The invention relates to a new method for converting metallic silicate minerals to silicon compounds and metal compounds via a conversion reaction.

The largest proportion of minerals by far in the earth's crust (about 90%) is a silicate mineral. Each silicate mineral is built up of a lattice comprising silicate groups (Si04) and metals. Positively charged metal atoms are present in each lattice opposite the negative charge of each silicate group (4-) . The type of lattice in which the silicate groups are arranged is subdivided into orthosilicates (isolated silicate groups), nesosilicates (chain of silicates) , phyllosilicates (silicates linked in sheets) and tectosilicates { 3-dimensional structure of silicates) . During the conversion of silicate minerals the lattice is broken, wherein separate silicon compounds and metal compounds are formed. Silicate minerals form as such an interesting source for forming of these silicon

compounds: silica or orthosilicic acid in particular are raw materials much used in industry. In addition, various metal compounds which have economic importance as for instance intermediate product for metal extraction, can be separated in the form of a salt.

Serpentine (a phyllosilicate) and olivine (a

nesosilicate) are silicates which comprise mainly magnesium as metal (and a lower content of iron) . A conversion process for these magnesium silicates is known in which the mineral reacts with carbon dioxide in an autoclave under increased temperature and pressure to form magnesium (bi) carbonate and silicic acid as end product.

Shown in simplified form the main reactions are:

(I) 2 Mg2Si04 + C02 + 2H20 => Mg3Si205 (OH) 4 + MgC03 (ii) Mg2Si04 + 2C02 + 2H20 => 2MgC03 + H4Si04

(iii) Mg2Si04 + 4C02 + 4H20 => 2Mg(HC03)2 + H4Si04 It is otherwise noted that the above reactions are shown in simplified form: the minerals serpentine and olivine not only comprise magnesium as metal but often also comprise, in a lower content, iron, manganese and nickel, and sometimes also titanium, calcium and aluminium.

The three main reactions above are exothermic, and this increasingly so. Particularly reaction ii, and to even greater extent reaction iii, will predominate here. A complete conversion of silicate mineral is obtained via these two reactions. In order to achieve sufficient

conversion speed for a complete conversion, both heat and an increased pressure are initially desirable when the reaction is performed in an autoclave. The energy consumption for performing the reaction is therefore considerable.

The invention has for its object to provide a method wherein the energy consumption is reduced and to achieve additional advantages.

According to a main aspect, the invention provides for this purpose a method for converting metallic silicate minerals to silicon compounds and metal compounds via a conversion reaction,

characterized in that the conversion reaction is performed in a gravity pressure vessel (GPV), wherein:

the GPV comprises two channels having separate entries on the upper side of the GPV and which channels are mutually connected on the underside of the GPV,

and wherein the method comprises the steps of:

- providing a dispersion of solid particles of silicate minerals in water;

carrying the provided dispersion through a first channel of the GPV in a descending direction so that a descending dispersion flow is obtained in the first channel;

- reacting the solid particles of silicate minerals in the dispersion with one or more reactants by adding the reactants to the descending dispersion flow;

- and carrying the silicon compounds and metal compounds formed during the conversion reaction away through a second channel of the GPV in an ascending flow. Surprisingly, it has been found that an efficient and energy-saving conversion reaction, with other additional advantages, is achieved by applying a gravity pressure vessel (GPV) and reacting the descending flow of silicate minerals with added reactants in the GPV.

A GPV is per se known from the American patient application US 2006/0086673 as a vertical, elongate, cylindrical vessel placed in the ground and having a length of several hundred metres. This GPV is constructed from a central inner channel and an outer channel which encloses the inner channel. Inner channel and outer channel are mutually connected on the underside of the GPV. Owing to this construction a flow introduced into the inner channel is carried to a lowest point inside the GPV and then carried upward through the outer channel to then leave the GPV.

Depending on the depth, an increased pressure is created at the lowest point by the weight of the column of material with which the channels are filled.

When the descending flow in the inner channel consists of a slurry of dispersed silicate minerals in water with a density of about 2 kg/litre a pressure increase of up to 20 bar can be achieved per 100 metre length of a GPV. From the lowest point inside the GPV the flow reverses and moves upward through the outer channel, leaving the GPV at the surface. The entry flow is here usually carried with some overpressure into the GPV. It is noted for the sake of clarity that in the GPV according to the invention the first channel through which the descending flow is carried is preferably embodied as inner channel. Vice versa, the second channel is preferably embodied as outer channel.

