TECHNICAL FIELD The present mvention relates to the manufacture and recovery of monodisperse silica sols from geothermal brine.

BACKGROUND ART

Geothermal resources are an attractive alternative energy source from economic and environmental points of view. However, geothermal power production in

New Zealand, and in some other parts of the world, is almost entirely from water dominated hydrothermal systems. Therefore, large quantities of "waste" water are produced from the geothermal reservoir along with the steam required for conventional steam turbine power production. As geothermal power production becomes a mature technology, attention is turning to the sustainable management and more eflBcient use ofthe total resource. This is a consequence of both the realisation that geothermal reservoirs are finite resources, as well as increasing public pressure for environmentally sensitive development. Consequently, there has been a growing interest in the utilisation and treatment of the hot water fraction that is produced during geothermal reservoir exploitation. This water fraction is commonly called a brine because of the presence of various dissolved salts and other minerals.

There are three aspects to the utilisation of geothermal brine. The first is the use ofthe available heat energy and this can take the form of power production through steam turbines or binary cycle plants as well as through direct use of the heat in such applications as district heating, aquaculture, and horticulture, for example. The second aspect concerns the disposal of the geothermal brine in an environmentally acceptable manner, bearing in mind the mineral content of the brine. The third aspect relates to the extraction of minerals, and of silica in particular, from the brine. The present invention particularly concerns the third of these aspects.

Silica is the name by which silicon dioxide, Si0 ₂ , is commonly known. One form of silica, the mineral quartz, is ubiquitous in rocks in the earth's crust. Indeed, silicon is the earth's second most abundant element after oxygen. Hot water in a geothermal reservoir is in contact with quartz and a chemical equilibrium is established where some of the quartz dissolves in the geothermal water where it generally exists as monomeric silicic acid, often simply called monomeric silica. The exact amount that

dissolves in the water is almost entirely a function ofthe temperature of the water; the hotter the water, the more quartz that is dissolved in it. The relationship between temperature and the resulting concentration of dissolved quartz, or silica, has been measured in the laboratory and is well known.

When geothermal fluid is brought to the surface for power production, the brine contains an amount of monomeric silica dependent on the temperature of the geothermal reservoir. As the temperature ofthe brine is lowered during the separation of steam for power production, the concentration of monomeric silica becomes greater than the equilibrium with quartz would allow. However, at temperatures below about

250°C, monomeric silica is in equilibrium not with quartz, but with other forms of silica such as amorphous silica. Under this new equilibrium, the solubility of silica with respect to amorphous silica is greater than with respect to quartz. When the temperature ofthe brine is lowered sufficiently it becomes oversaturated with respect to amorphous silica. Under these circumstances, solid silica would normally be expected to precipitate from the brine. However, silica is unusual, in that the individual molecules of silica, as monomeric silica, join together in a process of polymerisation to form nuclei that then grow by direct molecular deposition of silica to form colloidal silica particles and hence a colloidal suspension.

The individual colloidal silica particles are small in size. If nucleation occurs continuously over a period of time the silica particles formed are present in a range of different sizes. However, in a mixture of silica particles of different radii and where the radius is less than the critical radius, the smaller particles tend to dissolve and the dissolved silica is deposited by molecular deposition on the larger particles, this eventually resulting in a decrease in the number of particles and an equalisation of size.

At high cooling rates to near ambient temperatures growth occurs to produce particles of less than about 15 nm in diameter. At lower cooling rates growth may continue, to produce particles having larger sizes, for example, up to 5000 nm.

These particles, particularly the smaller particles, can remain suspended in solution for some time. If the colloidal particles are all of substantially the same size, this is called a "monodisperse" colloid. A solution containing particles of differing sizes is called a "polydisperse" colloid. In either case individual colloidal particles can come

together to form larger clusters or chains of particles. Such a process is called "aggregation" or "flocculation" or "coagulation".

In the pH range normally encountered in geothermal brines (pH 6-9) the colloidal particles carry a negative charge and the resulting repulsion helps to constrain the aggregation of the particles. At a pH of about 2 the particles have a zero surface charge and can more easily aggregate while at an early stage of polymerisation. However, the presence of "bridging" ions in solution can facilitate the aggregation of colloidal silica particles at any pH. Such "bridging" ions are usually naturally present, these comprising the ions of elements such as calcium, magnesium and iron, for example. In order to produce a high quality silica sol, incorporation of impurities during the growth of the colloidal silica is to be avoided, or at least minimized.

