GB2027685A - Precipitation of Dissolved Impurities - Google Patents

Abstract

A method and a device for removing from a liquid undesired components dissolved therein by converting such components into substantially insoluble compounds which are separated from the liquid wherein substantially inert particles are used which, when mixed with the liquid, will act as nuclei on which the substantially insoluble compounds formed will preferably deposit.

Description

SPECIFICATION A Method and a Device for Removing from a Liquid Undesired Components Dissolved Therein The invention relates to a method and a device for removing from a liquid undesirable components dissolved therein.

An example thereof is the treatment of waste water obtained when processing metal ores. The ores which are valuable for obtaining metals comprise these and other metals often in the form of sulphides, or are being converted into sulphides for further processing. From the ground ore the various metal compounds are separated from one another by flotation in water. The residues comprising i.a. sulphides which are no longer useful for further processing, are drained together with the water. This is generally done into natural or artificial ponds, which are large and relatively shallow (generally called, in English, "tailings ponds").Therein these substances can settle, and the dimensions of these ponds are so that the water introduced therein at one end, before being drained at the opposite end into a lake, watercourse or the like via an overflow, will have a sufficiently long residence time in the pond for allowing all the entrained ore residues to settle.

This type of operation has been in use for a long time already without any objections being met with. However it has appeared that, on the bottom of such settling lakes, bacteria will develop which convert the sulphides into soluble sulphates, and, at the same time, sulphuric acid is produced so that the pH will be lowered considerably. Thereby heavy metals such as lead and copper can arrive, together with the drained water, into the natural surface water, which may give rise to a substantial mortality among fishes and to other undesirable effects. Since the lakes are relatively shallow, sufficient oxygen is present which is needed by the bacteria for this conversion, and this activity increases at higher temperatures, so that only during a short period at the end of the winter the production of noxious dissolved substances decreases below the allowable level.

This has resulted in government regulations ordering to remove such noxious substances from the drained water, or to start this within a short term. This not only holds for settling ponds to which ore residues are still being supplied, but also for ponds which are no longer in use but which can still collect rain water, so that such undesirable substances will be permanently discharged over the weir.

In order to remove such substances from the water to be drained, the addition of a base such as, for example, calcium hydroxide, will already cause precipitation of heavy metals as a hydroxide. In order to obtain a possibly effective dosage, this should be done in reaction vessels, after which the available substances can be separated in a separation stage, but this appears not to provide an effective solution either.

This is, in the first place, a consequence of the fact that the hydroxides precipitate in an aqueous gel-like state which is difficulty to be separated, and the remaining liquid remains turbid because of more or less stably suspended hydroxide particles which cannot be removed effectively by means of the currently used separators.

Furthermore, when using calcium hydroxide as a base, it will form with the sulphate present insoluble calcium sulphate which will deposit as hard crusts on the walls of the reaction space and the separator, and can only very difficultly be removed therefrom. Even if another more expensive hydroxide will be used, the first problem will remain existent.

The invention provides a solution for these problems, allowing to remove effectively the noxious components from the supplied liquid, whereas, when using lime as a precipitation agent, no calcium deposition on the walls will take place.

According to the invention, besides the auxiliary substances serving to bring about precipitation of the undesired substances, a large number of substantially inert particles is added to the liquid in the reaction space, which are adapted to serve as nuclei on which the badly or not soluble compounds produced will deposit, which nuclei and deposits adhered thereto can be easily separated from the liquid.

In particular these nuclei can be completely or at least partially constituted by the silt consisting of components precipitated by said auxiliary substances, which is returned towards the reaction space either continuously or step-wise.

The effect of these nuclei appears to be the better as their number is larger, the only condition being that the separator used in the device is able to remove these nuclei from the water completely.

When returning the separated matter, only the increase thereof which is a consequence of the reactions taking place in the reaction space should be removed. This surplus matter is produced then in such a concentrated condition that often valuable metals and the like may be separated therefrom in an economically feasible manner, or other useful utilization thereof is possible.

In particular when the reactions may lead to the formation of hard deposits such as of calcium salts, it is advisable to construct the reaction space in such a manner that the reactions have ended before the liquid will reach the separator, in particular if a separator such as a plate separator is used, in which such deposits would lead to obstruction.

If necessary, separation promoting agents can be added and/or coalescence apparatuses can be used for improving the separation in the separator, and still other auxiliary substances may be added into the reaction space enhancing the precipitation of the separable substances, e.g. by influencing the solubility product of the compounds produced.

