METHOD AND APPARATUS FOR REMOVING MATERIALS IN SOLID FORM FROM SOLUTIONS

Abstract: A method of removing materials in solid form from solution in a solvent comprising the steps of collecting the solution in a deposition vessel, injecting directly into the solution in the vessel a liquid refrigerant material whereby the refrigerant vapourises in and cools the solution so that at least part the material is deposited from solution; drawing off the solution with the deposited solid material and separating the solid material therefrom and collecting the remnant solution to be recycled for use; extracting the refrigerant from the deposition vessel, liquifying it and storing it in a storage vessel for re-insertion into the deposition vessel; the refrigerant being chosen such that it is insoluble in the solution, is chemically inactive therewith and that, at the pressure of the deposition vessel, its vapourisation temperature is above the freezing temperature of the solution and below the solid material deposition temperature thereof.

Description: Method and apparatus for removing materials in solid form from solutions. This invention relates to the removal of materials in solid form from solution in a solvent, and more particularly, although not exclusively, to the removal of crystalline iron compounds from sulphuric pickle liquor of the kind used for cleaning steel sheet or strip or like steel products during processing in a steel plant. In practice it is found that pickle liquor, essentially comprising sulphuric acid at a dilute concentration, becomes saturated with iron sulphate and "spent" before all useable acid in the liquor has been utilised. It is, therefore, known to remove at least some of the iron sulphate from solution and then recycle the partially used pickle liquor for further service in the steel plant. It has been proposed that the separation of the iron sulphate be undertaken by reducing the tempera¬ ture of the pickle liquor so as to supersaturate the iron sulphate present whereby crystallisation takes place. Crystals so formed can them be removed from the liquor by means, for example, of a centrifuge. A current proposal for carrying out such a temperature reduction process utilises a vacuum process for the evaporation and temperature reduction of part of the pickle liquor by means of a steam jet acting in a venturi manner across the exit from a closed container of pickle liquor. Whilst such a process succeeds in separating iron sulphate crystals and allowing pickle liquor to be returned to the plant for subsequent re-usage, and additionally has the advantage of producing the iron sulphate in its useful hepta-hydrate form, considerable energy is consumed in the form of steam by the steam venturi which makes the process considerably expensive in operation. It is an object of the present invention to overcome or at least substantially reduce this problem. In accordance with one aspect of the invention there is provided a method of removing materials in solid form from solution in a solvent comprising the steps of collecting the solution in a deposition vessel, injecting directly into the solution in the vessel a liquid refrigerant material whereby the refrigerant vapourises in and cools the solution so that at least part the material is deposited from solution; drawing off the solution with the deposited solid material and separating the solid material therefrom and collecting the remnant solution to be recycled for use; extracting the refrigerant from the deposition vessel, liquifying it and storing it in a storage vessel for re-insertion into the deposition vessel; the refrigerant being chosen such that it is insoluble in the solution, is chemically inactive therewith and that, at the pressure of the deposition vessel, its vapourisation temperature is above the freezing temperature of the solution and below the solid material deposition temperature thereof. In accordance with another aspect of the invention, there is provided apparatus for carrying out the method as hereinbefore defined comprising a deposition vessel for collecting the solution, means for injecting directly into the solution in the vessel a liquid refrigerant material whereby the refrigerant vapourises in and cools the solution so that at least - . - part of the material is deposited from solution; means for drawing off the solution with the deposited solid material and separating the solid material therefrom and collecting the remnant solution to be recycled for use; means for extracting the refrigerant from the deposition vessel, liquifying it and storing it in a storage vessel for re-insertion into the deposition vessel. The refrigerant may be cooled during the liquifying stage or immediately therebefore or thereafter. Such cooling may be carried out by means of a water cooled heat exchanger tower arrangement. The liquid refrigerant storage vessel may include means for removing any liquor from the refrigerant incidentally carried over during removal of the refrigerant from the deposition container. Alternatively, the refrigerant in vapour form may be liquified by being added to the remnant solution after separation of solid material therefrom to form a mixed refrigerant/solution liquid, which may then be allowed to settle for separation of liquid refrigerant from the remnant solution. Where the solution is an aqueous solution, we have found that a suitable refrigerant is butane, n- butane has been found to be especially suitable in relation to refrigerant requirements with respect to the aqueous