While the individual colloidal silica particles can remain suspended in solution for some time, under certain circumstances they can deposit very rapidly. This deposition, as already indicated, involves aggregation of individual colloidal particles to form larger chains and clusters of particles. Aggregation is followed by settling whereby the particles form a silica scale, that being a coating of silica deposited on surfaces with which the brine is in contact. Monomeric silica may also deposit directly on such surfaces. The exact conditions required for deposition of silica scale are not fully understood, but many factors are involved. These include such parameters such as particle size, particle density, fluid flow conditions, temperature, pH and the presence of other ions.

Silica scaling is one of the most persistent problems facing geothermal development. It causes blocking of waste water pipes and can detrimentally affect the mjectivity of injection wells whereby waste brine is reinjected back into the ground. Furthermore, it can preclude the use of heat exchangers for low grade heat use, and can hamper the use of binary cycle power plants.

One way of dealing with the silica problem is to precipitate the silica from the waste brine into an open accessible drain or pond which can be mechanically cleaned. However, the resulting waste brine still presents a problem in that its discharge into surface waterways, such as rivers, may not be environmentally acceptable because of the presence of environmentally sensitive chemicals such as arsenic, mercury and boron in the brine. Current environmental philosophy often requires that the waste geothermal brine be reinjected back into sub-surface aquifers in the ground.

However, the disposal of waste geothermal brine by reinjection into the ground presents its own problems. In particular, the development and full utilisation of energy from liquid dominated geothermal fields will always be constrained by the need to dispose of waste brine oversaturated with silica. The major concern is silica fouling in plant, in transmission pipelines, and in injection wells and ground formations. The easiest method for avoiding silica deposition is to maintain the silica concentration below the amorphous silica solubility. However, that means maintaining the temperature of the waste brine above the temperature at which the brine would be saturated with monomeric silica and this can be very wasteful ofthe available heat energy. For example, at Wairakei, New Zealand, reinjection would have to be at a temperature of above 140°C, and at some of the high temperature New Zealand geothermal resources reinjection would have to be at temperatures around 200°C or higher.

If reinjection could take place at lower temperatures then more heat energy would be available for utilisation. Reinjection at lower temperatures, for example, ambient temperatures and at some of the higher temperatures can in the absence of treatment ofthe brine suffer from problems caused by silica scaling. Aggregated silica particles in the brine can and do cause plugging and blockage of underground formations. A sufficient fouling of injection wells means that they have to be replaced by new injection wells, that adding to the cost of disposing of the waste brine. The life of injection wells may be extended by back flushing or acid treatment of them to clear some of the blockages in the surrounding subterranean structure but that also adds costs and only delays the need for new injection wells.

Chemical treatment of waste geothermal brine can reduce silica scaling problems but introduces other kinds of problems. For example, a sufficient acidification ofthe brine can reduce the silica scaling problem but the greater acidity of the brine causes greater corrosion problems in plant and pipelines unless more expensive corrosion- resistant materials are used. Furthermore, acidic brines reinjected into the ground could facilitate the formation of clays which can be just as effective in blocking the underground aquifers. The cost of the chemicals used to treat the brine is another unwanted expense.

Thus, many ofthe processes leading to increased use of the heat energy contained in the waste geothermal brine require, as a pre-requisite, a solution of the silica

problem. A process whereby a saleable silica product was extracted would therefore have double benefits for geothermal development.

There are a number of different silica products that potentially can be extracted from geothermal silica. The most versatile silica products are almost exclusively those produced from colloidal suspensions. These give a product with particle sizes in the range of 4 nm to 50 nm. The current technology relies on dissolving pure silica in caustic soda to form a soluble sodium silicate (water glass) solution. This is then treated with acid under carefully controlled conditions to form a colloidal suspension of silica. By careful regulation ofthe reaction parameters, a very uniform product is obtainable. pH control is used to achieve a particular particle size. The main advantage of this method over the use of geothermal silica is that the initial silica solutions are much more concentrated (4 to 30% by weight compared with geothermal brines of about 0.04-0.15% by weight, for example). The main advantage of geothermal silica is that it is readily available at geothermal fields, presently as a waste product, and that moderately costly chemicals such as acid and alkali may not be required, or if required then be required in relatively small quantities.

For many uses of colloidal silica it is generally desirable and often necessary that the colloidal silica particles be of a specific size or of a specific range of sizes.

Colloidal silica having a specific range of particle sizes can be produced by mixing a plurality of different colloidal silica products, each having a different specific particle size. In any case, it is desirable to be able to obtain colloidal silica particles having substantially the one predetermined particle size. These products are monodisperse sols of high purity and high surface area.