It is further noted that the whole GPV preferably has an overall length of between 500 and 800 metres, wherein the first channel and second channel have corresponding lengths.

In addition to an intrinsically increased pressure, the GPV provides a very efficient heat use. Depending on the reaction to be performed in the GPV and the design of the two channels, the conditions can be chosen such that reaction products are formed substantially in the lowest part of the descending flow and the further part of the ascending flow so that more heat is created overall in the ascending flow. This makes the GPV eminently suitable for heat exchange.

The heat from the ascending flow can usefully be employed to heat the descending dispersion flow before it reaches the lowest point. When the first channel is embodied as inner channel, the outer channel can relinquish the formed heat to the inner channel via a separating wall which separates the inner channel from the outer channel. The separating wall then functions as heat exchanger.

It has been found in this respect that by applying the

GPV the operational costs for converting olivine (expressed per kilogram of olivine) are halved relative to a conversion reaction in an autoclave. This saving is a direct result of the above described efficient use of heat and the pressure build-up achieved in a GPV.

The reactants are introduced here at some point in the descending flow: this can be at the position of the entry on the upper side of the GPV, but can also be at some depth in the descending flow via a separate conduit. It is a further advantage in the method according to the invention for the solid particles of silicate minerals in dispersion to have an average diameter in the order of magnitude of 2-3 millimetres or smaller. This upper limit for the particle size is relatively high compared to the usual particle size for a reaction in an autoclave. In an autoclave a particle size is generally applied which is at least a factor of 100 smaller, and so lies in the order of magnitude of micrometres.

The dynamics of the silicate particles in the

descending dispersion flow in the GPV have the result that it is possible to suffice with a relatively large particle and that no further reduction in size is necessary. Due to the relatively long path covered by the silicate particles inside the GPV there is a constant erosion of the outer surface of each individual particle. The reactivity of the outer surface is constantly increased due to this constant erosion, which increases the efficiency of the process.

Since the eroding effect continues for a relatively long time, it is possible to bring about complete reaction of relatively large particles in the method according to the invention.

The content of the particles of silicate minerals in the dispersion preferably lies between 3% by weight and 30% by weight of the overall weight of the dispersion. On the one hand the process requires a minimum content in order to be economically attractive, while on the other it is a requirement that no undesired agglomerations of solid material, which can block or impede the dispersion flow, can take place in the dispersion.

In the method according to the invention the formed silicon compounds preferably comprise silica and/or

orthosilicic acid. Both compounds have an important economic value as raw materials for various applications. In the method according to the invention the formed metal compounds further comprise metal bicarbonates and/or metal carbonates. Soluble metal bicarbonates are a good nutrient for oil-producing algae, such as determined seaweeds and algae. The bicarbonates thus form an attractive alternative to the introduction of carbon dioxide gas into the medium in which the algae are cultivated, in addition, metal carbonates have for instance an advantageous function as absorbent in solution for absorbing carbon dioxide, with the formation of bicarbonates.

In the method according to the invention the formed metal compounds more preferably comprise metal sulphides. For the separation of valuable metals it is attractive to obtain for instance nickel and iron in sulphide form. The metal can then be extracted via known conversion reactions.

The silicate compounds and metal compounds formed during the conversion are separated from the slurry in usual ways, such as by filtration, precipitation or by applying a cyclone.

In the method according to the invention the metallic silicate minerals advantageously comprise olivine or serpentine. It has been demonstrated for these minerals that the operational costs for complete conversion thereof to silicate compounds and metal compounds can be halved per kilogram of starting material.

Use is advantageously made in the invention of water which is salt water. The higher concentration of dissolved salts results in a higher ion activity being achieved in salt water, this having a positive effect on the conversion reactions.

In the method according to the invention reactants are more preferably applied which are chosen from a group of acids comprising hydrochloric acid, sulphuric acid, carbon dioxide and bicarbonate. In gas form carbon dioxide can react with silicate minerals in accordance with known reactions, several examples of which have been given in the introduction.

Hydrochloric acid and sulphuric acid are known acids which (usually as liquid) react with silicate minerals, wherein respectively metal chloride and metal sulphate are formed in addition to silica. Bicarbonate in solution reacts with silicate minerals similarly to carbon dioxide.

