WO2018234398A1

WO2018234398A1 - Method for separation of boron from a mineral acid solution

Info

Publication number: WO2018234398A1
Application number: PCT/EP2018/066464
Authority: WO
Inventors: Christian Elbek
Original assignee: Cec Invest Aps
Priority date: 2017-06-21
Filing date: 2018-06-20
Publication date: 2018-12-27

Abstract

The present invention relates to methods useful in reduction of boron content in aqueous media. One first process relates to removal via precipitation with a strong base of mineral acids from boron containing solutions so as to facilitate production of a boron containing solution of hig h purity and a salt of the mineral acid(s). A second process relates to recovery of boron from a boron containing aqueous med ium, where the boron content is precipitated by use of a strong base.

Description

METHOD FOR SEPARATION OF BORON FROM A MIN ERAL ACID SOLUTION FIELD OF THE INVENTION

The present invention relates to the field of effective removal of boron from aqueous media . In particu lar, the invention relates to a process for a highly efficient removal of boric acid from an aqueous solution that in addition to boron may contain a mineral acid .

BACKGROUND OF TH E INVENTION

The problem and field of producing water with low boron content is known and is used in many d ifferent areas of industry. The 2 primary methods used industrially today are reverse osmosis (RO) and Ion exchange (IX) . A host of add itional methods have been contemplated and are experimented with. A comprehensive review is found in "Boron Separation processes" ISBN 978-0-444-63454-2, 2015 in regards to all available methods. In RO removal of boric acid, either very tig ht membranes are used or the water is treated with an alkaline substance to raise the pH to a level where the boric acid is efficiently rejected by the membrane. In both RO cases, the brine produced cannot economically be made to be less than 10% of the total water produced . This brine is easy to d ispose of at seaside, but the discharge is an insurmou ntable problem in inland areas where the brine cannot be d isposed off by transport as it is not economically viable due to the shear volumes involved . The brine cannot either be environmentally safely d ischarged into surface waters or sewage systems if these are available. As an example, a typical well producing water for ag ricultural irrigation produces 5,000 m³ per day, which will produce 50 m³ of brine per day that contains all salts in the pumped water, not just boric acid .

For IX, non-specific anion exchange technolog ies have the same problem as RO in that all natural salts are removed together with the boric acid and have issues with effluents used in connection with regeneration.

IX-technolog ies that specifically seek to adsorb boric acid have been used for decades. Products such as Purolite^® S108, Diaion^® CRB 03, Diaion^® CRB 05 and Amberlite^® IRA 743 are examples of resins used for this purpose. The advantage of these resins is that they exhibit hig hly specific adsorption of boric acid, so that all other ions in the water pass the resin freely. However, the resins need to be regenerated, at regular short intervals, by using strong acids and caustic agents that will then contain all the boron that is removed from the water. In a typical setup, a column removing 1 ppm boron from the water will need regeneration for every 2,000 bed volumes of water passing throug h it, producing an absolute minimum of 2 bed volumes of liqu id waste containing high levels of salts of the acid and base used as well as boron to the level of 1-3 g B/L that is toxic to plants and wild life. Again for a typical well for irrigation of 5,000 m³ /day this translates into a minimum of 5 m³ of waste/day. Even if it were possible to d ispose of this waste at zero cost, the shear logistics of access, storage, transport and d isposal means that this is not economically viable. As has been demonstrated above, many methods exist to remove boron from water, but all currently known methods produce waste streams with sig nificant environmental and economic drawbacks, as long as seaside d ischarge is not available.

One area of commercial use of low boron water, where seaside d ischarge of by-products is not read ily available, is ag riculture. Many fruits, berries and nuts are highly sensitive to high boron levels in the irrigation water on the order of > 0.75 ppm B. H igher levels can significantly reduce crop yields. For some producers of g rapes, desired boron levels in irrigation water is < 0.3 ppm boron. This requirement is a problem if the well water available for irrigation has a boron content that is >0.75 ppm as is the case in many areas of e.g . the central valley in California, USA. The use of boron-specific resin capture is attractive for this purpose, but only if it is possible to devise a way to hand le the brine waste produced by the regeneration of the resin in an economically feasible manner. As such, it is desirable to devise a process that will convert such brine by an economically efficient process into concentrated by-products that can be either easily d isposed off or - even better - find commercial use. OBJECT OF TH E INVENTION

It is an object of embod iments of the invention to provide improved methods for control of boron in aqueous media, in particular in aqueous media for use in ag ricu lture.

