WO1999015266A1

WO1999015266A1 - Method and apparatus for providing precipitation

Info

Publication number: WO1999015266A1
Application number: PCT/AU1998/000794
Authority: WO
Inventors: Ronald William Arthur; Clifford John Restarick
Original assignee: Geo2 Limited
Priority date: 1997-09-24
Filing date: 1998-09-23
Publication date: 1999-04-01

Abstract

A method of producing precipitation in a liquid containing a dissolved or suspended species to be precipitated including passing the liquid through a pipe reactor and introducing to the pipe reactor a precipitation agent adapted to reduce the solubility of or to agglomerate said species.

Description

METHOD AND APPARATUS FOR PROVIDING PRECIPITATION

The present invention relates to a method for producing precipitation of a component from a liquid and an apparatus for use in the precipitation method.

Precipitation processes of the type in which an agent, hereinafter referred to as the precipitation agent, is added to a liquid to induce the formation of a precipitate in the liquid are widely used in industry. Such processes are frequently used to remove a component from a solution or to improve the ease with which a component is removed from suspension in a liquid by agglomeration or by otherwise increasing the particle size of the component. The precipitation agent may act on the liquid or component to be precipitated to cause the component to be less soluble or more easily separated from the liquid. Examples of precipitation reactions of this type include the addition of acidic or alkaline precipitation agents to an aqueous solution to provide a pH at which one or more of the soluble components is rendered insoluble.

The invention will be described with specific reference to the use of alkaline precipitation agents, for example limestone, lime and dolomite in inducing the precipitation of metal species from aqueous solution. Such processes are used in the treatment of waste waters to remove metals. An example of a precipitation reaction of this type and which may be carried out using the process of the invention is described in International Patent Application PCT/AU97/00179 (GE02 Limited) the contents of which are herein incorporated by reference. Despite description of the invention with reference to this application it will be understood that the invention has a wide range of applications and is not limited to this exemplary application.

Precipitation reactions frequently require addition of the precipitation agent to be carefully controlled so that the appropriate insoluble species are formed with a particle size which will enable them to be readily removed from the liquid by filtration or settling. This frequently also involves controlling a number of steps during the precipitation process. Precipitation typically involves the formation of minute seeds which act as nuclei for the growth of larger particles. Whereas rapid dosing of precipitation agent often leads to a very fine dense floe which is difficult or impossible to filter, controlled addition of the precipitation agent during formation of nuclei and their subsequent growth may provide a commercially viable process. We have found that precipitation of metals from acid mine waste may be significantly improved by multi-step control in which pH in different steps is maintained in distinct ranges for example between a pH of 5 and 6 in a first step and between a pH of 8 and 9 in a second step.

Prior art methods of precipitation involving multi-step control have generally involved titrating the liquid with the precipitation agent in a stirred reaction vessel to provide careful control of the rate of addition. Efficient rapid mixing and slow addition is frequently necessary to avoid high localised concentrations within the liquid which can adversely effect the results. While it would be desirable to use a continuous process in which liquid is continually added and removed from a reactor vessel this produces a wide distribution of residence times within the vessel, significantly reduces control and can lead to poor results.

We have now found that efficient control of precipitation reactions can be obtained by passing the liquid in which precipitation is to be produced through a pipe reactor and introducing the precipitation agent to the pipe reactor to thereby induce precipitation. Preferably the precipitation agent is introduced to the pipe reactor at a plurality of points along the pipe reactor to thereby maintain a progressive change in conditions as liquid passes through the pipe reactor such that the change in conditions induces or facilitates precipitation.

In the preferred embodiment of the invention, the precipitation agent is an alkaline reagent and the alkaline reagent is preferably introduced to the pipe reactor at a plurality of points spaced along the pipe reactor to thereby progressively increase the pH in the pipe reactor.

