WO2020070119A1

WO2020070119A1 - Process for the manufacture of precipitated silica

Info

Publication number: WO2020070119A1
Application number: PCT/EP2019/076578
Authority: WO
Inventors: Sylvaine Neveu; Eric Garcin
Original assignee: Rhodia Operations
Priority date: 2018-10-04
Filing date: 2019-10-01
Publication date: 2020-04-09

Abstract

A process for the preparation of precipitated silica comprising the reaction of an alkali metal silicate with hydrochloric acid. The process comprises the conversion of the alkali metal chloride obtained as by-product in the silica precipitation reaction into hydrochloric acid and alkali metal hydroxide that can be reused in the silica precipitation process. The conversion comprises an electrolytic process.

Description

PROCESS FOR THE MANUFACTURE OF PRECIPITATED SILICA

Technical Field

[0001] The present invention relates to an integrated process for the manufacture of precipitated silica using an alkali metal silicate and hydrochloric acid. The integrated process combines a silica precipitation step with a transformation process performed on the water stream containing alkali metal chloride deriving from the precipitation step. Said transformation process comprises at least one electrolysis step and allows to obtain hydrochloric acid and alkali metal hydroxide both of which may be recycled into the process.

Background Art

[0002] Precipitated silica is a synthetic, finely divided, white, amorphous form of silicon dioxide. Precipitated silica is typically obtained via the reaction of an alkaline silicate solution with an acid in water. The resulting silica precipitate is then separated from the liquid phase, typically by filtration, washed and dried. In the most common process, sulfuric acid is used as the acid and sodium silicate as the alkali metal silicate.

[0003] The use of hydrochloric acid as the mineral acid in the manufacture of precipitated silica is known although not widely used at industrial level (see for instance Ullmann’s Encyclopedia of Industrial Chemistry,“Silica”, § 7.2, (2012) Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim). As in other precipitation processes, the process generates large volumes of wastewater containing dissolved alkali metal chlorides. The possibility to recover these alkali metal chlorides from the precipitated silica

wastewaters in an economically viable fashion would represent an advantageous improvement of the overall precipitated silica manufacturing process. Even more advantageous would be to transform said alkali metal chlorides into chemicals which can be reused into the silica manufacturing process, thus generating a virtuous loop in terms of materials and water consumption. Summary of invention

[0004] Objective of the present invention is to provide an integrated process for the preparation of precipitated silica starting from hydrochloric acid allowing for an effective recovery and reuse of the wastewater containing alkali metal chlorides generated in the silica precipitation process.

Description of invention

[0005] A first object of the invention is a process for the manufacture of

precipitated silica comprising:

reacting an alkali metal silicate with hydrochloric acid to obtain a precipitated silica suspension;

submitting said precipitated silica suspension to a solid-liquid separation step and optional washing to obtain a wet precipitated silica and a water stream Wo containing an alkali metal chloride; submitting said water stream Wo to a transformation process to provide an alkali metal hydroxide and hydrochloric acid, said transformation process comprising at least one electrolysis step.

[0006] The hydrochloric acid obtained at the end of the transformation process may conveniently be re-used in the reaction with the alkali metal silicate to prepare precipitated silica.

[0007] The expression“water stream Wo” is used herein to refer to water

containing alkali metal chloride and optionally other contaminants. The alkali metal chloride is the main product in water stream Wo, that is the product which is present at the highest concentration.

[0008] In the remainder of the text the expression“hydrochloric acid” is used in its conventional meaning to refer to a solution of HCI in water.

[0009] The reaction of the at least one alkali metal silicate with hydrochloric acid may be performed according to conditions well known in the art. Non- limiting examples of processes for the preparation of precipitated silica are disclosed for instance in EP520862, EP670813, EP670814, EP917519, WO 95/09127, WO 95/09128, WO 98/54090, WO 03/016215, WO

2009/1 12458 or WO 2012/010712. [0010] The preparation of precipitated silica by reaction of an alkali metal silicate with hydrochloric acid can be carried out according to any method, for instance by addition of hydrochloric acid to a feedstock of alkali metal silicate, or else by simultaneous addition, total or partial, of hydrochloric acid and of alkali metal silicate, to a feedstock of water, or of alkali metal silicate, or of hydrochloric acid. The reaction of the alkali metal silicate with hydrochloric acid takes place in an aqueous medium, typically water.