As a reaction space use can be made of a plurality of vessels connected in series and provided with stirring means, in which vessels the reagents and the supplied liquid will have a sufficiently long residence time for obtaining an effective development of the desired reactions, and it is also possible to use tubes as the reaction space, in which a so-called plug flow can be maintained, viz. a flow of which all portions have substantially the same residence time, and in which, nevertheless, an intermixing will be produced by turbulences. Stirred vessels have the advantage that the reactions therein can be easily adapted to varying conditions, and tubes with a plug flow therein have the advantage that the dosage of the substances to be added can be very accurately adjusted so that overdosing which is inevitable in stirred vessels can be avoided.

The separator preferably comprises two stages, viz. a first stage in which the greater part of the silt can be separated, and a second stage in which the liquid arriving from the first one can be submitted to a post-separation treatment, in order to remove therefrom the last remnants of separable components, and the silt removal from both stages can be adjusted or controlled corresponding to the relative yield. Also the pumps included in the silt circulating and discharge ducts are preferably adjustable, in order to allow the most favourable ratio between the circulated and discharged amounts of silt to be adjusted. Both above-mentioned stages of the separator can, in particular, be constituted by a supply chamber and the plate assembly respectively of a plate separator of current design, communicating with mutually separated silt collecting spaces.

Between the silt discharge of the separator and the return duct a buffer vessel can be included so as to ensure a uniform silt return flow, which buffer vessel can be provided with a stirrer for keeping the silt sufficiently in suspension.

When using this method and device for treating waste water resulting from processing sulphidic ores, the sulphur will be drained as soluble and harmless sulphate in the treated water, and often valuable substances can be recovered from the thickened silt, or another profitable use of the silt is possible.

The invention will be elucidated below by reference to a drawing, showing in: Fig. 1 a highly simplified schematical representation of a device according to the invention; Fig. 2 a corresponding schematical representation of another embodiment of this device; Fig. 3 a schematical representation of a modified embodiment of the separation part of the device of Fig. 2; and Figs. 4 and 5 schematical representations of two embodiments of the reactor part of such a device.

In Fig. 1 the structure of a device for executing the method according to the invention is shown in principle and in a highly simplified manner.

The liquid to be treated, e.g. water containing heavy metal and sulphate ions, and originating from a settling pond of an ore processing plant, is supplied at 1 to a reaction vessel 2. At 3 a substance is added thereto, causing precipitation of the undesirable substances. In the case under consideration, calcium hydroxide, e.g. as milk of lime, is used, causing, together with the metal ions present in the water, the production of poorly soluble hydroxide thereof. If only lime is added, more or less gel- or sol-like hydroxides will be produced, leading to difficultly separable sediment with a very high water content, and the remaining liquid will remain turbid because of the rather stably suspended hydroxide particles. An effective separation will become virtually impossible then.Moreover calcium sulphate will be formed, which will deposit as hard crusts on the walls of the various spaces and ducts.

According to the invention, as indicated at 3, a substance is supplied to the reaction vessel 2, the particles of which are suitable for serving as precipitation nuclei for the difficultly soluble substances produced by the reaction, the number of these nuclei being so large that all the precipitation will take place on these nuclei.

During deposition on such nuclei the influences which, otherwise, lead to the gel-like sediment or to more or less stable suspensions, and probably the water coats surrounding the hydroxide particles, appear to have no or a smaller effect.

The nuclei thus loaded by deposited substances become, then, sufficiently heavy for being easily separated by sedimentation from the liquid. Also insoluble sulphates etc. can precipitate on such nuclei, so that crustaceous depositions on walls and the like will be avoided.

If necessary additional substances can be supplied to the reaction space, as indicated at 5, which may enhance the separation of the difficultly soluble hydroxides, such as, for instance, Fe3+ ions, which can shift the solubility product of these hydroxides (so-called coprecipitation).

The liquid with the loaded nuclei suspended therein is discharged through a duct 6, into which, if required auxiliary substances can be supplied at 7 which enhance the separation or flocculation.

This duct leads towards a separator 8 in which the nuclei can settle as a silt layer 9, and the cleaned liquid can flow off at 10. This liquid is stripped of the undesired substances, but can still contain dissolved substances. In the case of ore waste under consideration, sulphur leaves the device as a soluble sulphate in the water drained at 10, and the metals as, for instance, lead and copper, and also iron and zinc, land as hydroxides in the silt 9. In the lower part of the precipitator 8 the silt will gradually be thickened, and can be removed at 11 continuously or discontinuously.