SUMMARY OF THE INVENTION

It has been found by the present inventor that it is possible to attain removal of boron from aqueous med ia while provid ing condensed waste products that can either be used for other purposes on-site or be used for production of borate containing end products. The present invention was initially devised on the background that boron is conventionally removed from water by use of an IX-based technology (cf. above), where the resins used have to be regenerated for repeated use. The direct by-product of the recovery process is typically an acid ic mixture of BA and a strong mineral acid . The present inventor has found that this type of by product can be subjected to a process (the "stage A" process) where the majority of the mineral acid is removed by precipitation and separation and made available in the form of a mineral acid salt of low solubility. This mineral acid salt will only contain a minor amount of boric acid . The remaining liqu id, which is now a boric acid solution with only neg lig ible amounts of mineral acid can after this process be subjected to a second process (the "stage B" process), which consist of a precipitation step that provides for a boron salt of low solubility. Both the mineral acid salt product of stage A and the boron salt of stage B are easy to utilise for secondary purposes (often in the same geog raphical area where the water is to be use) .

It is believed that the combined process of Stage A + Stage B, as well as the separate processes, constitute novel and useful inventions that are not limited to use when removing boron from regeneration of resins used in boron isolation - both of the Stage A and Stage B processes can be advantageously employed on suitable starting materials, but in combination they find a special and advantageous use in hand ling of boron contained in regeneration fluids from resins used in IX treatment of boron containing water. This invention hence relates to processes for a hig hly efficient removal of boric acid from an aqueous solution that may contain a mineral acid . This removal is achieved by a process using low cost raw materials and scalable processing methods, yield ing by-products in solid form that can be used for other purposes or d isposed of in an economical and

environmentally safe way. So, in a first aspect the present invention relates to a process for preparing a mineral acid salt containing precipitate from an initial acid ic aqueous solution, wherein the initial acidic aqueous solution comprises boric acid (BA) and a mineral acid (MA) and has a pH < 5, the method comprising precipitating the MA by add ition of a strong base (StBa) in an amount, which does not exceed the amount of MA acid ic equivalents to fully neutralize the solution, achieving a pH < 7, so as to produce a mineral acid salt precipitate and a BA containing (slightly acidic) aqueous solution, and subsequently recovering said mineral acid salt precipitate, which comprises less than 10% of the boron orig inally in the initial acid ic solution. In a closely related 2^nd aspect, the present invention relates to a process for preparing a boric acid (BA) containing aqueous solution from an initial acid ic aqueous solution, wherein the initial acid ic aqueous solution comprises BA and a mineral acid (MA) and has a pH < 5, the method comprising precipitating the MA by add ition of a strong base (StBa) in an amount, which does not exceed the amount of StBa, which does not exceed the amount of MA acid ic equivalents to fully neutralize the solution, achieving a pH < 7, so as to produce a mineral acid salt precipitate and a (slig htly acid ic) BA containing aqueous solution, and subsequently recovering the BA containing aqueous solution, which has a BA: MA ratio that is at least 10 times higher than the BA: MA ratio of the initial acid ic aqueous solution and/or which has a BA content of at least 90% of the BA content of the initial aqueous solution.

In a 3^rd aspect, the present invention relates to a process for preparing an aqueous solution having a low boron content from an initial acidic aqueous solution having a higher boron content, the method comprising 1) preparing a BA containing aqueous solution accord ing to the method of the 2^nd aspect of the invention, and 2) subsequently add ing to the BA containing aqueous solution a strong base (StBa) at a temperature of at least 60°C to precipitate boron from the BA containing aqueous solution, and subsequently recovering said aqueous solution having a low boron content. In a 4^th aspect, the present invention relates to a process for preparing a boron salt from an initial acid ic aqueous solution, the method comprising 1) preparing a BA containing aqueous solution accord ing to the method of the 2^nd aspect of the invention, and 2) subsequently add ing to the BA containing aqueous solution a strong base (StBa) at a temperature of at least 60°C to precipitate boron from the BA containing aqueous solution, and subsequently recovering boron containing precipitate comprising said boron salt from the solution.

In a 5^th aspect, the present invention relates to a process for preparing an aqueous solution of low boron content, comprising add ing to a BA containing aqueous solution, which comprises at least 0.90 g/L boron in the form of boric acid, a strong base (StBa) at a temperature of at least 60°C so as to precipitate boron from the starting solution, and subsequently recovering said aqueous solution of low boron content.