[0011] In a preferred embodiment of the invention, the hydrochloric acid used for reaction with the alkali metal silicate has been obtained by submitting a water stream containing an alkali metal chloride to a transformation process comprising at least one electrolysis step. Preferably the water stream containing the alkali metal chloride derives from a silica

precipitation process.

[0012] In said embodiment the inventive process comprises the steps of:

submitting a water stream containing an alkali metal chloride to a transformation process comprising at least one electrolysis step to obtain hydrochloric acid;

reacting an alkali metal silicate with said hydrochloric acid to obtain a precipitated silica suspension;

[0013] The alkali metal silicate used in the inventive process is advantageously selected from the group consisting of sodium and potassium silicate. The alkali metal silicate is preferably sodium silicate. Sodium silicate generally exhibits a ratio Si0₂/Na₂0 by weight between 2.0 and 4.0.

[0014] The alkali metal silicate is generally provided as a solution in water. The alkali metal silicate solution typically has a concentration, expressed in terms of the amount of S1O2 by weight, of between 3.0 wt% and 25.0 wt%, for example between 5.0 wt% and 23.0 wt%, in particular between 6.0 wt% and 21.0 wt%.

[0015] Generally only one kind of alkali metal silicate is used in the silica

precipitation reaction although more than one kind may be used.

[0016] The reaction of the alkali metal silicate with hydrochloric acid is usually performed in a reaction vessel equipped with adequate stirring and heating equipment.

[0017] The entire reaction of the alkali metal silicate with hydrochloric acid is generally performed at a temperature of between 40 and 96°C, in particular between 80 and 95°C. The entire reaction may be performed at a constant temperature, or, alternatively, the temperature at the end of the reaction is higher than the temperature at the start of the reaction.

[0018] At the end of the reaction a suspension of precipitated silica in water is obtained.

[0019] The precipitated silica suspension is subsequently subjected to a solid- liquid separation step.

[0020] The separation usually comprises a filtration, followed by washing, if

necessary. The filtration is performed according to any suitable method, for example by means of a belt filter, a rotary filter, for example a vacuum filter or a filter press.

[0021] At the end of the solid-liquid separation step a wet precipitated silica and a water stream containing an alkali metal chloride are obtained.

[0022] The wet precipitated silica may be, and preferably is, submitted to one or more washing operations to remove residual salts. The water recovered from the one or more washing operations may be merged with the water stream recovered from the step of separating the precipitated silica from the precipitated silica suspension. Water stream Wo comprises the water stream recovered from the step of separating the precipitated silica from the precipitated silica suspension and optionally the water recovered from the one or more washing operations. Water stream Wo may consist of the water stream recovered from the step of separating the precipitated silica from the precipitated silica suspension. [0023] In general the wet precipitated silica is subsequently dried. Drying may be performed according to means known in the art. Preferably, drying is performed by atomization. To this end, use may be made of any type of suitable atomizer, in particular a turbine, nozzle, liquid pressure or two- fluid spray-dryer.

[0024] The process for the preparation of precipitated silica typically comprises at least one additional step of drying the wet precipitated silica in a drying apparatus. The drying apparatus typically discharges a flow of hot gas as defined hereafter.

[0025] According to the inventive process, water stream Wo containing an alkali metal chloride is submitted to a transformation process comprising at least one electrolysis step. The transformation process provides an alkali metal hydroxide, generally as a solution in water, and hydrochloric acid.

[0026] The transformation process typically comprises multiple steps which may be performed before and/or after the electrolysis step.

