vessels (1a) or (1b) may be used for mixing the calcium oxide and water, whilst the other is used for storage of the product. This offers the advantage of enabling the process to be continued with even if there is a fault within the first mixing vessel. <IMAGE>

Description

APPARATUS AND PROCESS FOR PRODUCTION OF AQUEOUS DISPERSION OF CALCIUM HYDROXIDE.

This invention relates to an apparatus and process for the production of an aqueous dispersion of calcium hydroxide from calcium oxide and water.

The reaction, known as slaking, between calcium oxide (commonly known as, and referred to hereafter as quicklime), and water to produce an aqueous dispersion of calcium hydroxide, (commonly known as, and referred to hereafter as milk of lime) is carried out on a wide scale around the world. The reaction is very exothermic and it is therefore essential that the proportion of quicklime and water which are reacted is carefully monitored and controlled in order that the temperature within the vessel in which the reaction is effected is carefully controlled.

Furthermore, it is also necessary to provide the required proportions of quicklime and water in order to produce a milk of lime having the required solids content.

Conventional apparatus for slaking quicklime typically comprises a first vessel to which quicklime and water are introduced and mixed in controlled amounts and in which the reaction between quicklime and water is effected and a second vessel into which the product milk of lime is transferred from the first vessel and in which the milk of lime is allowed to stand and settle for a suitable period of time before being fed to downstream processes in which the milk of lime is employed.

Many vessels are known in which the quicklime and water are mixed, the design of the vessel depending at least to some extent upon the particular parameter which is chosen to be monitored in order to regulate the proportions of water and quicklime added to the reaction vessel.

Thus the reaction may be monitored and the proportions of calcium oxide and water added regulated, by following, for example, the temperature of the reaction mixture, or the viscosity of the reaction mixture, or a rotating vessel may be employed in which the torque of the vessel is monitored When the desired amount of water and quicklime have been added to the reaction vessel to give the required amount of product, the milk of lime is then transferred to the second storage vessel. The first reaction vessel may then be charged with further quicklime and water and the process repeated. Thus a continuous output of milk of lime may be provided from the storage vessel to a downstream process in which the milk of lime is employed.

In known apparatus, the reaction vessel is usually much smaller than the storage vessel in order that many batches of milk of lime produced in the reaction vessel may be stored in the storage vessel.

A disadvantage of known apparatus is however that upon failure of the reaction vessel, the whole apparatus may be out of service until the fault has been repaired. In order to alleviate this problem, a back-up apparatus, which comes into operation if the apparatus which normally operates fails, is sometimes provided. This however necessitates much duplication of apparatus components.

According to the present invention there is provided an apparatus for the production of an aqueous dispersion of calcium hydroxide from calcium oxide which apparatus comprises first and second vessels, means operable selectively for supplying calcium oxide and water to said vessels, means operable selectively for effecting mixing of calcium oxide and water in each of said vessels and means for effecting transfer of the product from one vessel to the other vessel thereby enabling either one of said vessels to be used for mixing while the other is used for storage of the product.

The present invention provides an apparatus for slaking quicklime in which the apparatus may be used continuously even where there is a fault with the reaction vessel and vhich avoids the need for duplicating all the apparatus components.

In typical embodiments of the invention the elements required for mixing quicklime and water, i.e. means for supplying quicklime and water, means for transferring contents between the vessels, and mixing means, are provided for both vessels of the apparatus. One or more of these elements may be common to both vessels or each vessel may be provided with its own set of elements.

Thus for example, the apparatus may comprise a single means for mixing quicklime and water which is transferable between the vessels, although we prefer to employ a separate means for mixing quicklime and water in the second vessel from the means for mixing calcium oxide and water in the first vessel. The means for mixing quicklime and water in the first and second vessels preferably comprises a planetary stirrer provided within each of the vessels.

In normal operation of the apparatus, one of said vessels is normally employed for effecting the slaking reaction and quicklime and water are fed to that vessel as required, the product of the vessel being transferred to the other vessel which is normally employed as the storage vessel.

However, where a fault occurs with the slaking vessel such that the slaking vessel may no longer be usefully employed for effecting the slaking reaction, the supply of quicklime and water may be transferred to the second vessel, and if the fault with the first vessel does not prevent its use as a storage vessel, then the contents of the second vessel may be transferred to the first vessel. If the fault with the first vessel prevents its use both as a slaking and storage vessel, then the product of the second vessel may he stored therein before being transferred to downstream processes.