In addition, the erosion of silicate particles in the descending dispersion flow can be further enhanced when gas bubbles are present in the flow which cause turbulent flows in the main flow. This can be achieved by introducing a reactant in gas form into the descending dispersion flow. If no reactant in gas form is introduced it is possible to consider introducing an additional inert gas so that the enhancing effect of the gas bubbles is nevertheless

achieved. In addition, the heat transfer within the GPV is improved by the presence of gas bubbles .

Because the silicate minerals have to be formed into less small particles in the process according to the invention, the operational costs of the process are further reduced.

In the method according to the invention one or more of the reactants are preferably added in gas form to the descending dispersion flow. This has the effect of creating turbulence in the descending dispersion flow, whereby the erosion of the particles of silicate mineral is enhanced, with the above stated resulting advantages. In addition, the heat transfer inside the GPV is improved by the presence of gas bubbles. The addition of the reactants in gas form can for instance be performed via a conduit provided wholly or partially in the first channel. The gas is brought under a suitable pressure here so that at the position of the injection point a slight overpressure is created relative to the hydrostatic pressure prevailing there in the first channel .

In the method according to the invention it is

particularly recommended to add the reactants at different positions in the first channel of the GPV. It has been found that there is an optimal position for the addition of each reactant to the descending dispersion flow. Assuming a GPV with a length of 500 to 800 metres, the preferred positions for the addition of reactants lie in a range from 50 metres to maximum depth (i.e. 500 to 800 metres). The same

preferred positions apply for the addition of an inert gas.

In respect of the addition of a gas to the descending flow there are some limitations to the speed of addition: the concentration of gas bubbles formed in the liquid flow must remain below a critical limit so that the gas bubbles do not accumulate into a large gas bubble which disrupts the continuity of the liquid flow. This is undesirable from the viewpoint of hydrodynamics, but also from the viewpoint of an optimal reaction surface area between the gas phase and the liquid phase.

The aim in practice is a gas content in the dispersion of between 10 and 50% by volume, preferably 20 to 40% by volume, for instance 30% by volume.

Since the gas phase has an intrinsically greater volume per quantity of reactant than the liquid phase, it is generally desirable, or even essential, to introduce the gaseous reactant distributed over the liquid flow {i.e.

distributed over the length of the first channel) for reaction thereof with the silicate mineral in a suitable mol ratio.

It is further recommended in the method according to the invention that the GPV comprises a heat exchanger, preferably a system of heat exchangers, and that with particular preference the heat exchanger herein comprises a water reservoir which encloses a part of the GPV as a casing and wherein a plurality of supply and discharge conduits for water are provided at different positions in the reservoir. The heat exchanger is for instance provided round the GPV as a casing which is in heat-exchanging contact with the channels of the GPV so that surplus heat - i.e. heat which is not used in the exchange between the channels in the GPV - can be discharged externally. The GPV advantageously comprises a system of heat exchangers which can be deployed actively at different heights of the GPV. The temperature inside the GPV can thus be actively controlled so that an optimum temperature for the conversion reaction can be maintained along the column in the GPV. Maintaining an optimum temperature on the underside of the GPV {at the highest pressure) is for instance very desirable for the purpose of controlling the conversion reaction. An optimum temperature is moreover often desirable for the exit flow on the upper side.

The heat discharged by the heat exchanger (s) can be used in other processes, for instance to produce

electricity.

The heat exchanger can optionally be used in reverse manner to supply heat to the GPV when heating of the slurry in the GPV is desired.

In a particular aspect the invention relates to a conversion reaction in which carbon dioxide or a derivative form thereof is applied as reactant, wherein the silicate mineral functions as sequestering agent for carbon dioxide or a derivative thereof. Sequestration in this context is a general term for storing carbon dioxide in a compound, whereby gaseous carbon dioxide is extracted from the atmosphere or release of formed carbon dioxide into the atmosphere is prevented.

For a preferred embodiment of the method according to the invention it is therefore the case that the reactants comprise carbon dioxide and/or bicarbonate, and the silicate mineral functions as sequestering agent for carbon dioxide and/or bicarbonate.

The reactants are introduced at some point in the descending flow: this can be at the position of the entry on the upper side of the GPV, but can also be via a separate conduit at some depth in the descending flow.