[0027] Electrolytic processes for the conversion of alkali metal chlorides, in

particular sodium and potassium chlorides, into an alkali metal hydroxide, hydrogen and chlorine are known. They are commonly referred to as “chlor-alkali processes”. Notable non-limiting examples of chlor-alkali processes are for instance the mercury, diaphragm or membrane cell electrolysis techniques. An outline of these processes is provided for instance in Ullmann’s Encyclopedia of Industrial Chemistry,

“Electrochemistry, 2. Inorganic Electrochemical Processes”, § 1.2, (2012) Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.

[0028] In a preferred embodiment of the inventive process, the electrolysis step is performed using a membrane cell electrolyser.

[0029] In a membrane cell electrolyser the anode and cathode are separated by an ion-conducting membrane. The solution containing the alkali metal chloride flows through the anode compartment, where chloride ions are oxidised to chlorine gas. The alkali metal ions migrate through the membrane to the cathode compartment, which contains an alkali metal hydroxide solution. Water is electrolysed at the cathode, releasing hydrogen gas and hydroxide ions. The alkali metal and hydroxide ions combine to produce alkali metal hydroxide. The alkali metal hydroxide is continuously removed from the circuit. The depleted alkali metal hydroxide solution, discharged from the anode compartment, may be re-saturated with salt or concentrated.

[0030] The ion-conducting membrane prevents the migration of chloride ions from the anode compartment to the cathode compartment. The ion-conducting membranes used in this process are commonly made of perfluorinated polymers, e.g. PTFE.

[0031] The products of the membrane electrolysis step are hydrogen, chlorine and alkali metal hydroxide, generally in the form of a solution in water.

[0032] The alkali metal hydroxide may be conveniently reused in the silica

precipitation process and/or in the membrane electrolysis of water stream Wo providing an economic and environmental advantage over the processes of the prior art.

[0033] It is known, in fact, that alkali metal silicates, in particular sodium or

potassium silicates, can be obtained by treatment of silicate precursors with strong bases such as sodium hydroxide or potassium hydroxide.

[0034] Notable, non-limiting examples of silicate precursors are sand, preferably natural quartz sand, biogenic silica-containing materials, olivine, as well as any other sources of amorphous silica. Among the biogenic silica- containing materials mention may be made of rice hulls, rice straw, rice husk ashes and the like. Natural quartz sand or biogenic silica-containing materials are preferred silicate precursors.

[0035] Accordingly, the alkali metal hydroxide obtained from the electrolysis

process, which is typically in the form of a solution in water, as such or after suitable adjustment of its concentration, may be used for the production of the alkali metal silicate used in the silica precipitation process.

[0036] In such an embodiment the inventive process thus comprises a step

wherein the alkali metal hydroxide obtained at the end of the

transformation process is reacted with a silicate precursor to obtain an alkali metal silicate. [0037] The other products of the electrolytic process of water stream Wo, in particular of the membrane electrolysis process, are hydrogen and chlorine.

[0038] The transformation process generally comprises a step wherein hydrogen and chlorine are reacted together to provide hydrochloric acid.

Any process known in the art may be used. Advantageously, the synthesis of hydrochloric acid is performed by combustion of hte and CI2 gas in a burner. In a typical set-up, chlorine and hydrogen are fed by separate tubes into a combustion chamber. After ignition, the chlorine burns in the hydrogen with a hot flame, for instance at more than 2000°C, producing gaseous HCI. The HCI gas is then absorbed in demineralised water to produce hydrochloric acid. The concentration of the hydrochloric acid generally does not exceed 33% by weight.

[0039] The hydrochloric acid thus produced may then be reused in the silica

precipitation process, either as such or after a dilution or a concentration step.