The pipe reactor is a tube whose length is generally at least about 50, preferably 100 times its average internal diameter and more preferably at least 200 times its average internal diameter. We have found that precipitation is enhanced when the pipe reactor is arranged so as to ascend in the direction of flow. Ascending flow reduces the amount of air which could otherwise form void spaces within the pipe resulting in air being entrained in the liquid particularly the first part of the pipe reactor. We have found that avoiding the entrainment of significant air is desirable because both the carbon dioxide and oxygen components of air may reduce the efficiency of the process. Carbon dioxide has the effect of altering the pH of an aqueous solution and also reacts with some precipitation agents. For example if lime is used as the precipitation agent it may react with carbon dioxide to produce calcium carbonate which is not as effective a base and is less soluble than lime. As a result significant aeration leads to increased formation of carbonate scale which tends to precipitate out on equipment such as pH electrodes. Calcium carbonate formed from lime also tends to form a scum on the surface of the water when settling the precipitate. The pipe reactor of the invention provides excellent control of the atmosphere under which precipitation takes place. If liquid is continuously passed through the reactor so that voids are eliminated the exposure of oxygen and carbon dioxide can be minimised. The use of non- oxidising conditions is important in conducting the precipitation reaction described in International Application PCT/AU97/00179. Alternatively, if in specific precipitation reaction the addition of a gas such as oxygen or carbon dioxide is required then the proportion of these gases can be carefully controlled by injection into the pipe reactor. The pipe reactor used in the method of the invention preferably includes one or more coil sections. We have found that an efficient and convenient pipe configuration comprises a plurality of substantially planar spiral sections in flow communication. The planar spirals may be horizontally disposed with ascending flow connection between vertically spaced spiral sections. The configuration of the pipe may provide flow from the inside of a spiral to the outside of the next spiral section or from the inside of one spiral section to the inside of the next, from outside to outside or combinations of these configurations. The spiral sections may be arranged to enable the precipitating agent to be introduced to a plurality of spirals. The precipitating agent may introduce one or more points spaced along the pipe.

The flow of liquid through the pipe reactor will preferably be turbulent so that settling of precipitate within the tube is inhibited. Typically the flow of liquid will provide a Reynolds number of at least about 4000 and more preferably at least about 6000. Most preferably the Reynolds number is at least about 8000.

The method and apparatus of the invention are particularly useful in the treatment of waste waters such as acid mine drainage, to remove metal species including heavy metals. Metal species can be precipitated from acid mine waste by increasing the pH with an alkaline reagent. Lime is particularly useful because of its content of calcium ions and relatively low cost. The use of alkaline agents as the precipitating agent in this reaction generally causes the metal species to precipitate, in the form of the metal hydroxide species or double cation hydroxy salts such as in the process of our copending International Application.

Typically the acid waste water treated in the preferred embodiment of the invention will have a pH in the range of from 2 to 4 and the addition of alkaline reagent in the pipe reactor will provide a pH of at least 5 and in many cases from 7.5 to 12. The method of the invention will typically include isolating the precipitate from liquid which has passed through the reactor. The precipitate may be isolated by allowing it to settle in a settling tank, by filtration or by other solid liquid separation processes known in the art.

Settling of fine precipitate may be enhanced by addition of a flocculating agent. The flocculating agent may be added prior to delivery of the liquid to an apparatus used in collecting the precipitate such as a filter or settling tank. It is particularly preferred that flocculant is added to the pipe reactor in a region near to the end of the pipe.

It is preferred that the pipe reactor is also provided with condition monitors, such as pH monitors, to monitor the effect of the precipitation agent on the condition of the liquid. The monitors may, for example, be located down stream of each of the inlets for the precipitation agent such as an alkaline reagent. The monitors may be used as part of a feed back and or feed forward control loop to maintain the contents of sections of a pipe reactor within a certain range of conditions such as a specific pH range by regulating the rate of addition of the precipitation agent in response to a change in the conditions. Preferably successive sections of the pipe reactor maintain an incremental increase in pH.

The precipitation agent used in the process of the invention is generally at least partly water soluble and can be added to the reactor in the form of an aqueous solution or particulate slurry. It is highly desirable that the precipitation agent is injected into the pipe reactor as a continuous stream under conditions which provide efficient high shear mixing and yet avoid aeration. While the precipitation reagent may simply be pumped into the pipe reactor we have found that in a pipe reactor the pulsed flow produced by commercially available metering pumps results in the precipitation agent being introduced in spaced sections of high concentration. This may lead to a reduction in control and to a precipitate which is more difficult to isolate. We have now found that the precipitation is enhanced if the precipitation agent is mixed with the liquid stream in a cyclone disposed in the line (herein after referred to as an in-line cyclone).