DISCLOSURE OF THE INVENTION

It is therefore an object ofthe present invention to provide a method of manufacture of and apparatus for the recovery of monodisperse silica sols from a geothermal brine. By monodisperse silica sol is meant a concentrated, substantially unaggregated colloidal silica, the particles of which are of substantially the same size, and preferably of high purity. To achieve this object, the major problems to be overcome are:

1. Measuring colloidal silica particle sizes in situ;

2. Controlling the polymerisation process to produce a monodisperse silica colloid ofthe required particle size, and stopping aggregation of the colloidal silica particles;

3. Minimising adsoφtion of impurities; and

4. Concentrating the monodisperse colloidal silica particles in the brine (which is initially very dilute in silica), without changing the properties of the particles, to provide the monodisperse silica sol.

In one aspect, the present invention broadly consists in a method of manufacture and recovery of a monodisperse silica sol from a geothermal brine which has a total silica concentration such that the brine is oversaturated with respect to amoφhous silica solubility at lower temperatures, the method comprising the steps of: a primary cooling step wherein the brine undergoes a primary cooling from a temperature above its saturation temperature to a selected temperature below its saturation temperature at a rate of cooling such that no significant polymerisation of the silica takes place during this primary cooling step; a primary ageing step wherein the brine is aged at about the selected temperature to initiate nucleation ofthe silica and to fix the number density ofthe silica particles formed by the nucleation and thereby determine the final particle size of the monodisperse silica sol to be produced; a secondary ageing step wherein the brine is aged and optionally undergoes a secondary cooling so that substantially no further nucleation of the silica occurs but growth ofthe previously formed silica particles does occur so as to form a colloidal suspension of monodisperse silica particles in the brine; an optional primary stabilising step wherein at least one stabilising agent is optionally added to the brine to inhibit aggregation of the monodisperse colloidal silica particles; a concentrating step wherein the monodisperse colloidal silica particles in the brine are concentrated by ultrafiltration ofthe brine to provide a concentrated monodisperse silica sol; an optional diafiltration step wherein the concentrated sol is optionally washed to lower the concentration of ions in solution; and an optional secondary stabilising step wherein an alkali is optionally added to the sol to further stabilise the sol at a desired pH.

For geothermal brines the amoφhous silica saturation temperature will usually be greater than 100°C.

According to the preferred method, the selected temperature to which the brine is cooled by the primary cooling step and the subsequent ageing and optional secondary cooling of the brine are chosen to produce monodisperse colloidal silica particles having a substantially uniform particle size in the range from about 10 nm to about 100 nm in diameter, more preferably in the range from about 10 nm to about 100 mm in diameter.

For Wairakei geothermal brine, the selected temperature to which the brine is cooled by the primary cooling step is preferably a temperature at which the brine has a monomeric silica saturation index of greater than about 1.2. This is because for Wairakei brine a saturation index of about 1.2 must be reached before nucleation will occur. Again for Wairakei brine, this is usually achieved if the selected temperature is less than about 80 ^C C. However, it should be realised that the relevant saturation index and the associated selected temperature may differ for brines having different compositions and pH. Therefore the relevant saturation index may differ from field to field, but in any case can be derived by experiment.

At the end ofthe primary ageing step, the silica saturation index ofthe brine will normally be less than that required for further nucleation to occur and throughout the secondary ageing step the silica saturation index is maintained at such a lesser value to avoid further nucleation of the brine. Usually the brine will undergo secondary cooling during at least part of the secondary ageing step. In this case, the objective is to lower the temperature so that the experimentally derived solubility index at which further nucleation will occur is not exceeded. Growth ofthe nucleated silica particles continues during the secondary ageing step until polymerisation of the silica in the brine has substantially ceased.

The brine is preferably cooled to a temperature of less than about 40 °C by the end ofthe secondary ageing step, whether by primary cooling alone or by a combination of primary cooling and secondary cooling.

The method preferably includes the primary stabilising step and this step preferably follows the secondary ageing step and precedes the concentrating step. The primary stabilising step becomes increasingly important as the particle size of the

monodisperse colloidal silica particles in the brine decreases. The preferred stabilising agent comprises an organic stabilising agent.

A desirable object of the concentrating step is to produce a concentrated monodisperse silica sol comprising at least about 30% by weight of monodisperse silica particles. This may be achieved by passing the brine through one or more ultrafiltration stages. At the same time, the diafiltration step may be performed, there preferably being at least one diafiltration stage intermediate the beginning and the end of the ultrafiltration stage(s).