Whilst this will reduce the efficiency of operation of the apparatus, it will be readily understood that the apparatus may still be usefully employed to produce a milk of lime.

According to a further aspect of the invention there is provided a process for the production of an aqueous dispersion of calcium hydroxide, which process comprises supplying quicklime and water to a first vessel, mixing the contents therein, transferring the contents to a second vessel, monitoring operation of the first vessel, and in the event of malfunction, continuing said process by supplying quicklime and water to the second vessel and mixing the contents therein. For the part of the time that quicklime and water are fed to the second vessel, the contents of the second vessel may be transferred to the first vessel or they may be discharged directly from the second vessel to a downstream process, depending as described previously upon the particular fault with the first vessel.

In order that the first and second vessels may operate interchangeably as storage and reaction vessels, we prefer that the vessels are of substantially the same volume in order that product may be transferred as required between the vessels.

The means for supplying quicklime to the first and second vessels may take any suitable form, and may for example be a screw conveyor leading to each vessel from a quicklime storage bunker, commonly known as a silo. In operation of the apparatus, one screw conveyor only operates at any one time.

As previously described, it is necessary that the addition of quicklime and water to whichever vessel is being employed as the vessel within which mixing of the quicklime with water takes place, is carefully controlled. The addition of the components is preferably controlled by monitoring the temperature of the mixture within the vessel. Thus, a starting heel of water may be charged to the vessel and quicklime may be added until a predetermined temperature is reached. flore water may then be added until the temperature has fallen to a lower predetermined temperature.

Addition of water and quicklime, repeating this process and thereby maintaining the temperature within certain pre-set limits, may be continued until the desired batch quantity of milk of lime has been produced. Thus, both the first and second vessels are preferably provided with means for monitoring the temperature within the vessel, for example a thermometer, whereby the supply of water and calcium oxide to the vessel is regulated according to the temperature sensed by the temperature detection means The means for transferring the contents of the first and second vessels between the first and second vessels may comprise a two-way pump and valve combination provided within a pipe extending between the first and second vessels.Preferably however, the means for transferring the contents of the first vessel to the second vessel and the second vessel to the first vessel each comprise a pump and valve combination within separate pipes extending between the first and second vessels.

In a preferred embodiment of the invention, each vessel is mounted on load cell means in order to monitor the amount of material within each vessel. Monitoring of the amount of material within each vessel enables automatic regulation of the transfer of material between the vessels and co-ordination of the production of each batch of milk of lime within the reaction vessel with the discharge of milk of lime from the storage vessel to downstream processes.

Co-ordination of operation of the apparatus may be controlled automatically by the provision of suitable electronics and computer software linked to the means for monitoring temperature and amounts of materials within the vessels.

The invention is illustrated with reference to the following figures in which: Figure 1 is a plan view of an apparatus according to a preferred embodiment of the invention, and Figure 2 is a side elevation of the apparatus in Figure 1.

EXAMPLE The apparatus illustrated in the figures comprises two identical vessels la and ib. Each vessel is provided with a planetary stirrer 2a, 2b, a temperature sensor 3a, 3b, a vent 4a, 4b, an inlet for water 5a, 5b, and inlet for calcium oxide 6a, 6b.

Screw conveyors 7a, 7b are provided from a quicklime silo 8 to the calcium oxide inlets of each vessel, the outlets from each screw conveyor having a flap valve 9a, 9b.

A pipe 10, for the transfer of material from vessel la to vessel ib extends from an outlet in vessel la to an inlet in vessel Ib. The pipe is provided with a valve 11 and pump 12. A second pipe 13 for transfer of material from vessel Ib to vessel la extends from an outlet in vessel ib to an inlet in vessel la. The pipe 13 is provided with a pump 15 and valve 14. Each vessel is further provided with an outlet pipe 16a, 16b, having pumps 17a, 17b and valves 18a, 18b respectively, for discharge of material from the apparatus to external plant.

The vessels la, ib are mounted upon load cells 19a, 19b.