The invention therefore also provides a method of adding a reagent to a pipe reactor comprising: feeding the liquid stream to the tangential inlet of a cyclone; feeding the outlet stream from the vortex finder of the cyclone; and introducing to the cyclone at the apex orifice a fluid slurry of the reagent material, to provide mixing of the reagent and stream.

The cyclone, which is generally a hydrocyclone, is known in the art and generally includes an tangential inlet, a conical section which commonly narrowing tapers towards an apex outlet at the end remote from the inlet, an axial vortex finder outlet sometimes referred to as the overflow at the same end as the inlet. The apex outlet is conventionally referred to as the underflow outlet. It will be understood in the art that the terms overflow and underflow are defined with reference to the cyclone oriented with the vortex finder at the top. However, since the centrifugal force in a cyclone may be many times that of gravity the cyclone may be disposed with any orientation. Indeed it is particularly preferred in the process and apparatus of the invention that the apex orifice or underflow orifice is directed upward. Accordingly, to avoid confusion we have used the term vortex finder flow and apex flow rather than "overflow" and "underflow". The reagent added to the pipe reactor may be any suitable reagent to be reacted with the liquid stream however it is preferred that the reagent is a precipitation agent. When, in the more preferred embodiment, the liquid stream is acid mine waste the precipitation agent will be an alkaline reagent preferably in the form of a fine particulate slurry. Aqueous slurries of lime or limestone are most preferred.

Hydrocyclones generally provide separation of a liquid into two outlet flows, one outlet flow from the apex carrying particles larger than a predetermined size and the other outlet flowing from the vortex finder and carrying finer particles. We have found that if the apex outlet flow is restricted or contained and a reagent is introduced to the apex orifice then particularly efficient high shear mixing of the reagent and liquid is provided. Preferably there is no continuous flow out of the apex orifice. The most preferred hydrocyclones for use in-line have a chamber which encloses the apex orifice and contains the outflow so that there is no net flow out of the apex orifice. When the reagent is introduced under pressure to the chamber adjacent the apex orifice we have found that a zone of high shear mixing is established inside the cyclone adjacent the apex orifice. The cyclone system used in the invention provides efficient mixing of the particulate material with the fluid stream. The above cyclone configuration provides high shear mixing of the incoming reagent with the fluid streams and as a cyclone is operated without an air space or moving parts the centrifugal force provides highly efficient and rapid mixing without significant aeration. This is to be contrasted with impeller mixing which leads to aeration from the vortex created at the surface and also from cavitation or small air pockets produced at the edge of the impeller. Mixing in a cyclone also avoids the pulsing effect common with metering pumps.

In a further embodiment of the invention we provide a method of mixing an aqueous liquid stream with an aqueous slurry of particulate material comprising the steps of: passing the liquid stream through a hydrocyclone via the tangential inlet and vortex finder outlet; restricting the liquid flow out of the apex outlet and preferably enclosing the apex outlet so that there is no net outflow from the apex; and introducing particulate slurry at the apex orifice of the hydrocyclone continuously with the flow of the liquid stream such that the particulate material is mixed with the fluid by the centrifugal forces of the cyclone.

When, as in the preferred precipitation process, the precipitation agent is introduced in the form of a particulate slurry of water soluble material, such as lime, the use of a hydrocyclone in accordance with this further embodiment provides excellent results. Particles of water soluble and partly water soluble material are rapidly and efficiently mixed with the incoming liquid stream to provide dissolution.

In accordance with a further embodiment of the invention there is provided a pipe reactor apparatus for carrying out a precipitation reaction comprising: a pipe for providing a continuous flow of the liquid in which precipitation is to be induced; a plurality of injecting points for introducing a precipitation agent adapted to induce precipitation in the liquid, the injection points comprising a hydrocyclone in-line with the pipe such that the initial or upstream portion of pipe feeds into the tangential inlet and the vortex finder outlet feeds into the later or downstream portion of the pipe and wherein the apex outlet flow of the cyclone is restricted, and preferably enclosed such that there is no net outlet flow at the apex; and means for introducing the precipitation agent to the hydrocyclone at the apex orifice to provide continuous mixing of the incoming precipitation agent with the liquid fed into the tangential inlet. The pipe reactor may include a plurality of cyclones spaced along and inline with the pipe.