Finally, in a 6^th aspect, the present invention relates to a process for preparing a precipitate containing a boron salt from a BA containing aqueous solution, which comprises at least 0.90 g/L boron in the form of boric acid, the method comprising add ing to the BA containing aqueous solution a strong base (StBa) at an elevated temperature of at least 60°C so as to precipitate boron, and subsequently recovering the boron containing precipitate. LEGEN DS TO TH E FIGURE

Fig . 1 : Diag ram showing an example of a two stage (A and B) process of the invention. In stage A, mineral acid is precipitated by calcium hyd roxide and filtered . The filtrate from this stage is subsequently let to stage B to be precipitated with calcium hydroxide forming a calcium borate. This latter process step is preferentially done at a temperature of 80°C or above. The precipitate is filtered off leaving a filtrate containing approximately 100 mg B/L.

Fig . 2 : Reproduction from page 45 in "Boron Separation processes" ISBN 978-0-444-63454- 2, 2015.

Showing the different species involved in the d issociation of boric acid as a function of pH . DETAILED DISCLOSURE OF TH E INVENTION Definitions

The term "boric acid" is in the present context a common expression for a number of acid ic species of which BH₃0₃ is the fully protonated version. As the boric acid becomes less protonated, more complex species appear, of which the most prevalent is H₂B₄0₇ (when pH increases above 5), but also other species such as H B₃0₃(OH)₄. Further, boric acid at these pH values may appear in ionic clusters, agg regates, or crystals. For the purposes of the present invention, "boric acid" is intended to refer to any and all of these species, but for practical purposes, any quantitative ind ications concerning boric acid are provided by referring to the content of atomic boron, B. When expressing amounts of B in percentages in the present description and claims, it is - unless otherwise ind icated - an ind ication of the amount of B relative to the amount present in the relevant boron containing solution prior to treatment.

The term "precipitate" has its usual meaning in the art of chemistry, i.e. an insoluble substance that separates (typically in the form of settled material) from a solvent (in this case from an aqueous solvent) .

The expression to "fully neutralize" a solution with an amount of StBa is in the present context intended to mean that the amount of StBa does not bring pH above 7.0. In practice, the StBa is used in considerable lower amounts, meaning that pH prior to recovery of the mineral acid salt precipitate or the BA containing aqueous solution does not exceed 6.9 or 6.8 or 6.7 or 6.6 or 6.5 or 6.4 or 6.3 or 6.2 or 6.1 or 6.0 or 5.9 or 5.8 or 5.7 or 5.6 or 5.5 or 5.4 or 5.3 or 5.2 or 5.1 ; in most cases the pH does not exceed 5 after add ition of the StBa to the initial aqueous solution and prior to recovery of the mineral acid salt precipitate or the BA containing aqueous solution.

Specific embodiments of the invention General considerations

Aqueous brine consisting of boric acid and a mineral acid can, according to the invention, efficiently be converted into a hig hly insoluble solid calcium salt of the mineral acid with only minute amounts of boric acid, a hig hly insoluble solid calcium borate salt and an aqueous filtrate that has the vast majority of boric acid removed . The discovery of the process is a combination of two ind ividual d iscoveries. The first discovery is the ability to precipitate and process a solution of e.g . boric acid and su lfuric acid by the add ition of calcium hydroxide (or other strong bases, see below) in such a way that < 10% of the boron (boric acid) present initially in the solution is confined to the solid calcium sulphate that is filtered from the process u pon removal of >90% of the sulfuric acid as calcium su lphate. The key to obtain this is to keep the pH of the precipitation solution below the pH at which boric acid starts to deprotonate.

The second d iscovery is that an aqueous brine consisting of boric acid in a concentration of more than 0.9 g B/L and preferentially more concentrated, can be very efficiently precipitated (>97%) by add ition of calcium hydroxide (or other strong bases) at reaction temperatures of >60°C and preferentially at about 80°C or above to produce a calcium borate salt. The key in this d iscovery rests upon the realization that the basic driver for efficient removal of boric acid in this process lies in having the concentration of the boric acid solution being as high as feasibly possible. If no mineral acid is present in the initial solution, only stage B is used, which has merit on its own. By the combination of the two ind ividual inventions, a process has been devised that very efficiently separates boric acid out from brine consisting of boric acid and a mineral acid . This combined process can conveniently be used in treating the acid regeneration brine produced when regenerating specific boric acid IX type resins such as Purolite S108, Diaion CRB 03, Diaion CRB 05, Amberlite 743 and similar resins. Such resins, can depend ing on operation parameters, hold up to 1.5-3.0 g B/L of resin and with proper acid elution generate mineral acid solutions of 40-100 g mineral acid/L with a boric acid content of - typically - 1,5-3.0 g B/L. However, the source of the incoming solution can essentially be coming from anywhere. Stage A. Precipitation of the mineral acid

Stage A covers the 1 and 2 aspect of the present invention - the only d ifference between these 2 aspects being the separation product after the treatment of the incoming liqu id .