If necessary a stage 12 can be included in the duct 6, in which stage coalescence of suspended particles can take place if this is conducive to the separation thereof in the separator 8.

For the nuclei materials can be used which can be divided into particles having dimensions which are suitable for the deposition of the compounds in question, and having surface characteristics which are suitable for their separation, such as, for instance, sand, chalk, coal waste, ore residues, mine stone, lava and the like. The choice will be determined, in the first place, by the availability of such materials.

All the material introduced at 4, augmented by the compounds deposited thereon, is to be removed at 11. This can lead to difficulties, in particular because of the amount of silt, and because of the fact that the greater part thereof consists of the added material which can lead to difficulties when further processing the silt. The character of the substances separated from the liquid can, furthermore, hamper the draining of this -material.

It has now appeared, however, that the discharged silt can be used again as material for the nuclei. In particular it appears not to be necessary to use therefor foreign matter since it is possible to use as nuclei the difficultly or not soluble substances themselves formed by the reaction. The structure of a device suitable for this purpose is shown in Fig. 2, in which the same reference numerals have been used as in Fig. 1.

The silt discharge 1 1 of the separator 9 now connects, on the one hand, to a silt discharge duct 13 with a discharge pump 14, and, on the other hand, to a silt return duct 15 with a return pump 16, which duct 15 is connected again to the inlet of the vessel 2. The pumps 14 and 16 are adjusted in such a manner that a considerable portion of the silt removed through the duct 1 1 is returned to the vessel 2, which portion is mixed with the supplied liquid, the silt particles then forming the aforesaid nuclei.

If, initially, as indicated at 4, foreign matter is supplied, this supply can be interrupted as soon as a sufficient number of nuclei is present in the cycle. If at 13 so much silt is discharged as corresponds to the increase of matter caused by the reactions in the vessel 2, the number of circulated nuclei will remain the same. The matter initially supplied at 4 will then gradually disappear from the cycle, and in the long run only reaction products will remain as nuclei in the cycle. It is also possible to omit the supply of foreign matter completely, and to allow a gradual growth of the silt amount to take place during a starting period.

The amount of silt discharged at 13 comprises, for instance, only 1% of the amount circulated through the vessel 2 and the separator 8. This is only an example, but is indicative for the order of magnitude which may be used in practice. As the circulated quantity becomes larger, the number of nuclei will be larger accordingly, and, as has appeared in practice, the operation of the device will become better too. The separator 8, however, should be adapted to cope with the large amounts of circulated silt or other matter used for the nuclei, which is the main limitation imposed on increasing this amount.

As mentioned above, a thickening ofthe separated silt will take place in the lower part of the separator 8. This part can be made conical to that end. For this thickening, however, a certain residence time is required. If a large amount of silt is circulated, and has suitable residence time determined by the discharge flow through the duct 13, the silt level will possibly rise, but this rise should not be so large that the discharge outlet 10 is reached thereby. If, on the other hand, the return flow by means of the pump 16 is increased, the residence time will be shortened, which may lead to a dilution of the silt bed 9, causing this bed to expand, so that, then, the silt may reach the discharge outlet 10 too. Careshould, therefore, be taken that the silt level will not rise too much. This can be easily determined in practice, or can be continuously established by measurements.

The pumps 14 and 16 are preferably adjustable so as to allow to adjust the most favourable discharge and return flows respectively. In any case the silt should be thickened as much as possible so as to circulate a possibly large amount of silt at a high throughput, and, thus, to obtain a large number of nuclei, and, on the other hand, the discharge and/or further processing of the silt is facilitated if it contains less water. The thickened silt can, for instance, be returned to the ore processing plant, in which useful metals may be recovered therefrom. Furthermore the silt can often be utilized in another manner.In the case of settling ponds which are no longer in use, when locally processing of the silt is generally no longer possible, the silt can be returned to the settling pond, which is not harmful since the separated metals are now present in the hydroxide form and not in the sulphide form, and these hydroxides will favourably influence the acidity, thus counteracting the bacterial activity.

Fig. 3 shows a modified embodiment of the separation part of Fig. 2. The liquid outlet 10 of the separator 8 is, now, connected to a postseparator 17 in which still separable remnants can be removed from the liquid, and purified liquid leaves the device at 18. The separated silt is discharged through a silt discharge duct 19 connecting to the discharge duct 1 1 of the separator 8. The ducts 1 1 and 9 are provided with a valve or stop-cock 20 and 21 resp., which are adjusted in such a manner that the flow rates therethrough correspond to the amount of silt separated in the separator 8 or postseparator 17 respectively. It is also possible to use periodically opened and closed valves of which the open-period lengths correspond to the amounts of silt produced in the separators in question.Controlling these valves can, if necessary, be done by means of sensors establishing the silt level in the separator 8 or 17 in question.