Where the precipitation agent is a particulate slurry such as a lime or limestone slurry the in-line cyclone mixer may also be used to provide a degree of particle size control so that particles larger than a predetermined size are retained in the cyclone. However, as the in-line cyclone mixers are primarily used in providing efficient mixing without aeration it is preferred that the slurry is efficiently mixed prior to reaction with the liquid stream.

The formation of a slurry from a solid reagent such as lime or limestone is preferably carried out so as to remove impurities which could interfere with precipitation and to regulate the size of particles in the slurry. Impurities which are present in industrial chemicals may have a deleterious effect on precipitation or provide problems in isolating the precipitate. The presence of large particles of precipitation agent can also lead to localised high concentrations and further reduce control of the process. It is difficult to produce uniform particle sizes because even when compositions are precision milled the formation of agglomerates of particles produces an effective particle size much larger than the size of individual particles produced by milling. Agglomeration may also be exacerbated when the particles are mixed with water. Preparation of the slurry should also preferably be carried out in a manner which will minimise aeration of the composition.

We have found that excellent results are provided if the slurry of the precipitation reagent is prepared by passing a crude aqueous mixture of the precipitation agent through a premixing cyclone wherein the premixing cyclone is adapted to retain particles larger than a predetermined size adjacent the apex orifice.

Preferably the cyclone is provided with a chamber about the apex orifice. When the apex orifice is enclosed in a chamber oversize particles circulate within the chamber until they are reduced in size by dissolution or disrupting agglomerates to provide particles sufficiently fine to be delivered to the vortex finder. The chamber may be provided with a sealable opening or continuous bleed system to allow the impurities and oversize material to be removed. In a preferred embodiment of the pipe reactor, the reactor is provided with a precipitation agent premixing apparatus comprising a premixing cyclone having a tangential inlet, a chamber enclosing the apex orifice and a discharge outlet from the chamber for providing discharge of oversized material and impurities from the chamber; a premix vessel for receiving solid precipitation agent and mixing liquid such as water to form a crude slurry, preferably without significant mechanical mixing or agitation, within the vessel; a premix vessel outlet for feeding crude slurry preferably from adjacent the base of the vessel to the tangential inlet of the cyclone; a recycle line for returning the vortex finder outflow of the cyclone to the premix tank preferably adjacent the base; preferably an open ended conduit vertically disposed within the tank, which may preferably extend from above the liquid level to adjacent the bottom, for receiving oversize particles and impurities from the chamber discharge of the cyclone; at least one dosing line in fluid communication with the recycle line for providing a flow of liquid to the inlet of the in-line cyclone; and valve means adapted to divert flow from the vortex finder outlet to the dosing line.

The valve means may provide diversion of liquid to a plurality of dosing lines. We have found that by recycling crude slurry via the hydrocyclone a consistent particle size can be provided without the need for mechanical mixers in the tank. Larger particulate material tends to settle in the tank and be drawn off into the cyclone which produces high shear mixing to disrupt agglomerates and also enables impurities and oversize material to be expelled via the apex orifice. The oversize material and impurities may be disposed of or may be recycled to the tank as described in the above preferred aspect or ground to a smaller size in an adjacent communication device.

The dosing line is preferably provided with a dosing pump and means for regulating the rate of dosing into the in-line cyclone. In a particularly preferred embodiment the pipe reactor is provided with in-line condition monitoring means, such as pH electrodes, downstream of the in-line cyclone and the regulator is provided with feed-back means adapted to change the rate of delivery to the inline cyclone in response to a change conditions, such as pH, within the pipe to thereby regulate the conditions. We have found that the feed-back system described above enables the slurry to be dosed without the need to precisely maintain the concentration of the slurry as the feed-back mechanism may be used to automatically compensate for variations in conditions. Thus the premix vessel may be topped up from time to time with fresh liquid and particulate material without precise quantity measurement.