The purpose of stage A is to separate out the mineral acid as a solid with only minor amounts of boric acid, thus providing a range of advantages. It is worth noting that boric acid may be removed solely by the stage B process, but this would provide for a mixture of a calcium mineral salt and a calcium borate salt, which has no obvious immed iate use. First of all, this combined end-product would constitute a large amount of boron containing "waste" that would be of questionable use, except as landfill or as cement filler or the like. Also, it would require heating a relatively concentrated acid solution to e.g . 80°C, (see example 3), which poses severe long-term corrosion issues for all equipment, especially for any economically relevant plant. If, in contrast, the mineral salt is separated out as in shown in example 5 and 6, the level of the boron content is so low that the mineral salt can be used d irectly for a range of purposes. In an agricultural context, the formed mineral salt can for instance, be used as either a fertilizer or a soil improver. This has value not only in its use, but also in the economics of the log istics of handling the waste.

If the mineral acid in the brine is phosphoric acid, stage A generates CaHP0₄ 2H₂0 as a solid by-product. This product can be used either d irectly as a phosphate fertilizer or be further processed to a more solu ble phosphate fertilizer. If the mineral acid is sulfuric acid, stage A generates CaS0₄ 2H₂0 (gypsum), which can be used for a variety of purposes (soil improver, gypsum wall board etc.) . Because of the very low level of boric acid in the isolated solids, these solids can without further processing be used as either fertilizer or soil improver in an ag ricultural context. A key property of the used mineral acid is that it produces a relatively insoluble calcium salt that can be filtered off. As such, HCI is not practical as the formed CaCI₂ is a hig hly solu ble salt.

Examples 1 and 4 show how operation above pH = 5 starts precipitation or bind ing of the boric acid to the formed mineral acid salt. Hold ing this up against d iagram 2, this makes sense as boric acid is un-d issociated below pH = 5 and as such will not be inclined to be part of any ionic clusters, agg regates or crystals. A person skilled in the art will appreciate that the above will hold for any mineral acid . And of particular interest any mineral acid that will also form a highly insoluble calcium salt by reaction with calcium hyd roxide or calcium oxide.

Stage B. Precipitation of the borate The purpose of this stage (to which, aspects 5 and 6 of the invention belong) is to separate out the boric acid as e.g . a calcium borate by add ition of calcium hyd roxide (or other suitable strong base) and heating of the solution (e.g . to 80°C), as shown in Irawan C et al. (2011), Desalination 280, pp. 146-151. In this article a range of precipitations are made with calcium hyd roxide using a boric acid solution brine with a concentration of 750 mg B/L. From this article a ratio 4g of boric acid to lOg of Calcium hydroxide is found to be optimal.

Add ition of more calcium hydroxide does not lead to a hig her level of precipitation. For the 750 mg B/L brine solution it was shown that it was possible to precipitate at 80°C, approximately 87-90% of the boric acid, thereby leaving approximately 10- 13% in solution. At 60°C the reaction progresses to the same equilibrium as the reaction at 80°C, but over a much longer timespan (8 hours vs. 2 hours) and at 45°C, the reaction is so slow that it is not of commercial interest.

Example 2 is a reproduction of this best case demonstrated in Irawan ef al. Example 3 on the other hand uses the exact same process, but a substantially higher starting boric acid concentration of approximately 3.0 g B/L. This surprising ly resulted in a precipitation leaving only 3% of the initial amount of boron in solution. As d iscussed in detail in example 3, this must rest upon the fact that in both cases (examples 2 and 3) a saturated calcium borate solution is obtained . This is likely also the reason, why no improvement in precipitation was observed when add ing higher amounts of calcium hyd roxide in Irawan et al. - beyond the saturation point, the concentration of calcium and hydroxide ions do not change as a function of added amounts. As such, it can be concluded that the precipitation efficiency is primarily determined by the concentration of the ingoing boric acid containing solution, which preferably shou ld be > 0.9 g B/L and preferably higher. This insig ht is new and of substantial value. The produced calcium borate can be used either as a fertilizer by itself, be further processed to a more soluble fertilizer source or be further processed as an alternative to colemanite, which is a primary mined mineral for the production of a variety of borate compounds used for a large variety of industrial purposes, such as ceramics, detergents, g lass, insulation, fiberg lass and flame retardants. A person skilled in the art will appreciate that instead of calcium hydroxide - calcium oxide, magnesium oxide, mag nesium hydroxide, zinc oxide and zinc hydroxide and many other alkaline metal oxides and hyd roxides can be used instead .