[0040] The inventive process thus allows for the recovery and reuse of the by- products generated in the silica precipitation process. The inventive process may thus comprise the steps of:

submitting said precipitated silica suspension to a solid-liquid separation step and optional washing to obtain a wet precipitated silica and a water stream Wo containing an alkali metal chloride; submitting said water stream Wo to a transformation process to provide an alkali metal hydroxide and hydrochloric acid said transformation process comprising at least one electrolysis step;

reacting the hydrochloric acid obtained at the end of the transformation process with an alkali metal silicate to prepare precipitated silica; and/or

reacting the alkali metal hydroxide obtained at the end of the transformation process with a silicate precursor to obtain an alkali metal silicate. [0041] The transformation process may comprise other steps in addition to the electrolysis and HCI generation steps described above.

[0042] Non-limiting examples of said additional steps are for instance pre- treatment and/or concentration steps performed before submitting water stream Wo to the electrolysis step or any dilution or concentration steps performed on the alkali metal hydroxide or hydrochloric acid solutions obtained after the electrolysis step.

[0043] The concentration of the alkali metal chloride as well as the nature and respective amounts of other ions or compounds present in water stream Wo may vary broadly and may depend on different factors, such as the process for the preparation of the precipitated silica itself, which may involve different ratios for the reactants, the presence of specific electrolytes or additives, the nature of the alkali metal silicate or other variations in the process.

[0044] Additional ions which may be present in water stream Wo are typically Ca, Mg, Al, Fe and sulfate ions. Water stream Wo typically also contains silica, in suspended and/or in dissolved form.

[0045] The concentration of alkali metal chloride in water stream Wo may vary and it is not a limiting factor on the ability of the inventive process to effectively recover hydrochloric acid and/or alkali metal hydroxide. Alkali metal chloride concentration in water stream Wo, Co, may be as low as 0.5 wt%, even as low as 1.0 wt%. Concentration Co may be as high as 20.0 wt%, even 15.0 wt%, or typically 10 wt%. Typical values of concentration Co may be between 1.0 to 5.0 wt%.

[0046] Water stream Wo is typically subjected to preliminary treatments before the electrolysis step. Preferred preliminary treatments are those that reduce the amount of suspended and/or dissolved silica in water stream Wo.

Water stream Wo is typically also submitted to treatment to reduce the amount of sulfate, Mg or Ca ions and in general of all those ions which could interfere with the membrane electrolysis process.

[0047] A physical and/or chemical treatment can be used for the removal of silica and/or other ions from water stream Wo. Said physical and/or chemical treatment generally involves the addition of certain reagents, notably coagulants, which act to destabilize the solids present in water stream Wo. Sodium carbonate and sodium hydroxide may be added to water stream Wo to precipitate calcium and magnesium ions as calcium carbonate and magnesium hydroxide. Metals (iron, titanium, molybdenum, nickel, chromium, vanadium, tungsten) may also precipitate as hydroxides during this operation.

[0048] The amount of sulfate ions can be controlled by adding calcium chloride or barium salts (BaCOs or BaCh) which promote the precipitation of calcium sulfate or barium sulfate.

[0049] Precipitates formed at this stage can be subsequently removed from the water stream by means known in the art, e.g. in a liquid-solid separation unit, prior to further treatment.

[0050] Further purification steps, e.g. treatment in ion exchange membranes may be performed on water stream Wo.

[0051 ] Ultrafiltration processes may be also applied to water stream Wo for the removal of suspended solids, such as silica.

[0052] In an embodiment of the process, the concentration of the alkali metal chloride in water stream Wo is increased before treatment in the

electrolysis unit.

[0053] In one aspect of this embodiment a nanofiltration unit may be employed to concentrate water stream Wo. Any known type of nanofiltration equipment may be used to increase the concentration of alkali metal chloride in water stream Wo, typically polymeric nanofiltration membranes may be employed. Known polymeric nanofiltration membranes may be comprised of polyethersulfone (PES), polyacrylonitrile (PAN) or polyvinylidenefluoride (PVDF) with a typical pore diameter of 1 to 10 nm.

[0054] In another aspect of this embodiment of the process, reverse osmosis may be used to increase the concentration of alkali metal chloride in water stream Wo. Any known type of reverse osmosis equipment may be used. Use is generally made of semi-permeable membranes, typically made of polyamides.