The preferred monodisperse silica sol has at least one, and preferably all, of the following properties: a sodium content of less than about 150 ppm, a chloride content of less than about 100 ppm, an arsenic content of less than about 2 ppm, and a SiO ₂ /Na ₂ O weight ratio from about 50 to about 550.

In a second aspect, the present invention broadly consists in a concentrated monodisperse silica sol made by the method defined above.

In a third aspect, the present invention broadly consists in an apparatus for manufacture and recovery of a monodisperse silica sol from a geothermal brine which has a total silica concentration such that the brine is oversaturated with respect to amoφhous silica solubility at lower temperatures, the apparatus comprising: primary cooling means whereby the brine undergoes a primary cooling from a temperature above its saturation temperature to a selected temperature below its saturation temperature at a rate of cooling such that no significant polymerisation of the silica takes place during this primary cooling of the brine; primary ageing means whereby the brine is aged at about the selected temperature to initiate nucleation ofthe silica and to fix the number density of the silica particles formed by the nucleation and thereby determine the final particle size of the monodisperse silica sol to be produced; secondary ageing means with optional secondary cooling means whereby the brine is aged and optionally undergoes a secondary cooling so that substantially no further nucleation of the silica occurs but growth of the previously formed silica particles does occur so as to form a colloidal suspension of monodisperse silica particles in the brine;

optional primary stabilising means whereby at least one stabilising agent is optionally added to the brine to inhibit aggregation of the monodisperse colloidal silica particles; concentrating means whereby the monodisperse colloidal silica particles in the brine are concentrated by ultrafiltration of the brine to provide a concentrated monodisperse silica sol; optional diafiltration means whereby the concentrated sol is optionally washed to lower the concentration of ions in solution; and optional secondary stabilising means whereby an alkali is optionally added to the sol to further stabilise the sol at a desired pH.

The respective means of the preferred apparatus are interconnected to provide for a continuous processing of the geothermal brine.

The preferred apparatus further comprises respective temperature control means associated with each ofthe primary cooling means, the primary ageing means, and the secondary ageing means with optional secondary cooling means, so as to provide control respectively over the primary rate of cooling of the brine, the selected temperature to which the brine is cooled by the primary cooling and at which the brine undergoes the primary ageing, and the temperature of the brine during the secondary ageing and optional cooling of the brine.

The preferred primary cooling means comprises a liquid-cooled heat exchanger in which the primary cooling of the brine takes place at a rate determined by the dimensions ofthe heat exchanger, the flow rate and the initial temperature of the brine entering the heat exchanger, and the flow rate and temperature of the cooling liquid entering the heat exchanger, and for a heat exchanger of chosen dimensions the temperature control means associated with the primary cooling means controls the rate of primary cooling of the brine by controlling at least one of said flow rates and said temperatures.

The preferred primary ageing means comprises a primary ageing tank through which the brine flows and in which nucleation of the silica in the brine takes place, the temperature control means associated with the primary ageing means maintaining the temperature of the brine in the primary ageing tank at about the selected temperature.

The preferred secondary ageing means comprises at least one secondary ageing tank in which the nucleated particles of silica grow to form a colloidal suspension of monodisperse silica particles in the brine, the temperature control means associated with the secondary ageing means being usable either to maintain the temperature of the brine in the secondary ageing tank at about the selected temperature or to effect secondary cooling of the brine. More preferably, the secondary ageing means comprises a plurality of secondary ageing tanks connected in series through which the brine flows, the temperature control means associated with the secondary ageing means comprising a separate temperature control means associated with each of the secondary ageing tanks.

The temperature control means associated with each of the primary and secondary ageing tanks preferably comprises a liquid-cooled heat exchanger, and preferably also comprises stirring means to stir the brine in each tank.

The apparatus preferably includes the primary stabilising means in order to add at least one stabilising agent, preferably an organic stabilising agent, to the brine between the secondary ageing means and the concentrating means.

The concentrating means preferably comprises a plurality of ultrafiltration stages and the diafiltration means, when present, preferably acts to wash the concentrated sol intermediate the beginning and the end ofthe ultrafiltration stages. The apparatus also preferably includes the secondary stabilising means whereby an alkali is added to the sol during the diafiltration of the sol.