Although, in Fig. 3, the separators 8 and 17 are shown as separate elements, they can be united if required, and then the separator 8 can be the supply chamber of a plate separator having a plate assembly which acts as the postseparator 17, which supply chamber and plate assembly are each communicating with a separate silt collector. Also in the case of separate elements the postseparator can be constructed as a plate separator which, then, can be completely adapted to removing relatively small silt amounts still present in the preclarified liquid.

Fig. 3 shows, moreover, a buffer vessel 22 connected to the silt discharge ducts and provided with a stirrer 23, in which vessel the silt originating from the separators 8 and 17 is collected before being drained or recirculated.

Such a buffer vessel can also be used in the case of a simple separator 8 of Fig. 2. Thereby it is ensured that a sufficient amount of silt is available for being recirculated, so that fluctuations in the supply can be smoothed. Furthermore a quantity of silt can be stored therein, which, after interruption of the operation of the device, can be used for starting the process again. The stirrer 23 serves to keep the silt sufficiently in suspension.

The reaction part 2 of the device can be realised in various ways. Fig. 4 shows a number of vessels 24 each provided with a stirrer 25, and forming together the reaction space 2. It is known that, although in a vessel provided with a stirrer a good mixing is obtained, the residence times of the components mixed with one another are rather widely spread, since a small part, because of the complete mixing, appears already immediately at the outlet. By connecting a plurality of stirred mixing vessels in series, it can be insured that all parts have a given minimum residence time which is sufficient for the progression of the reaction. The auxiliary substance required for precipitation is, for instance, added to the first vessel 24, as indicated at 3, and the auxiliary substances serving for coprecipitation are added to the subsequent vessels 14, as indicated at 5.The number of these vessels is, of course, not restricted to the number shown.

Fig. 5 shows another embodiment of the reaction part constituted by one or more tubes 26, which tube or tubes can be straight as shown, but can also be curved, this, of course, depending on the available space and/or the distance to be bridged. Such a tube should be chosen in such a manner that therein such a turbulent flow will occur that a given supplied quantity will move through said tube substantially as a plug, i.e. that all parts thereof will have substantially the same residence time, although therein a certain mixing will be brought about by turbulences. Such a tubular reaction vessel has the advantage that the dosage of the substances to be added can be done very accurately, this in contrast to the embodiment of Fig. 4. As indicated again at 3 and 5, this supply can take place in various points of the tube 26.Such a tube, however, is adapted to a definite flow quantity, so that larger variations therein may be harmful for the operetion. Stirred vessels according to Fig. 4, on the other hand, allow an adaptation to divirgent amounts, but the dosage is less accurate therein.

It will be clear that the device of the invention can be modified in many ways. Moreover the method and the device of the invention are not restricted to processing ore waste, but can be used in all cases where comparable conditions are met with. Also non-aqueous liquids can be treated in this manner, if the components present therein can be made precipitating by adding substances reacting therewith, and if the precipitation can be promoted by the introduction of nuclei.

The silt needs not to be recirculated continuously towards the inlet of the reaction space. In the embodiment of Fig. 1 , for instance, the silt collected at 11 can be recirculated batchwise, or can be lead towards a vessel arranged near the reaction space, from which the silt can be drawn off as required.

Claims (34)

Claims

1. A method for removing from a liquid undesired components present, and in particular dissolved, therein, in which, in a reaction space, additional substances are added, converting these components in not or badly soluble compounds, which compounds are then separated from the liquid before the liquid is drained, characterised in that a large number of substantially inert particles is supplied to the liquid in the reaction space, which particles can serve as nuclei on which the formed not or badly soluble compounds will deposit, which nuclei with the deposits adhered thereto can be easily separated from the liquid.

2. The method of claim 1, characterised in that the quantity of the material for the nuclei is at least several times larger than the amount of undesired substances present in the supplied liquid.

3. The method of claim 2, characterised in that the ratio between the amount of the material for the nuclei and the amount of supplied undesired substances is chosen as large as possible as is compatible with the operation of a settling stage in which this material is separated from the liquid.

4. The method of any one of claims 1 to 3, characterised in that at least a part of the material for the nuclei is formed by the silt separated from the liquid and recirculated towards the reaction space.