In normal operation of the apparatus, a heel of water is charged to the vessel employed as the slaking vessel, in this case vessel Ia. Quicklime is added to vessel la via the screw conveyor and the mixture is stirred. Addition of quicklime is continued at a slow rate until the temperature of the mixture reaches a pre-determined temperature, in this case 850C. At this temperature, further water is added until the temperature falls below a second pre-determined temperature, in this case 0 80 C. At this lower set temperature, the addition of water is stopped and quicklime is added until the higher set point is reached and this process is repeated thus maintaining the reaction at a steady temperature. The process is stopped when the load cells on vessel la indicate that the vessel is about a quarter full.At this point further water is added to vessel la until the load cells register that the vessel is half-full. The diluted mixture is agitated for a short time, for example 15-20 minutes, and after this time pump 12 and valve 11 operate together to transfer the contents of vessel la through pipe 10 into the storage vessel ib. Valve 11 closes and pump 12.

stops when the load cells 19a on the slaking vessel la register that it contains the Rheel" of material for start up of the next batch. The storage vessel ib holds 2 batches of product from the slaking vessel la and the batch sequence for the slaking vessel la is initiated by the storage vessel load cells lb registering less than half-full.

Discharge of product from the storage vessel through outlet pipe 16b is controlled by pump 17b and valve 18b which are controlled by the demand of the downstream process to which product from the storage vessel is fed.

In the event of a breakdown in the slaking vessel la, the storage vessel 1b may be used as the slaking vessel. In this event, the second screw conveyor 7b may be operated to feed quicklime to vessel ib and the sequence of controlled operations described above are initiated in storage vessel lb. If the breakdown in vessel la does not prevent its use as a storage vessel, then it may be so used and product from vessel 1b may be transferred to vessel la through pipe 13 by pump 14 and valve 15. If however, the breakdown prevents any use of vessel la, then pump 17b and valve 18b may be operated manually to discharge product from vessel ib direct to the downstream process.

In the event of a breakdown in the storage vessel lb, the product is allowed to stand in the vessel la, and pump 17a and valve 18a may be activated manually to discharge product from vessel 1b directly to the downstream process.

Claims (18)

1. Apparatus for the production of an aqueous dispersion of calcium hydroxide from calcium oxide, which apparatus comprises first and second vessels, means operable selectively for supplying calcium oxide and water to one or other of said vessels, means operable selectively for effecting mixing of the calcium oxide and water contained in one or the other of said vessels to provide the calcium hydroxide product and means for effecting transfer of the product from the one vessel where mixing has been effected to the other vessel, thereby enabling either one of said vessels to be used for mixing while the other is used for storage of the product.

2. Apparatus according to Claim 1 wherein first means are provided for supplying one said vessel and second means are provided for supplying the other said vessel.

3. Apparatus according to Claim 1 or 2 wherein means for supplying the first and/or second vessels include one or more screw conveyors.

4. Apparatus according to any preceding claim wherein first means are provided for effecting mixing of calcium oxide and water contained in one said vessel and second means are provided for mixing in the other said vessel.

5. Apparatus according to Claim 4 or 5 wherein means for effecting mixing in the first and/or second vessels comprise one or more planetary stirrers.

6. Apparatus according to any preceding claim which includes means for measuring the temperature within the first and/or second vessel.

7. Apparatus according to Claim 6 which includes means for regulating supply of calcium oxide and/or water to a vessel in dependence upon the temperature within the said vessel.

8. Apparatus according to any preceding claim which includes a means for determining the amount of material within the first and/or second vessel.

9. Apparatus according to Claim 8 wherein each vessel is mounted on a load cell to enable the amount of material within each vessel to be determined.

10. Apparatus according to Claim 8 or 9 wherein means for effecting transfer of product from one vessel to another vessel are selectively operable in dependence upon the amount of material within said one vessel.

11. Apparatus according to any preceding claim wherein the first and second vessels have substantially the same volume.

12. Apparatus according to any preceding claim wherein first means are provided for effecting transfer of product from said first vessel to said second vessel and second means are provided for effecting transfer from said second to said first vessel.

13. Apparatus according to any preceding claim wherein means for transferring product from one vessel to another vessel comprise a pump and valve combination.

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

15. Process for production of an aqueous dispersion of calcium hydroxide, which process comprises supplying calcium oxide and water to a first vessel, mixing the calcium oxide and water to provide the calcium hydroxide product, transferring product to a second vessel, monitoring operation of the first vessel and, in the event of malfunction, supplying calcium oxide and water to the second vessel, mixing the calcium oxide and water therein and transferring the product to the first vessel.

16. Process according to Claim 15 which includes monitoring the temperature within the vessel in which calcium oxide and water are mixed and regulating subsequent supply of calcium oxide and/or water to the vessel in dependence upon the temperature within the vessel.

17. Process according to Claim 15 or 16 which includes monitoring the amount of material within the vessel in which calcium oxide and water are mixed and regulating transfer of product from the vessel in dependence upon the amount of material in said vessel.

18. Process substantially as hereinafter described with reference to the accompanying example.