BRIEF DESCRIPTION OF THE DRAWINGS

The above broadly defines the present invention, a preferred embodiment of which will be described with reference to the accompanying drawings in which:

Figure 1 is a flow diagram showing the steps of a preferred method for manufacturing and recovering stable concentrated monodisperse silica sols from colloidal silica particles formed within a geothermal brine as a result of a reduction in temperature;

Figure 2 is a graph showing by way of example the particle size of the monodisperse silica particles in the sol plotted against the selected temperature to which the brine was cooled by the primary cooling step and at which nucleation of the silica particles occurred during the primary ageing step;

Figure 3 is a graph showing the narrow particle size distribution characteristic of monodisperse colloidal silica particles for a brine cooled by the primary cooling step to a selected temperature of 20°C;

Figure 4 is a graph similar to that of Figure 3 for a brine cooled by the primary cooling step to a selected temperature of 40 °C;

Figure 5 is a schematic diagram of a pilot plant suitable for carrying out the steps of the method shown by the flow diagram of Figure 1; and

Figure 6 is a graph showing by way of example the change in monomeric silica concentration in the brine from the primary cooling step through to the end of the secondary ageing step.

MODES FOR CARRYING OUT THE INVENTION

As already indicated in the introduction to this specification, the cooling of geothermal brines to a temperature below the saturation temperature of amoφhous silica induces nucleation and polymerisation of monomeric silica dissolved in the brine. Nucleation may be either a homogeneous or heterogeneous process though the end point is the same in both cases, that is, colloidal silica particles. These particles carry a negative surface charge and may adsorb small quantities of other elements or ions such as sodium, calcium, potassium, arsenic and chloride. The resulting colloidal silica particles are of a size and number which is dependent upon brine chemistry, the degree of silica saturation and the cooling rate.

At Wairakei, New Zealand, the monomeric silica content of fresh geothermal brine is approximately 550-600 ppm. After temperature reduction to ambient temperatures and aging for up to 24 hours, the monomeric concentration reduces to approximately

140 ppm; the balance being recoverable colloidal silica. A number of other elements are also found in the brine, some of which affect the silica recovery process.

A typical analysis of Wairakei geothermal brine is as follows:

Silica 550-600 ppm (parts per million)

Sodium 1100 ppm

Potassium 140 ppm

Calcium 30 ppm Magnesium 0.1 ppm

Rubidium 2 ppm

Caesium 2 ppm

Arsenic 4.5 ppm

Chloride 1900 ppm

Bicarbonate <5 ppm

Boron 25 ppm pH 8.4

As has been indicated previously, colloidal silica forms as a natural consequence of temperature reduction from silica found naturally in solution in geothermal brines. However, as also indicated previously, the colloidal silica may aggregate, that leading to the formation of colloidal silica particles of a range of sizes which particles tend to trap impurities. The present invention involves controlling the process conditions to allow the formation of monodisperse colloidal silica in the brine and then preventing its aggregation. The monodisperse colloidal silica particles are then concentrated by ultrafiltration.

In order to form a substantially stable suspension of monodisperse colloidal silica particles in geothermal brine it has been found that the two most important factors are controlling the initial or primary cooling rate ofthe brine so that there is no significant polymerisation ofthe silica during this initial cooling step and selecting a temperature to which the brine is cooled by the initial cooling step at which selected temperature the brine undergoes a primary ageing whereby polymerisation ofthe silica occurs, that then largely deterrnining the number of nucleation sites that form in the brine and hence the ultimate size of the monodisperse silica particles after a secondary ageing ofthe brine at the selected temperature or at other temperature(s) below the saturation temperature, and usually below the selected temperature. The conditions under which the secondary ageing occurs are such that substantially no further nucleation of the silica occurs but growth of the previously formed silica particles does occur so as to form a colloidal suspension of monodisperse silica particles in the brine. Thus, for a particular geothermal brine, cooling to different selected temperatures at which polymerisation and nucleation of the silica is allowed to occur can be used to obtain monodisperse colloidal silica particles of different sizes. For any particular geothermal brine, experimentation with different selected temperatures allows the monodisperse colloidal silica particle size of the sol to be determined for each particular selected temperature. The results of such experimentation may be plotted on a graph; see Figure 2, for example. Once that is known, use of the appropriate selected temperature can be used to obtain monodisperse colloidal silica particles of a predetermined size, these then being concentrated by ultrafiltration. Of course, if

there should be changes made to the process conditions, for example, in the pH of the brine, then the selected temperatures to achieve particular monodisperse particle sizes may alter and need to be re-determined by further experimentation. This will usually be necessary for geothermal brines from different geothermal fields as they will usually have differing chemical compositions and/or pH.