1^st and 2^nd aspects of the invention As ind icated above, both of these processes entail the steps of precipitating MA from a solution also containing a mineral acid . This is accomplished by addition of a strong base (StBa) in an amount, which does not exceed the amount of MA acidic equivalents necessary to fully neutralize the solution, to achieve a pH < 7. If selecting the strong base so as to ensure that the MA will indeed precipitate as a salt of low solubility, the precipitate can be recovered by simple and conventional means (filtration or centrifugation are exemplary choices) . Thereby a mineral acid salt precipitate and a BA containing aqueous solution are produced, and aspects 1 and 2 of the invention each focus on the production of these separable end products (and when combined, both end products are isolated) . The mineral acid salt precipitate comprises less than 10% of the boron present in the initial acid ic solution and the BA containing aqueous solution consequently has a lowered content of BA (and thereby of boron) .

Hence in effect the 1^st and 2^nd aspects of the invention both constitute methods for removal of mineral acid from a mixture of mineral acid and BA. Typically, the mineral acid salt precipitate obtained comprises less than 4% of the boron in the initial acidic solution (i. e. of the boron present in BA, but as shown in the examples, it is possible to attain lower values, such as less than 3.5%, and even less than 3.3% . In other words, the mineral acid salt precipitate obtained has a boron content which is sufficiently low for it to be usefu l as a fertilizer or soil improving agent, dependent on the exact type of mineral acid salt. As demonstrated in the examples, it is even possible to precipitate the MA with hig her specificity, meaning that the mineral acid salt precipitate comprises 0.5% - 3%, such as 0.5%-2%, 0.5%-1.5%, and 0.5%-l% of the BA in the initial acidic solution.

Likewise, the BA containing aqueous solution has a BA: MA ratio of at least 10 times the BA: MA ratio of the initial acid ic aqueous solution. Put differently, the amou nt of mineral acid remaining after the treatment with the StBa is neglig ible : the ratio is hence at least 15 times, such as at least 20, at least 25, at least 30, at least 35, at least 40, at least 45, and at least 50 times the BA: MA ratio of the initial acid ic aqueous solution.

3^rd and 4^th aspects of the invention

These aspects combine the Stage A and Stage B processes to obtain on the one hand a boron salt which has further uses as well as a purified aqueous med ium (water) with boron content, which is sufficiently low to allow it to be either bled into the product water or returned for resin capture to constitute a zero liquid d ischarge process. Both processes include the step of preparing a BA containing aqueous solution as described in the 2^nd aspect of the invention, and 2) su bsequently add ing to the (typically slightly acidic) BA containing aqueous solution at a temperature of at least 60°C, a strong base (StBa), thereby precipitating boron from the BA containing aqueous solution. Subsequently, recovery of the aqueous solution (3^rd aspect) or the boron salt (4^th aspect) takes place. Obviously, both end products may be recovered in the same process by simple separation (centrifugation or filtration) . The temperature in the step of add ing the strong base is not essential, but will typically be at least or about 65°C, such as at least or about 70°C, at least or about 75°C, and at least or about 80°C. These temperatures are g iven for reactions taking place at atmospheric pressure (1 bar) but higher temperatures can be used if e.g . operating under elevated pressure. It is also possible to facilitate the precipitation reaction by other means, e.g . by applying ultrasound or microwaves or any other external stimu lation that may speed up the precipitation reaction.

Embod iments of the i^st-4^th aspects

A number of process parameters and reagents can be used in all the first 4 aspects :

For instance, instead of calculating the maximum amount of StBa to add in the process step for precipitation of the mineral acid, the amount can be simply controlled by not add ing further StBa when the pH is at most 7, such as at most 6, preferably at most 5. Under normal circumstances, the pH = 5 is interesting because substantially no precipitation of BA will take place at pH values below 5 whereas the MA may become (almost) neutralized . However, depend ing on the exact composition of the incoming mixture of BA and mineral acid, titration up to higher pH values than 5 can be acceptable, as long as no substantial amounts of BA are precipitated .

The pH of the initial acid ic aqueous solution is as a consequence of the mineral acid present, typically quite low such as <4, such as <3.5, < 3.0, < 2.5, <2.0, and < 1.5.

At any rate, the amount of StBa added to the mixture between MA and BA preferably does not exceed the amount needed to neutralize the MA.