5. The method of claim 4, characterised in that a part of the separated silt is discharged, said part corresponding to the increase caused by the reactions with the additional substances.

6. The method of claim 5, characterised in that the ratio between the amount of recirculated and discharged silt is adjustable.

7. The method of any one of claims 1 to 6, characterised in that a part of the additional substances is adapted to influence the solubility product of the formed not or badly soluble compounds in the sense of a reduction of the solubility.

8. The method of any one of claims 1 to 7, characterised in that, before supplying the liquid to the separation stage, substances promoting the separation are added.

9. The method of any one of claims 1 to 8, characterised in that, before supplying the liquid to the separation stage, a coalescence treatment is performed thereon.

10. The method of any one of claims 1 to 9, characterised in that the separation takes place in two stages, and in that the silt discharge from these stages takes place substantially proportional to the silt separation in the stage in question.

1 The method of any one of claims 1 to 10, characterised in that the reactions take place in a plug flow, into which the dosed reagents are added in consecutive points.

12. The method of any one of claims 1 to 10, characterised in that the reactions take place in a plurality of consecutive, and particularly stirred, vessels into which the dosed reagents are added.

13. The method of any one of claims 1 to 12, used for treating a liquid containing metal ions with not or badly soluble hydroxides, in particular heavy metals, characterised in that the additional substances contain soluble hydroxides.

14. The method of claim 13, characterised in that the additional substances for influencing the solubility product contain Fe3+ ions.

15. The method of claim 13 or 14, characterised in that the liquid is waste water originating from processing sulphidic ores, and containing heavy metal and sulphate ions.

16. The method of claim 1 5, characterised in that calcium hydroxide is added as an additional substance, the formed calcium sulphate also being deposited on the nuclei.

17. A device for executing the method of any one of claims 1 to 16, comprising a reaction space for treating the supplied liquid, provided with means for adding thereto additional substances adapted to convert the undesired components into not or badly soluble compounds, characterised by means connected to the reaction space for supplying thereto material that is adapted to serve as precipitation nuclei, and by a separator connected to this reaction space designed to remove said nuclei with the compounds deposited thereon from the liquid.

18. The device of claim 17, characterised in that the reaction space and the separator are adapted for processing amounts of material for the nuclei which are substantially larger than the amount of substances in the supplied liquid which are to be removed therefrom.

19. The device of claim 1 7 or 18, characterised in that it is designed for recirculating the silt separated in the separator towards the reaction space.

20. The device of claim 19, characterised by a return duct connected to the silt discharge space of the separator, leading towards the inlet of the reaction space.

21. The device of claim 20, characterised in that the return duct is provided with a branch duct through which a part of the silt discharged from the separator can be removed from the device.

22. The device of claim 21, characterised in that the return and branch duct are each provided with a pump.

23. The device of claim 22, characterised in that the ratio between the pump yields is adjustable.

24. The device of any one of claims 17 to 23, characterised in that the reaction space is provided with means for supplying thereto still other additional substances for influencing the solubility of the formed compounds.

25. The device of any one of claims 17 to 24, characterised in that the connection between the reaction space and the separator is provided with means for supplying separation promoting agents.

26. The device of any one of claims 17 to 25, characterised in that the connection between the reaction space and the separator is provided with means for executing a coalescence treatment.

27. The device of any one of claims 17 to 26, characterised in that the separator consists of two stages with interconnected silt discharge outlets.

28. The device of claim 27, characterised in that the silt discharges are each provided with an adjustable valve.

29. The device of claim 28, characterised in that the adjustment of the valves can be controlled in conformity with the silt level to be expected or actually attained in the separator in question.

30. The device of any one of claims 27 to 29, characterised in that both stages are a part of a single separator with separated silt discharge spaces for both stages.

31. The device of any one of claims 20 to 30, characterised by a buffer vessel between the silt discharge outlet of the separator and the return duct.

32. The device of any one of claims 17 to 31, characterised in that the reaction space consists of one or more tubes provided with means for introducing reagents in suitable points.

33. The device of any one of claims 17 to 31, characterised in that the reaction space consists of a plurality of stirred vessels connected in series, provided with means for supplying reagents thereto.

34. A device substantially as hereinbefore described with reference to the accompanying drawings.

34. The device of any one of claims 17 to 33, characterised in that the inlet is connected to a settling pond for waste products of ore processing.

33. A method substantially as hereinbefore described with reference to the accompanying drawings.