On the basis of the already stated advantages according to the main aspect of the invention, the advantages are thus gained of a considerable reduction in operational costs because an efficient energy consumption is realized by the GPV, both with a view to the heat required and the pressure required for performing the reaction.

In the method according to the invention the metallic silicate minerals advantageously comprise: olivine or serpentine. With these minerals a reduction in operational costs can be achieved of about 50%. These minerals moreover have a high absorbing capacity for sequestration of carbon dioxide: about 1 kilogram of olivine is able to absorb or store 1.2 kilograms of carbon dioxide.

In the method according to the invention the reactants preferably comprise carbon dioxide and the carbon dioxide is added in gas form. This has the advantage of a more

turbulent flow which enhances erosion of the particles of silicate mineral, whereby better reaction kinetics are achieved.

In the method according to the invention the carbon dioxide is preferably obtained from a regeneration of absorbent amines or from flue gas from a bioethanol plant. The advantage hereof is that such carbon dioxide has a high concentration in gas form, which enhances the kinetics of the reaction. A high concentration of 80%, preferably 90% or higher, is generally desirable for good reaction kinetics. In this respect flue gas from an ammonia plant can also be used. According to a subsequent preferred variant of the method according to the invention, the reactants comprise dissolved bicarbonate, this bicarbonate originating from the reaction of gaseous carbon dioxide with calcium carbonate and/or magnesium carbonate. This method provides the advantage that a concentrated flow of reactant can be supplied when use is made of a flue gas with a low

concentration of carbon dioxide (for instance lower than 50%} .

A specific advantage is achieved according to the above described embodiment in which use is made of a heat

exchanger or a system of heat exchangers. When these heat exchangers are coupled to a regenerator for absorbent amines, the surplus energy obtained from the conversion reaction and via the heat exchanger or exchangers can be sufficient to regenerate amines and thus obtain a highly concentrated flow of gaseous carbon dioxide for the purpose of the conversion reaction in the descending dispersion flow.

The following theoretical calculation applies as an example of this assertion:

With throughput in the GPV of 100 mVhour of slurry comprising 25 tons/hour of olivine, about 15 tons of C02 per hour can be absorbed while forming 11.7 MW of heat. Of this quantity of energy 8 MW can be usefully employed outside the GPV. 6.7 MW is the average needed for the process of absorbing 15 tons of C02 from flue gases in amines and subsequently regenerating a concentrated flow of C02 from amines. The GPV process hereby produces sufficient energy to concentrate a diluted flue gas in a separate step for the purpose of the GPV process itself.

Methods known at the moment for sequestration of C02 have in contrast a negative energy efficiency: insufficient energy becomes available here from the sequestration reaction for the purpose of regenerating amines with

absorbed C02.

The method thus provides a method for sequestration of carbon dioxide, wherein the heat produced can be utilized to form a concentrated gas flow of carbon dioxide prior to sequestration.

In the method according to the invention gaseous carbon dioxide is preferably added at different positions in the first channel of the GPV. The same advantages apply here as stated above in respect of the main aspect of the invention.

In the method according to the invention it is further recommended that unreacted silicate minerals present in the ascending flow in the second channel are carried back to the descending dispersion flow in the first channel.

Depending on the embodiment of the GPV and the process conditions applied, it may be that the silicate minerals have not yet reacted completely, whereby the capacity of the mineral has not been wholly utilized. A complete reaction can still be achieved by feeding back unreacted material. For this purpose the unreacted particles have to be

separated in a suitable manner from the ascending flow, for instance by filtration.

In the method according to the invention the solid particles of silicate minerals advantageously have an average diameter in the order of magnitude of 2-3

millimetres or smaller. The same advantages apply here as stated above in respect of the main aspect of the invention.

Water that is salt water is advantageously applied in the invention. The higher concentration of dissolved salts results in a higher ion activity being obtained in salt water, which has a positive effect on the conversion

reactions .

The invention will be further elucidated on the basis of examples and an accompanying drawing, in which: Fig. 1 shows a cross-sectional schematic view of a GPV in which a method according to the invention is performed.

Fig. 2 shows a variant of the GPV of fig. 1, in which the heat exchanger is modified.