As mentioned, the nature of the mineral acid mainly depends on the source of the incoming liquid, but the method finds particular use when 2 different features characterize the mineral acid . First o fall, it is highly convenient if the mineral acid in the initial solution forms a highly insoluble salt when reacted with calcium hydroxide or oxide. Further, it is obviously very convenient if the thus formed insolu ble salt has a commercial or at least practical use. For these reasons, it is preferred that the mineral acid is phosphoric acid or sulphuric acid . 5 and 6 aspects of the invention

As will be clear from the above, these aspects constitute an improvement of the technology for boron precipitation d isclosed in Irawan C et al. (2011) . The realization that the incoming BA concentration is the single determining feature for the efficacy of relative boron removal provides that the process can be employed with a very hig h efficiency on incoming BA containing material (such as that produced in the Stage A process) .

The two related processes of the 5^th and 6^th aspects both entail that a BA containing aqueous solution, which comprises at least 0.90 g/L boron in the form of boric acid, is reacted with a strong base (StBa) at a temperature of at least 60°C so as to precipitate boron from the starting solution, and subsequently recovering 1) said aqueous solution of low boron content and/or 2) the boron containing precipitate.

The temperature for the boron precipitation reaction is typically at least or about 65°C, such as at least or about 70°C, at least or about 75°C, and at least or about 80%. In this connection, the remarks made under the d iscussion of the 3^rd and 4^th aspects apply equally. The starting solution is preferably substantially free from mineral acid - this has no substantial impact on the quality of the aqueous end product, but it is as mention herein of hig h value that the boron salt is highly pure, thus provid ing for fewer problems with waste products.

The pH of the starting solution is typically at least 4.5, such as at least 5 .0 - in other words, the pH is such that no appreciable amounts of mineral acid are present, meaning that the only substantial precipitation reaction will be the boron precipitation.

Features common for all aspects of the invention.

The preferred StBa used in the invention is an alkaline metal oxide and preferably the same StBa is used in both the Stage A and Stage B process (although this is not a necessity) . The alkaline metal oxide is typically selected from the g roup consisting of calcium hyd roxide, calcium oxide, magnesium oxide, mag nesium hyd roxide, zinc oxide, zinc hydroxide,

Strontium hyd roxide, strontium oxide, barium hydroxide, barium oxide, Fe(II) hyd roxide, Fe(II) oxide, aluminium hyd roxide, and aluminium oxide.

Finally, as already mentioned, it is preferred that the BA containing aqueous solution has hig h boron content, for instance at least 1.0 g/L boron, such as at least 1.5 g/L, at least 2.0, at least 2.5, and at least 3.0 g/L. The latter value constitutes the normal upper boron concentration limit in regeneration liquid from IX material subjected to cou nter flow regeneration using mineral acids, but it is clear that other sources of BA can provide for hig her concentrations of boron. Since the Stage B process becomes increasing ly effective with increasing boron content in the BA containing aqueous solution, it is hig hly

advantageous to apply the presently d isclosed methods on such source materials.

As demonstrated in Examples 7-11, the process kinetics in both Stage A and B can be considerably improved by ensuring that the StBa is micronized, thus exhibiting a larger surface area to volume ratio. This is in particular relevant when the StBa is Ca(OH)₂, but in general it is preferred to ensure that the StBa is present as small particles (if the StBa is read ily solubilised) during the precipitation reactions disclosed herein. Hence, it is preferred that the StBa (preferably the calcium hydroxide) is either

1) in particulate form where the particles are of a size sufficiently small to provide for precipitation of at least 97% of MA within 20 minutes after add ition of the StBa, or

2) subjected to a micronizing influence, which is capable of ensuring that at least 97% of MA is precipitated within 20 minutes after add ition of the StBa,

when a process as defined in claim 1 is carried out by using an initial acid ic aqueous solution containing 2 g/L B, 65 g/L H₂S0₄ as the MA, and add ing 99.3% of the StBa 99.3% necessary to fully neutralize the H₂S0₄.

As also d iscussed in the examples, commercially available StBa products with small particle sizes are available. It is also possible to use larger particles as starting materials and then micronize these by methods known per se (milling, crushing etc.) and then carry out the stage A and/or B process. Furthermore, the stage A and/or B processes can be carried out using a micronizing influence during the precipitation process, where the micronizing influence preferably is u ltrasound treatment. EXAMPLE 1

Precipitation of phosphoric acid from a mixture of boric acid and phosphoric acid