solution. In order that the invention may be nore readily understood, two embodiments thereof will now be described by way of example with reference to the accompanying drawings in which:- Figure 1 is a diagrammatic representation of the operation of one form of apparatus in accordance with the invention; and Figure 2 is a diagrammatic representation of the operation of second form of apparatus in accordance with the invention. Referring to Figure 1 it will be seen that spent pickle liquor 1 from a steel plant enters on a continuous basis a deposition vessel 2. Commonly pickle liquor, on entry, will be at approximately 42°C. Disposed towards the lower end of the separation vessel are a plurality of injection nozzles 3 connected to a pipe 4 fed from a pump 5 and arranged to inject liquid butane directly into the pickle liquor contained in the deposition vessel. The injection of the liquid butane and its vapourisation therein reduces substantially the temperature of the spent pickle liquor 1 to about 6°C and causes crystal formation of iron sulphate in the hepta-hydrate form, since at the lower temperature now produced the pickle liquor constitutes a supersaturated sulphate solution. Liquor containing the crystals, usually in the form of a slurry, is withdrawn from the base 6 of the deposition vessel. The withdrawn liquor is passed to a centrifuge (not shown) for separation of the iron sulphate crystals and return of the pickle liquor for use in the steel plant after replenishment with sulphuric acid to retain the liquor at a desired overall acid concentration of approximately 15 to 17?-. The iron sulphate hepta-hydrate crystals collected can be sold and used for any desired commercial function. One such use, for example, is as a moss killer for lawns. Considering Figure 1 in greater detail, it is to be noted that the butane exists in the cycle in four distinct thermodynamic states:- State A State B State C State D Fluid State Liquid Vapour Liquid Vapour Temperature °C 32.2 4.2 4.2 32.2 Pressure (bar gauge) 2.0 0.2 0.2 2.0 The above are representative values, some variations being acceptable in the values of temperature and pressure, though not in the fluid state. The liquid butane in State A emerges from the heat exchanger 9 and is passed to the butane storage vessel 12. Here, the pressure is suddenly lowered, and the butane flash evaporates to produce some vapour at State B and some liquid at State C. For temperature reduction purposes vapour is drawn from the top of vessel 12 via line 15 as described later. Any contamination of the butane by liquor mist results in deposition in the storage vessel 12 of the liquor. It is drained off from the bottom 13 by means of a float controlled valve 14. The liquid butane at State C is passed through line 4 by pump 5 (with negligible increment in pressure) to the deposition vessel 2. Here the liquid butane droplets evaporate to vapour, obtaining the necessary latent heat energy for its evaporation by taking both the sensible heat of the pickle liquor and the heat of crystallisation of the compound (in The crystal slurry at approximately 6°C is led away from the bottom 6 of the vessel to the separation process. The butane vapour at State B leaves the top 7 of the deposition vessel. The vapour leaving the top of the deposition vessel at State B mixes with the butane at State B from the butane storage vessel 12 and passes to compressor 8 where the butane is compressed to State D. The butane at State D then passes to the heat exchanger 9 where, by heat exchange with cooling water, the butane is condensed to State A, thus completing the butane cycle. The cooling water is driven by pump 10 into the bottom of the heat exchanger at an approximate temprature of 27°C and exists from a line 11 at the top of the exchanger at a temperature of approximately 32°C. n-Butane has been found to be ideal for use with the invention in that it is relatively inexpensive, it has a boiling point at atmospheric pressure of 0°C so that with the depressed freezing point of the pickle liquor, caused by the presence of sulphuric acid and iron sulphate, no accidental freezing of the deposition vessel can occur during operation. The arrangement of Figure 2 is similar to that of Figure 1 except that, in general terms, it operates at somewhat higher pressures and temperatures, and, as a consequence, the water cooled heating tower can be dispensed with. In this arrangement the butane exists in the cycle in five distinct thermodynamic states:- State E State F State G Fluid State Liquid Vapour Vapour Temperature °C 32 42 52 Pressure 2.0 0. .2 4.2 (bar gauge) State H State I Fluid State Mixed Liquid Liquid Temperature °C 52 52 Pressure 4.2 4.2 (bar gauge) As with the arrangement of Figure 1, spent pickle liquor 21 from a steel plant enters a deposition vessel 22 at about 42°C, butane from storage/ flash vessel 23 in State E is injected by pump 24 and vapourises thereby reducing the liquor temperature and causing crystal formation. Crystal containing liquor is passed to crystal separation vessel 25 from which the crystals are removed at 33. Remnant liquor is removed via pipe X. Butane vapour from the vessel 22 and the storage/flash vessel 23 in State F is compressed by a compressor 26 to State G and is fed to a mixing column 27. T,he remnant liquor from vessel 25 is also fed, via pipe X, to the top of column 27 and intimate mixing with the compressed butane stream takes place over a packed bed. The result is to condense the butane in the liquor stream thus forming an "emulsion" type