The premix cyclone system described above has been found to provide efficient size regulation and removal of impurities so that the slurry delivered to the in-line cyclone comprises a high proportion of particles below a predetermined size. Preferably at least 80% of particles will be less than 150 microns more preferably less than 50 microns and more preferably less than 20 microns even more preferably less than 10 microns. The effective size separation is of course dependent on variables such as the flow rate, tangential feed diameter and inlet shape, vortex finder length and diameter and vortex finder orifice diameter. The vortex finder diameter is particularly important. All particles above a predetermined size are delivered to the apex orifice and remaining fine particles are discharged through the vortex finder outlet. A person skilled in the design and use of cyclones will readily be able to choose parameters to provide the desired particle size constraints.

The invention will now be described with reference to the attached drawings. In the drawings:

Figure 1a is a side perspective of the pipe reactor of the invention. Figure 1 b is a side elevation of the pipe reactor of the invention.

Figure 1c is a pipe reactor assembly according to a preferred embodiment of the invention.

Figure 2 is an in-line hydrocyclone used in the pipe reactor of the invention. Figure 3 is a schematic representation of a premix cyclone system for use with the in-line cyclone of Figure 2.

Referring to the drawings Figures 1a and 1b show a pipe reactor (1) which provides a continuous flow of liquid from the inlet (2) to the outlet (3). The pipe reactor (1) includes a substantial section in the form of a multiplicity of planar spirals (4) interconnected by ascending pipe sections (4a, 4b) between adjacent spirals (4). The ascending pipe sections include inner ascending pipe sections

(4a) and outer ascending pipe sections (4b) alternating between adjacent spirals.

The inlet (2) of the pipe is generally provided with a pump (6) for providing a continuous flow of liquid through the pipe reactor (1). Typically the pump will provide turbulent flow within the pipe reactor (1 ).

The pipe (1) is provided with and preferably at least one or more than one hydrocyclone dosing sites (7) for introducing the precipitation agent via an injection line (8) under pressure from a pump (9). The pipe reactor (1) may be provided with clear sections 7a for visual monitoring of the flow and precipitation downstream of the dosing sites (see Figure 1c).

In the preferred embodiment shown in Figure 1c the pipe reactor is also provided with a vessel (100) for receiving liquid to be treated, a pretreatment section (101) which may for example remove any oversized material, a flow meter (102) downstream of the pump for monitoring the flow rate to maintain turbulent flow in a predetermined range and pH electrodes (103). The pH electrodes (103) provide a feed-back to the dosing pump (9) to regulate flow of alkaline agent to thereby maintain the pH within a predetermined range. A dosing pump (104), is provided for introducing a flocculating agent near the end of the pipe reactor.

The pipe reactor may optionally be provided with by-pass lines (105) each provided with a by-pass valve (106) to allow the length of the pipe reactor to be shortened if desired. Referring to Figure 2 there is shown a schematic cross section of a hydrocyclone in-line dosing site for mixing the precipitation agent with the liquid. The hydrocyclone (7) has a lower cylindrical part (10) and an upper conical part (11) the apex of which is provided with an orifice (12) which according to usual hydrocyclone terminology may be referred to as the underflow orifice, The lower cylindrical part of the hydrocyclone is provided with a tangential inlet (13) and a vortex finder outlet (14). The inlet is continuous with the upstream portion of the pipe reactor and the vortex finder (14) is continuous with the downstream portion. A chamber (15) is provided about the apex orifice (12) so that there is no net apex discharge. The chamber (15) is further provided with an inlet (8) for introducing the precipitation agent and optionally an outlet (16) which may be adapted to allow periodic removal or continuous bleeding of impurities or oversized material.

In the preferred embodiment of the process of the invention acid mine waste containing heavy metal contaminants is pumped via inlet pump (6) through the pipe reactor (1) at a rate adapted to provide turbulent flow in the pipe (1). The liquid flows through a plurality of in-line hydrocyclone dosing sites spaced apart in the pipe line. The liquid flows through the hydrocyclones via the tangential inlet (13) and vortex finder outlet (14). A particulate lime slurry is introduced to each hydrocyclone via inlet (8) while the acid mine waste is passing through the hydrocyclone (7) to cause the lime slurry to mix with the acid mine waste in the hydrocyclone. If desired primary or additional doses of lime may be introduced by injecting the lime into the pipe reactor without using a cyclone.