In a 0.5 litre beaker with a stir bar, 205.04 g of M iliQ water, 3.45 g of boric acid (BA) (>99%) and 15.59 g phosphoric acid (86w%) were mixed to produce a homogenous, clear aqueous solution at room temperature with approximately 15.4 g boric acid/L (equivalent to 2.7 g boron (atomic) per I) and 65 g phosphoric acid/L. The pH of the solution was measured continually by a calibrated pH-meter, non temperature compensated . The phosphoric acid was neutralized accord ing to the reaction scheme : H₃P0₄ + Ca(OH)₂→ CaHP0₄. Calcium hydroxide was added in small aliquots up to and above equivalence for neutralization. The equivalent amount of calcium hyd roxide for full neutralization for the above solution is 10.12 g calcium hyd roxide. Prior to each new add ition of calcium hydroxide, the pH of the solution was allowed to equilibrate and a 4 ml sample of the solution was taken and filtered throug h a 0.45 micron Teflon filter. This solution was subsequently d iluted and analysed for boric acid content by the Azomethine H method as described in IOSR Journal of Applied Chemistry (IOSR-JAC) e-ISSN : 2278-5736.Volume 7, Issue 3 Ver. I. (Apr. 2014), PP 47-51. In table 1 below results are tabu lated :

TABLE 1

Time Sample # Accumulated pH BA remaining

(min) add ition of Ca(OH)₂ in solution

(%) (%)

1 1.13 98

0 31 1.77

5 2 64 2.68 98

30 3 93 3.25 101

85 4 105 5.62 101

165 5 118 9.40 42

960 6 9.92 31

As more and more calcium hyd roxide was added, a slurry developed due to the low solubility of the CaHP0₄ formed (0.2 g/L at 20°C) and the temperature rises modestly to approximately 35-40°C. The table shows that boric acid is not co-precipitated out of solution until slightly more than equivalent amounts of calcium hydroxide have been added and the pH turns alkaline.

EXAMPLE 2

Borate precipitation In a 300 litre beaker with a stir bar, 199.0 g of MiliQ water and 0.926 g of Boric acid (BA) (>99%) were mixed to a homogenous clear solution at 80°C. Accord ing to Irawan C et al. (2011), Desalination 280, pp. 146-151, fast precipitation of calcium borate is possible using a weight ratio of minimum 4 g BA to 10 g calcium hyd roxide.

3.71 g calcium hydroxide was added to the hot solution at 80°C and the solution was left to stir for 2 hours. A sample of 4 ml was taken from the solution and filtered throug h a 0.45 micron Teflon membrane, properly d iluted and analysed for boron content by the Azomethine H method as described in Example 1. The BA remaining in solution constituted 11.4% of the orig inally added BA. This value is very close to what was found by Irawan et al. where approximately equivalent conditions provided for 10-13 w% of orig inally added BA in solution. EXAMPLE 3

Borate precipitation

In a 300 litre beaker with a stir bar, 196.8 g of MiliQ water and 3.43 g of boric acid (BA) (>99%) were mixed to provide a homogenous clear solution at 80°C. 10.61 g calcium hyd roxide was added to the hot solution at 80°C and the solution was left to stir for 2 hours. A sample of 4 ml was taken from the solution and filtered throug h a 0.45 micron Teflon membrane, properly d iluted and analysed for boron content by the Azomethine H method as described in Example 1. The BA remaining in solution constituted only 3.3 w% of the orig inally added BA. After slow cooling to room temperature and sitting overnight, a second sample was taken that was prepared and analysed as the first sample. This second sample showed 3.0 w% of the orig inally added BA in the solution.

The results show that by applying the above-described technique, it is possible to precipitate a stable insolu ble calcium borate salt out of solution, thereby precipitating 97% of the total boron content in solution .

In example 2, the equilibrium borate concentration is 532 mg BA/L, for the present example, the equilibrium borate concentration is approximately 513 mg BA/L. As a consequence of the desig n of the experiments, this unequivocally demonstrates that the solutions are saturated in both instances - i.e. the liquid phase concentration of boron is the same, and this is irrespective of the amount of the solid phase.

Therefore, a key goal to obtain effective removal of boron from a boric acid solution in this manner is to provide a hig h concentration of boric acid in the starting solution, preferably a concentration which is as high as possible. Below in table 2 is calculated the effect of the precipitation efficiency (= amount of atomic boron precipitated as calcium borate under the above conditions) as a function of the in-going boron concentration as B g/L = (1 : 5.72) x BA g/L.