In the flow diagram shown in Figure 1, designed particularly for use at the Wairakei geothermal field, the geothermal brine has a silica concentration of about 550 ppm. The brine is fed at a temperature of about 130°C, which is above its saturation temperature, to a primary cooling step where the brine undergoes a primary cooling to a selected temperature below its saturation temperature at a rate of cooling such that no significant polymerisation of the silica takes place during this primary cooling step. It is the cooling rate which determines whether nucleation will occur. It seems to be the case that a certain monomeric silica saturation index at any particular temperature must be reached before nucleation will occur. For Wairakei brine that figure is about 1.2 but for other brines, having different compositions and pH it may be different. In any case the relevant saturation index can be derived experimentally. The selected temperature to which the brine is cooled by the primary cooling step is a temperature at which the brine has a silica saturation index of greater than the relevant saturation index at which nucleation will occur. Wairakei brine may be initially cooled to a selected temperature of about 80°C, for example, or other temperature, dependent on the particle size desired for the monodisperse silica particles to be formed.

Following the primary cooling step there is a primary ageing step where the brine is aged at about the selected temperature to initiate nucleation ofthe silica and to fix the number density of the silica particles formed by the nucleation and thereby determine the final particle size of the monodisperse silica sol to be produced. Comparing the results of a primary ageing at a lower selected temperature compared with that at a higher selected temperature, the monodisperse silica sol which is produced has a greater number of monodisperse silica particles in the sol but these are of a smaller size. Reference is again made to Figure 2 which shows the silica sol particle diameter obtained versus the nucleation or selected temperature used. The primary ageing step is relatively short and effectively ends when further nucleation of the silica in the brine substantially ceases.

After the primary cooling step to the selected temperature where polymerisation and nucleation of the silica in the brine is initiated and after the relatively short primary ageing step where the nucleation of silica particles in the brine is substantially completed, the brine undergoes a secondary ageing step. The brine is aged at a temperature or at temperatures below its saturation temperature and for a time such that the nucleated silica particles grow to form a colloidal suspension of monodisperse silica particles in the brine. The brine is preferably aged at a temperature or at temperatures at or below the selected temperature to which the brine is initially cooled. It is preferred that the brine is given a secondary cooling during at least part of the secondary ageing step and at a rate of cooling less than that of the primary cooling step. The secondary cooling rate must be a sufficiently slow rate such that it does not cause further nucleation but allows growth of the previously formed silica particles. With the substantial completion of the nucleation at the end of the primary ageing step, the brine typically has a monomeric silica saturation index which has dropped to about or lower than the relevant experimentally derived index at which nucleation will occur. To prevent further nucleation occurring during the secondary ageing step, the rate of secondary cooling is such that the saturation index is maintained at a value of less than that relevant index. The rate of secondary cooling affects the growth rate of the existing silica particles and therefore this and the secondary ageing time that is allowed are also factors affecting the resulting particle size of the monodisperse silica. According to the preferred process, the brine is cooled by the secondary cooling to a temperature of less than about 40°C and the ageing is continued until growth of the silica particles has substantially ceased. At some stage during or at the end of the ageing process the brine may be cooled to ambient temperature.

The primary cooling rate, the selected temperature to which the brine is initially cooled and the ageing regime of the brine are chosen to produce monodisperse colloidal silica particles having a substantially uniform particle size in the range of from about 5 nm to about 150 nm in diameter, more preferably in the range from about 10 nm to about 100 nm in diameter, and still more preferably in the range from about 10 nm to about 60 nm in diameter.

The cooled and aged geothermal brine with its load of monodisperse colloidal silica particles is fed to an ultrafiltration step which acts to concentrate the monodisperse colloidal silica particles in the brine. To produce a desired approximately 30 percent by weight concentrated silica sol from Wairakei geothermal brine, a concentration

factor of about 800 times is required. This may be achieved through the use of hollow fibre ultrafiltration with automatic control. An appropriate number of ultrafiltration stages is provided according to the process conditions and, in particular, the desired concentration of the end product. In Figure 1, three ultrafiltration stages are shown.

Aggregation of the colloidal silica particles formed in the brine is to be avoided, firstly to maintain the monodisperse nature of the particles, and secondly to reduce entrapment of impurities. The preferred method therefore includes at least one, and preferably two, stabilising steps, these being referred to as the primary and secondary stabilising steps respectively. The preferred primary stabilising step comprises adding one or more preferably organic dispersants to the brine. One suitable dispersant is produced by Rohm and Haas and sold under the trade name ACUMER 5000. In trials on Wairakei brine, 5 ppm of that dispersant was found to be quite effective. The dispersant is preferably added to the brine after the secondary ageing step so as not to inhibit growth ofthe monodisperse silica particles during that step. The dispersant is also preferably added to the brine before the ultrafiltration step. This is because the increasingly closer contact of the monodisperse silica particles as they are concentrated during ultrafiltration will tend to force them to aggregate. In Figure 1, the addition of the organic dispersant in the primary stabilising step is shown as occurring between the secondary ageing step and the first stage of the ultrafiltration step.