The pipe reactor will generally include at least one or two in-line hydrocyclone mixers (7). Introduction of the alkaline precipitation agent at the first hydrocyclone will preferably provide a lower pH condition for example in the range of from 6.0 to 7.5 and a further addition will provide a higher pH condition of for Example 9 to 9.5. The liquid may be delivered from the pipe reactor outlet (3) to means for isolating the precipitate from the liquid such as a filter press or settling tank.

Figure 3 shows a premix apparatus (200) for preparing a slurry of particulate precipitation reagent, particularly lime or limestone for mixing with the liquid stream in the in-line hydrocyclone mixers (7). The premix apparatus which delivers the premixed slurry to the in-line cyclone via feed lines (8) includes a crude slurry tank (201) for receiving periodic top up of liquid and crude particulate solid, a pump for feeding crude slurry to a cyclone (203) from an outlet 201(a) adjacent the bottom of the tank to the tangential inlet (204) of the cyclone (203). The cyclone (203) is of similar construction to the in-line mixing cyclone (7) and includes a cylindrical part (205), a conical part (206) the apex of which is provided with an orifice (207) and a chamber (208) about the apex orifice (207). The vortex finder (209) provides a continuous outlet for the cyclone (203). The chamber is provided with an outlet (210) which is regulated by valve (211) which is generally closed during cyclone operation but which may be opened permanently or periodically to return oversized material and impurities to tank (201) via a oversize return tube (212). The oversize return tube has open ends and extends from above the fluid line of the tank (213) to adjacent the bottom of the tank (214). The vortex finder outlet (209) of the cyclone (203) is fed to a manifold (215) which may recycle the flow to the tank recycle flow inlet (216) via recycle line (217) when recycle valve (218) is open. Manifold (215) is also provided with dosing feed valves (219a) and (219b) which allow the flow to feed lines (8) to in-line mixing cyclone (7) to be controlled.

When both valves (219a) and (219b) are open the manifold circuit (220) is purged. Closure of valves (219b) and (218) provides flow to in-line cyclone (7) via feed lines (8).

The premix system provides efficient sizing of the particulate slurry without significant aeration of the slurry. The tank (201) is not agitated and hence significant air entrainment is avoided. Clumping and agglomerates present in the slurry are generally disrupted by high shear mixing in the cyclone (203) and oversize particles and impurities delivered to the apex orifice of the cyclone and circulate within the chamber (208). On dissolution oversize particles and disruption of agglomerates the particles may become fine enough to be delivered to the vortex finder outlet.

One embodiment of the invention will now be demonstrated by an example. It will be understood that the Examiner does not limit application of the invention. Example

This example demonstrates the use of the process of the invention in precipitation of metals from acid mine waste and compares the process with a corresponding process in which precipitation is carried out in a vessel.

Simulated acid mine waste of known composition was prepared containing Magnesium (1736ppm) as MgSO₄«7H₂O and Manganese 320ppm as MnS0₄*H₂O. The magnesium sulphate and manganese sulphate were dissolved in deionised water and the pH adjusted to 2.8 with sulphuric acid. Comparison using Agitated Batch Cell

The simulated acid mine waste was contained in a vessel and a slurry of commercial builders lime of concentration 4.9% by weight was added to provide a pH of about 11.5. The system was mixed with a bleached stirrer. Embodiment of the Invention The acid mine waste was pumped through a pipe reactor depicted in

Figures 1 to 3 using the following conditions: • flow rate 10.65 L/min. • length of pipe in each spiral approx. 50m.

• internal pipe diameter 2.5cm.

• residence time approx. 30 min.

• dosing points as shown in Figure 3 with a difference in residence time of approximately 5 minutes. Flocculant was not used.

• Hydrocyclones were used to provide a vortex finder outflow of particle sizes less than about 10 microns.

Lime was dosed at a rate of 1.4 L/min at the first cyclone to provide a pH of 10.5 at the first pH meter and at a rate of 0.8 L/min. at the second cyclone to provide a pH of approximately 11.5 at the second cyclone.

The precipitates prepared by the comparison and method of the invention were each collected by filtration and the filtrate analysed for Calcium, Magnesium, Manganese and Sulphate. The results are shown in Table 1.

TABLE 1

The comparison shows a significant reduction in metals and sulphate remaining in the liquid when the method of the invention is used.