Figure 1 shows a GPV 1 with an inner channel 3 and an outer channel 5, separated from each other by the inner wall 7 which encloses inner channel 3. The length of the GPV is in reality much greater than shown in the figure. The flow direction inside the channels is shown with arrows.

A slurry of silicate mineral in water is introduced into inner channel 3 via entry 9. An injection pipe 10 is inserted into inner channel 3 for the purpose of introducing reactant via entry 11, in this case gaseous carbon dioxide. Gas bubbles of carbon dioxide which are formed in the slurry are shown at the bottom end of pipe 10. At the lower outer end of inner channel 3 the slurry flow reverses direction and the flow continues through outer channel 5. At this lowest point the reaction mixture is under the highest pressure and a significant part of the conversion reaction will take place. The ascending flow in outer channel 5 will consist largely of wholly converted silicate mineral, i.e. of separate silicate compounds and metal compounds. The exiting flow 14 is further processed in order to separate and process the produced compounds. If desired, the flow direction inside the GPV can also be the other way round, provided the supply channels and discharge channels also take a reversed form.

The GPV has an insulating outer casing 16 to limit heat loss to the environment, although the insulating property is not always necessary in practice depending on the ground composition. Provided in the outer casing are two heat exchangers consisting of a spiral-shaped water conduit 18 through which water can be pumped from inlet 20 to outlet 22. The heat exchangers are placed at different levels: one at the top and the other at the bottom. Depending on the reactions taking place in the GPV, surplus heat is created at one or both levels. As soon as this is detected (for instance using heat sensors (not shown) ) water is pumped through the heat exchanger and heat is thus extracted from the system. This heat can be used elsewhere in any desired manner as energy source for various processes, such as generating electricity.

The heat flow through the heat exchanger can otherwise also take place in the reverse direction, wherein heat is now carried to the GPV for the purpose of heating the reaction mixture in the GPV.

Figure 2 shows a GPV with similar parts as in figure 1, wherein the corresponding components have the same

numbering.

The heat exchanger in fig. 2 consists of a water reservoir 30 filled with water. A main conduit 32 with a valve 34 for water feed is connected to water reservoir 30. Protruding into water reservoir 30 are three pipes 36 of different lengths and with a valve 38, which opens or closes the pipes and with which it is possible to switch between water discharge or water feed. Diverse combinations can thus be made of water flows between the outer ends of pipes 36. A flow as indicated with arrows is optimal during use of the GPV wherein the reaction of olivine with C02 is in progress and sufficient heat is produced.

Water is introduced here into the reservoir via main conduit 32 and discharged via the shortest pipe 36 for the purpose of cooling discharge flow 14. Water is also

introduced via the medium-length pipe 36, and water is discharged via the longest pipe 36. Reaction heat is thus discharged efficiently from the low section of the GPV.

The heat exchange can be controlled with more variation during use of the GPV due to the arrangement of the heat exchanger according to figure 2. Example 1

A GPV of one of the two types as shown above is applied with the following specifications: The length of the GPV shaft is 600 m, the diameter of inner channel and outer channel respectively 20 cm and 30 cm.

Depending on the capacity and embodiment, the diameter can be in the order of magnitude of several metres.

A slurry of olivine particles of 2-3 mm in salt water is introduced at about 10 bar into the inner channel. The density of the slurry is about 1.9 kg/litre. During the descending transport of the slurry the particles are slowly worn down by erosion.

Carbon dioxide gas (concentration: 90% partial

pressure) is introduced at about 100 bar into the inner channel via an injection pipe with a length of 500 m. A temperature between 170-250 °C is maintained at the bottom of the GPV.

A complete conversion according to reaction ii) of olivine to magnesite (MgC03) takes about 2 hours at a pressure of 150 bar and a temperature of 185°C, and at a particle size of 75 pm.

At a diameter of the inner pipe of 1.5 metres and over a length of the inner pipe of 10 m it is possible to achieve a conversion of 746 tons of carbon dioxide in 2.42 hours.

This quantity of carbon dioxide is comparable to the emission of a.600 MW coal-fired power station.

The carbon dioxide gas is obtained in a concentration of 90% from the regeneration of amines which have absorbed carbon dioxide. The energy obtained via the heat exchangers of the GPV is sufficient to carry out this regeneration, to produce sufficient carbon dioxide gas for the conversion reaction in the GPV.