[0055] In another aspect of this embodiment of the process, membrane distillation may be used to increase the concentration of alkali metal chloride in water stream Wo. Membrane distillation is a thermally driven process in which separation is enabled due to phase change: a membrane, typically a porous hydrophobic membrane, provides a barrier for the liquid phase but it allows the vapour phase (e.g. water vapour) to pass through the membrane’s pores condensing on the other side of the membrane. The driving force of the process is given by a partial vapour pressure difference triggered by a temperature difference between the two sides of the membrane.

[0056] In still another aspect, the concentration of the alkali metal chloride in

water stream Wo is increased by evaporation. Suitable evaporation techniques are for instance vapor-compression evaporation or multiple- effect evaporation.

[0057] Physical and/or chemical treatments and concentration steps may be

carried out independently one of the other.

[0058] In an advantageous embodiment of the process a physical and/or

chemical treatment step is carried out before submitting water stream Wo to a concentration step.

[0059] The transformation process may thus comprise: at least one electrolysis step and at least one pre-treatment step wherein water stream Wo is treated to reduce the amount of at least one of suspended silica, dissolved silica, sulfate ions, Ca ions and Mg ions contained in said water stream Wo, said pre-treatment step being performed before water stream Wo is submitted to the electrolysis step.

[0060] The transformation process may further comprise the step of increasing the concentration of alkali metal chloride in water stream Wo before the electrolysis step.

[0061] The transformation process may advantageously comprise:

at least one electrolysis step,

at least one pre-treatment step wherein water stream Wo is treated to reduce the amount of at least one of suspended silica, dissolved silica, sulfate ions, Ca ions and Mg ions contained in said water stream Wo, said pre-treatment step being performed before water stream Wo is submitted to the electrolysis step; and at least one concentration step;

wherein the concentration step is performed either between the pre- treatment and electrolysis steps and/or after the electrolysis step.

[0062] The concentration techniques described for the concentration of water stream Wo may be equally employed for the concentration of the alkali metal hydroxide solution or the hydrochloric acid solution obtained at the end of the electrolysis step.

[0063] Preferably the transformation process further comprises the step of

increasing the concentration of the alkali metal hydroxide solution and/or hydrochloric acid obtained after the electrolysis step by means of membrane distillation or evaporation.

[0064] In a particularly advantageous embodiment of the process of the invention, the residual heat discharged by a drying apparatus is used for the operation of a concentration process. Concentration processes which may be operated using such a residual heat are for instance a membrane distillation process or an evaporation process such as those described above.

[0065] In a preferred embodiment of the inventive process, the drying apparatus is a dryer wherein wet precipitated silica is dried to afford precipitated silica. Any of the dryers described above for the drying of precipitated silica may be used. The drying apparatus can be any conventional dryer which discharges a flow of hot gas which can be used in a heat-exchanger and hence employed in one of the concentration steps described above. The expression“hot gas” is used herein to indicate a gas which has a temperature higher than ambient temperature and in particular higher than 70°C. The temperature of the hot gas is preferably of at least 85°C, more preferably of at least 100°C. The temperature of the hot gas typically does not exceed 150°C.

[0066] In such an embodiment the inventive process advantageously comprises:

reacting an alkali metal silicate with hydrochloric acid to obtain a precipitated silica suspension; submitting said precipitated silica suspension to a solid-liquid separation step and optional washing to obtain a wet precipitated silica and a water stream Wo containing an alkali metal chloride; drying the wet precipitated silica in a drying apparatus, said apparatus discharging a flow of hot gas;

submitting said water stream Wo to a transformation process to provide an alkali metal hydroxide and hydrochloric acid said transformation process comprising:

- at least one electrolysis step, and

- at least one concentration step performed before or after the electrolysis step which is a membrane distillation or an evaporation process operated using the flow of hot gas discharged from the drying apparatus; and

optionally reacting the hydrochloric acid obtained at the end of the transformation process with an alkali metal silicate to prepare precipitated silica; and/or optionally reacting the alkali metal hydroxide obtained at the end of the transformation process with a silicate precursor to obtain an alkali metal silicate.