A comparison of invention carried out with and without cyclone premixing and in-line mixing was found to reduce the presence of particulate impurities and carbonate scale in the precipitate.

Claims

1. A method of producing precipitation in a liquid containing a dissolved or suspended species to be precipitated including passing the liquid through a pipe reactor and introducing to the pipe reactor a precipitation agent adapted to reduce the solubility of or to agglomerate said species.

2. A method according to claim 1 wherein the said liquid is an acid aqueous solution of metal species and said precipitation agent is an alkaline reagent.

3. A method according to claim 2 wherein the reactor pipe ascends in the direction of flow to thereby exclude air from the liquid.

4. A method according to claim 2 wherein the liquid has a pH of from 2 to 4 and the alkaline precipitation agent is added to provide a pH in the range of from

7.5 to 12.

5. A method according to claim 2 wherein the alkaline precipitation agent is added to the pipe at a plurality of points spaced along the pipe.

6. A method according to claim 1 wherein the precipitation agent is added to the pipe by means of a cyclone located in line with the pipe which cyclone includes a tangential inlet, a vortex finder outlet and an apex orifice and the method includes: feeding the liquid to the tangential inlet of the cyclone; feeding an outlet stream from the vortex finder of the cyclone; and introducing the precipitation reagent to the cyclone at the apex orifice to provide mixing of the liquid and precipitation agent by centrifugal forces in the cyclone.

7. A method according to claim 6 wherein the apex orifice is enclosed by a chamber and the reagent is introduced to the chamber under pressure.

8. A method according to claim 6 wherein the precipitation agent is in the form of a particulate slurry of water soluble material.

9. A pipe reactor appartus for conducting precipitation reactions including: a pipe for providing a flow of liquid in which precipitation is to be induced; one or more points spaced along the pipe for introducing a precipitation agent adapted to introduce precipitation in said liquid; the points comprising a cyclone in-line with the pipe such that upstream portions of the pipe feeds into the tangential inlet of the cyclone and the vortex finder outlet of the cyclone feeds into the downstream portion of the pipe; and wherein flow from the apex orifice of the cyclone is restricted and the apex orifice is provided with means for introducing the precipitation agent to the cyclone at the apex orifice to thereby provide continual mixing of the precipitation agent with liquid passing through the cyclone.

10. An apparatus according to claim 9 wherein the apex outlet is enclosed by a chamber so that the outflow from the apex orifice is restricted.

11. An apparatus according to claim 9 further including a premixing cyclone for preparing an aqueous slurry of the precipitation agent and which is adapted to retain particles larger than a predetermined size adjacent the apex orifice of the premixing cyclone.

12. An apparatus according to claim 13 wherein the premixing cyclone is provided with a chamber about the apex orifice for restricting flow from the apex orifice.

13. An apparatus according to claim 9 wherein the pipe reactor is provided with a precipitation agent premixing apparatus comprising a premixing cyclone having a tangential inlet, an apex orifice, a chamber enclosing the apex orifice and a discharge outlet from the chamber for providing discharge of oversized material and impurities from the chamber; a premix vessel for receiving solid precipitation agent and mixing liquid such as water to form a crude slurry, preferably without significant mechanical mixing or agitation, within the vessel; a premix vessel outlet for feeding crude slurry to the tangential inlet of the cyclone; a recycle line for returning the vortex finder outflow of the cyclone to the premix tank; at least one dosing line in fluid communication with the recycle line for providing a flow of liquid to the inlet of the in-line cyclone; and valve means adapted to divert flow from the vortex finder outlet to the dosing line.

14. An apparatus according to claim 11 wherein the premix cyclone provides a slurry in which at least 80% of particles are on size less than 50 microns.

15. A method of mixing an aqueous liquid stream with an aqueous slurry of particulate material including the steps of: passing the liquid stream through a cyclone having a tangential inlet and vortex finder outlet and an apex orifice wherein the stream is fed to the tangential inlet and out of the vortex finder outlet; restricting the liquid flow out of the apex orifice; and introducing the aqueous slurry of particulate material to the apex orifice under pressure such that the particulate material is mixed with the fluid by the centrifugal forces in the cyclone.

16. A method according to claim 15 wherein the flow of out of the apex orifice is restricted by a chamber enclosing the apex orifice.