liquid of butane and liquor. The liquid is passed from the bottom of column 27 in State H to a separation vessel 28 where the two components are allowed to separate into two imiscible layers 29 and 30. The liquor (the bottom layer 30) is allowed to leave by means of a float controlled valve 31 to return at a temperature of 52°C to the steel plant for further use after strength replenishment. The liquid butane is transferred in State I via a float controlled valve 32 to the storage/flash vessel 23 where the pressure is reduced to 0.2 bar gauge, and the ' butane is ready for a repetition of the cycle. We have found that the application of the invention particularly described is of extremely useful economic value. Thus, we have found in one commercial plant a 90$ reduction in recycling costs or better can be achieved compared with previous steam/vacuum arrangements. In the common instance where the cost margins required for profitability of plant are critical, such a saving is of most considerable importance. At a typical works a recovery system of the kind to which the invention relates would be rquired with pickle liquor removing approximately 0.5 tons of iron per hour from which approximately 2.5 tons of iron sulphate per hour could be expected by means of the arrangement of the present invention. The invention has been particularly described hereinabove in relation to the removal of iron sulphate from pickle liquor. However, the invention is equally applicable for removing any material capable of depositing solids from aqueous solution, for example, whose solubility decreases with decreasing temperature. Thus, the invention could be applied for the removal of sugar from a sugar solution, and ferric chloride from a hydrochloride acid solution, for example.

Claims: . CLAIMS 1 A method of removing materials in solid form from solution in a solvent comprising the steps of collecting the solution in a deposition vessel, injecting directly into the solution in the vessel a liquid regrigerant material whereby the refrigerant vapourises in and cools the solution so that at least part of the material is deposited from solution; drawing off the solution with the deposited solid material and separating the solid material therefrom and collecting the remnant solution to be recycled for use; extracting the refrigerant from the deposition vessel, liquifying it and storing it in a storage vessel for re-insertion into the deposition vessel; the refrigerant being chosen such that it is insoluble in the solution, is chemically inactive therewith and that, at the pressure of the deposition vessel-, its vapourisation temperature is above the freezing temperature of the solution and below the solid material deposition temperature thereof. 2 A method as claimed in claim 1 wherein the refrigerant in vapour form is liquidised by being added to the remnant solution after separation of solid material therefrom to form a mixed refrigerant and solution liquid. 3 A method as claimed in claim 2 wherein the refrigerant and solution are combined in a mixing column . 4 A method as claimed in claim 2 or claim 3 wherein the mixed refrigerant and solution liquid is passed to a settlement tank for separation of the refrigerant and the remnant solution. 5 A method as claimed in any one of claims 2 to 4 wherein the refrigerant is passed through a compressor prior to being added to the remnant solution, the refrigerant outlet of the deposition vessel and the refrigerant storage vessel being connected to the compressor input as to be maintained at low pressure. 6 A method as claimed in claim 1 wherein the refrigerant is cooled just before, during or just after the liquifying stage. 7 A method as claimed in claim 6 wherein the cooling is by means of a water cooled heat exchanger tower. 8 A method as claimed in claim 6 or 7 wherein means are provided for removing any liquor from the refrigerant carried over from the deposition container. 9 A method as claimed in any one of the preceding claims wherein the refrigerant is pumped from its storage vessel into the deposition vessel thereby increasing its pressure and temperature. 10 A method as claimed in any one of the preceding claims in which the solution is an aqueous solution, and the refrigerant is butane. 11 A method of removing materials in solid form from solution in a solvent substantially as hereinbefore described with reference to Figure 1 or Figure 2 of the accompanying drawings. 12 Apparatus for carrying out the method as claimed in any one of the preceding claims comprising a deposition vessel for collecting the solution, means for injecting directly into the solution in the vessel a liquid refrigerant material whereby the refrigerant vapourises in and cools the solution so that at least part the material is deposited from solution; means for drawing off the solution with the deposited solid material and separating the solid material therefrom and collecting the remnant solution to be recycled for use; means for extracting the refrigerant from the deposition vessel, liquifying it and storing it in a storage vessel for re-insertion into the deposition vessel. 13 Apparatus for removing materials in solid form from solution in a solvent substantially as shown in and as hereinbefore described with reference to Figure 1 or Figure 2 of the accompanying drawings.

Inventors: JENKINS DAVID PAUL, LEWIS BARRIE, JENKINS, DAVID, PAUL, LEWIS, BARRIE
Applicants: BRITISH STEEL CORP, BRITISH STEEL CORPORATION
Application year: 1985
Main classification: B01D9
IPC codes: B01D9