To improve the purity of the concentrated monodisperse silica sol end product, where this is required, the preferred method includes a diafiltration step wherein the concentrated sol is washed to lower the concentration of ions in solution. Where the ultrafiltration step comprises a plurality of ultrafiltration stages, the diafiltration step, which may comprise one or more diafiltration stages, takes place between the beginning and the end ofthe ultrafiltration stages. In Figure 1, the diafiltration water is shown as being added between the second and third stages of the ultrafiltration step though the diafiltration actually takes place in the third ultrafiltration stage.

Diafiltration ofthe concentrated brine is preferably performed until the concentrated monodisperse silica sol end product has a sodium content of less than about 150 ppm, a chloride content of less than about 100 ppm, and an arsenic content of less than about 2 ppm. Furthermore, the preferred end product has a SiO ₂ /Na ₂ θ weight ratio of between about 50 and about 550. In cases where the concentration of contaminant ions in the sol is not important the diafiltration step may be omitted.

The secondary stabilising step is preferably provided because as the smaller sizes of monodisperse silica particles become increasingly concentrated during the concentrating step, there is an increasing tendency for aggregation to occur. The secondary stabilising step comprises adding an alkali to the concentrated brine, preferably by way of the diafiltration water, as shown in Figure 1. The preferred alkali is sodium hydroxide. The pH of the brine is preferably adjusted to between about 8.5 and about 11, depending on the product required. In experiments on Wairakei brine, pH adjustment to 9.5 with sodium hydroxide produced a stable monodisperse silica sol.

The importance ofthe first and second stabilising steps increases as the particle size of the monodisperse silica sol end product decreases. At a particle size of about 50 nm for example, stable sols have been produced without either ofthe stabilising steps being necessary. However, for a particle size of about 10 nm, both stabilising steps have been found to be necessary.

To check particle sizes, sizing was carried out using a Leeds and Northrup Microrrac UPA particle size analyser. This analyser allows the sizes of particles suspended in the brine to be determined.

The result of the ultrafiltration and diafiltration is a concentrated (preferably concentrated to at least about 30 percent by weight) monodisperse silica sol, the particle size of the colloidal silica particles in the sol depending on the selected process conditions and, in particular, on the selected temperature at which nucleation of the silica in the brine occurred. As already mentioned, for any particular geothermal brine the process conditions required to achieve a particular particle size can be determined by appropriate experimentation. Figures 3 and 4 are graphical analyses of monodisperse silica sols obtained from the particle size analyser. Figure 3 shows the colloidal silica particle size distribution in a brine wherein the initial cooling was to a temperature of 20°C producing a silica particle mean size of about

8 nm. Figure 4 shows the particle size distribution in a brine wherein the initial cooling was to a temperature of 40°C, that producing a monodisperse silica particle mean size of about 21 nm. In each case the particle size distribution is relatively narrow, characteristic of monodisperse colloidal silica.

The apparatus shown in Figure 5 represents a pilot plant designed particularly for use at the Wairakei geothermal field with provision made for the production of

monodisperse silica sols by either continuous or batch processes. The primary flow path for the continuous process is shown in bold.

In Figure 5, geothermal brine having a temperature of about 130°C, a silica concentration of about 550 ppm, and at a flow rate of about 3300 litres per hour is fed to the primary cooling means 10 which is a liquid-cooled heat exchanger. The preferred heat exchanger is a water-cooled plate heat exchanger. The heat exchanger effects primary cooling ofthe brine at a rate determined by the dimensions of the heat exchanger, the flow rate and the temperature of the brine entering the heat exchanger and the flow rate and temperature of the cooling liquid entering the heat exchange.

The heat exchanger has temperature control means associated with it which controls the rate of primary cooling by controlling at least one of the said flow rates and temperatures.

The brine at the selected nucleation temperature is fed to a primary ageing tank 1 through which the brine flows and in which nucleation of the silica in the brine takes place. In the pilot plant, the primary ageing tank has a capacity of 2000 litres and the average time the brine spends in the tank is determined by both the capacity of the tank and the flow rate of the brine through it. The tank has associated temperature control means to maintain the temperature ofthe brine in the tank at about the selected temperature.