[0067] The process as detailed above may further comprise at least one pre- treatment step, performed before water stream Wo is submitted to the electrolysis step, wherein water stream Wo is treated to reduce the amount of at least one of suspended silica, dissolved silica, sulfate ions, Ca ions and Mg ions contained in said water stream Wo.

[0068] The inventive process thus allows for an efficient and environment-friendly recovery and re-use of the by-products generated in the precipitated silica manufacturing process. In some embodiments of the process not only the alkali metal chloride obtained from the precipitation process is transformed into hydrochloric acid and alkali metal hydroxide but also the waste heat generated by the drying of the precipitated silica may be recovered and used.

[0069] A system for carrying out the inventive process comprises a reactor for the manufacture of precipitated silica, a unit for the filtration and optional washing of precipitated silica, and a chlor-alkali electrolysis unit, preferably a membrane cell electrolysis unit. Typically, the system further comprises at least a drying apparatus for drying the precipitated silica and

advantageously at least one of a membrane distillation unit or an evaporator which may be operated using the flow of hot gas generated by the drying apparatus. A burner for reacting hydrogen and chlorine for the preparation of hydrochloric acid is also generally present in the system.

Claims

1. Process for the manufacture of precipitated silica comprising:

submitting said precipitated silica suspension to a solid-liquid separation step and optional washing to obtain a wet precipitated silica and a water stream Wo containing an alkali metal chloride;

submitting said water stream Wo to a transformation process to provide an alkali metal hydroxide and hydrochloric acid said transformation process comprising at least one electrolysis step.

2. The process of claim 1 wherein said hydrochloric acid has been obtained by submitting a water stream containing an alkali metal chloride to a

transformation process comprising at least one electrolysis step.

3. The process of anyone of claims 1 or 2 wherein the electrolysis step is

membrane cell electrolysis.

4. The process of anyone of the preceding claims wherein the electrolysis step converts the alkali metal chloride into an alkali metal hydroxide, hydrogen and chlorine.

5. The process of anyone of the preceding claims wherein the transformation process further comprises the step of reacting hydrogen and chlorine obtained from the electrolysis step in a burner to obtain hydrochloric acid.

6. The process of anyone of the preceding claims wherein the hydrochloric acid obtained at the end of the transformation process is reacted with an alkali metal silicate to prepare precipitated silica.

7. The process of anyone of the preceding claims wherein the alkali metal

hydroxide obtained at the end of the transformation process is reacted with a silicate precursor to obtain alkali metal silicate.

8. The process of claim 7 wherein the silicate precursor is selected from natural quartz sand or biogenic silica-containing materials.

9. The process of anyone of the preceding claims wherein the transformation process further comprises at least one pre-treatment step wherein water stream Wo is treated to reduce the amount of at least one of suspended silica, dissolved silica, sulfate ions, Ca ions and Mg ions contained in said water stream Wo, said pre-treatment step being performed before water stream Wo is submitted to the electrolysis step.

10. The process of anyone of the preceding claims wherein the transformation process further comprises at least one concentration step performed either between the pre-treatment and electrolysis steps and/or after the electrolysis step.

11. The process of claim 10 the concentration step is a membrane distillation or an evaporation.

12. The process according to anyone of the preceding claims further comprising the steps of drying the wet precipitated silica in a drying apparatus, said apparatus discharging a flow of hot gas; using the flow of hot gas to perform the concentration step claimed in claim 11.

13. The process according to anyone of the preceding claims wherein the alkali metal is sodium or potassium, preferably sodium.

14. Use of a system comprising a reactor for the manufacture of precipitated silica, a unit for the filtration and optional washing of precipitated silica, and a chlor- alkali electrolysis unit for the manufacture of precipitated silica according to the process of anyone of the preceding claims.