From the primary ageing tank, the brine is fed through at least one secondary ageing tank. In the pilot plant of Figure 5, the brine is fed through a series of secondary ageing tanks 2, 3, 4 and 5, each having a capacity of 2000 litres. The secondary ageing means in this pilot plant also includes a 1000 litre capacity cushion tank 6, and two 34,000 litre ageing vessels 7 and 8 which complete a loop based on the fifth ageing tank 5 (i.e. the fourth of the secondary ageing tanks). These ageing vessels provide additional ageing time if necessary. However, they can be bypassed.

As for the primary ageing tank 1, each of the secondary ageing tanks 2-5 has a respective temperature control means associated with it so as to be able to control the temperature, and optional cooling, of the brine throughout at least part of the secondary ageing step. The temperature control means for each of the primary and secondary ageing tanks comprises a water-cooled heat exchanger 9. Each tank is also preferably fitted with a stirrer 11 whereby the brine entering each tank is stirred to ensure that uniform temperatures are maintained throughout the tank and that no dead

spots develop where unwanted nucleation and growth may take place. The secondary cooling rate of the secondary ageing step is controlled by controlling the flow of cooling water delivered to the heat exchanger on each secondary ageing tank by way of a temperature controlled flow valve 12. The cooling regime over the secondary ageing period is controlled to provide cooling at a rate which avoids further nucleation but allows growth of the already formed silica particles in the brine.

Prior to the brine entering the first stage of ultrafiltration, an organic dispersant from tank 13 is added at 14, thereby preventing unwanted aggregation as the silica sol particles are concentrated up from a concentration of approximately 0.04% to about

30% by weight. As already mentioned, approximately 5 ppm of ACUMER 5000 has been found to be effective for Wairakei brines.

Following the ageing steps, the brine is fed by way of a flow controlled feed pump 15 and a 100 micron bag filter 16 to the ultrafiltration means. In the pilot plant, this comprises three stages of ultrafiltration 17, 18 and 19 whereby the concentration of the silica is increased to about 30% by weight. Circulation of the brine through each stage of the ultrafiltration is maintained by centrifugal pumps 20,21 and 22.

Diafiltration with preferably pH adjusted potable water is achieved from the diafiltration tank 23 from where water is pumped by pump 24 into the third stage ultrafiltration flow via valve 25. As already indicated, the pH adjustment is preferably achieved by adding an alkali such as sodium hydroxide to the diafiltration water.

The final 30% by weight monodisperse silica sol is pumped by pump 26 from ultrafiltration stage 3 at a rate which will maintain a 30% by weight sol in that stage.

The pilot plant of Figure 5 shows an ammonium hydroxide tank 27 from which ammonium hydroxide may be fed to the brine prior to the primary ageing tank. This may be used for more acidic brines than normal, but in the usual case is not necessary.

An undesirable increase in pH at this stage increases the silica solubility and may undesirably inhibit growth of the silica particles and therefore reduce recovery.

The drawing also shows an anti-bacteria tank 28 from which a bactericide may be added to the brine prior to the primary ageing tank. However, use of this is not necessary where the incoming geothermal brine is sterile, as is usually the case, and where the plant is kept clean.

The graph of Figure 2 which shows silica sol particle size versus nucleation temperature is derived from four runs of geothermal brine through the pilot plant. Each of these runs included a primary stabilisation step involving the addition of 5 ppm of ACUMER, but no secondary stabilising step involving pH adjustment. The temperature regimes were as follows, the tank numbers referring to the primary and the secondary ageing tanks:

1. 21 °C - no cooling due to low nucleation temperature.

2. 46 °C - Tank 1 46 (nucleation tank)

Tank 2 44

Tank 3 43

Tank 4 42

Tank 5 24.

3. 56°C - Tank 1 56

Tank 2 56

Tank 3 54

Tank 4 53 Tank 5 36.

4. 70°C - Tank 1 70

Tank 2 66

Tank 3 63 Tank 4 61

Tank 5 36.

The graph of Figure 6 shows the change in monomeric silica concentration in the brine throughout the process from the heat exchanger 10 where primary cooling takes place through to the end of the secondary ageing step. As can be seen, at the end of the secondary ageing step, the silica monomer concentration has substantially stabilised, that indicating that no further growth of the particles is occurring.

A specific example obtained from the pilot plant is as follows:

Nucleation temperature = 40 °C No secondary cooling (just ageing)

ACUMER 5 ppm

Alkali stabilisation to pH 9.3 (alkali added to diafiltration water)

Particle size = 20 nm

Specific gravity - 1.21

Si0 ₂ /Na ₂ O ratio = 91

Viscosity = 2.4 cp@30°C

29% silica sol.