TABLE 2

B concentration Precipitation efficiency

(q/L) %

3.00 97

1.50 94

1.00 91

0.50 82

0.20 55

0.10 10 EXAMPLE 4

Precipitation of phosphoric acid, with measurements

In a 0.5 litre beaker with a stir bar, 195.9 g of a phosphoric acid solution (0.0871 g H₃P0₄/g solution) and 3.451 g of boric acid (BA) (>99%) were mixed to produce a homogenous, clear solution at room temperature, thus provid ing an aqueous solution with approximately 0.0174 g BA/g solution (equivalent to app 3.0 g B/L) . The pH, conductivity, and temperature of the solution were measured continuously by calibrated meters. For pH and conductivity, this was done without temperature compensation.

The phosphoric acid was neutralized accord ing to the reaction scheme : H₃P0₄ + Ca(OH)₂→ CaHP0₄, calcium hydroxide was added in small aliquots up to slig htly below full neutralization of acid . The equivalent amount of calcium hydroxide for full neutralization for the above solution is 12.88 g calcium hydroxide.

Prior to each new add ition of calcium hydroxide, the pH and conductivity of the solution was allowed to equ ilibrate and a 4 ml sample of the solution was taken and filtered throug h a 0.45 micron Teflon filter. This solution was subsequently d iluted and analysed for Boric acid content by the Azomethine H method in example 1. Below are the experimental cond itions and results in table 3.

TABLE 3 Accumu lated BA

add ition of remaining in rime Ca(OH)₂ Conductivity Temperature solution min) Sample # (%) PH mS/cm °C (%)

0 1 0.77 51.90 23.2 100

25 2 50.4 2.49 26.70 32.0 97

80 3 75.5 2.63 16.58 30.1 97

127 4 85.5 2.84 12.49 28.1 98

175 5 90.5 3.04 9.84 27.1 100

230 6 95.6 3.62 4.96 26.8 101

330 7 98.0 4.72 1.86 25.1 98

1200 N/A 4.82 1.76 19.7

Table 3 shows that up to 98% neutralization is possible, leaving only≥ 2% of the orig inally added BA in the solution and possibly less (due to the measurement variation) . Table 2 also shows that the very last part of neutralization happens very slowly and that the end point found at 330 minutes and 98% of Ca(OH)₂ add ition relative to 100% neutralization does not change by sitting overnig ht. Thus, true equilibration is found . The pH of 4.8 was an intended end target as pH = 5 is the point where boric acid begins to deprotonate and build various anionic species, which likely is the primary cause for the result in example 1 at pH = 9.92.

EXAMPLE 5

Precipitation of phosphoric acid, measurement of BA in washed cake process In a 0.5 litre beaker with a stir bar, 200.4 g of a phosphoric acid solution (0.0871 g H₃P0₄/g solution) and 3.481 g of boric acid (BA) (>99%) were mixed to produce a homogenous, clear solution at room temperature, thus provid ing an aqueous solution with app. 0.0171 g Boric acid/g solution (equivalent to approximately 3.0 g B/L) . The pH, conductivity, and temperature of the solution were measured continuously by calibrated meters. For pH and conductivity, this was done without temperature compensation.

The phosphoric acid was neutralized accord ing to the reaction scheme : H₃P0₄ + Ca(OH)₂→ CaHP0₄. 12.91 g calcium hydroxide was added in one single portion to the solution and the pH, conductivity and temperature measured continuously as in previous examples. The 12.91 g calcium hydroxide constituted 98% of the equ ivalent amount of calcium hydroxide for full neutralization.

The reaction was stopped after 270 min with pH and conductivity equilibrated . A sing le sample of 4 ml solution was taken and filtered throug h a 0.45 micron Teflon filter. This solution was subsequently d iluted and analysed for Boric acid content by the Azomethine H method in Example 1.

Results are as shown in table 4 below :

TABLE 4

Time Conductivity Temperature BA in solution (min) pH (mS/cm) (^CJ (%, w)

0 0.81 49.80 20.0

270 3.85 4^05 24 3 103

The slurry of CaH P0₄ thus formed was subsequently filtered on Sartorius 392 g rade paper using a Buchner funnel, filtration flask and vacuum pump. The slurry filtered very fast leaving a completely clear filtrate.

The filter cake was subsequently washed with 240 ml 0.01 M H₃P0₄ solution and finally with 120 ml of M iliQ water. The filter cake was sucked dry, removed and weighed . A total of 33.21 g of wet filter cake was found . 1.118 g of the wet filter cake (3.37% of total wet cake) was re-d issolved by re-slurring in 75.0 ml MiliQ water and by add ition of 6.65 g of phosphoric acid solution (0.0871 g H₃P0₄/g solution) to produce a completely clear and homogenous solution with no particulates. A 4 ml sample was su bsequently taken from this solution and analysed as in Example 1. The result found was that 1.3% of the orig inally added boric acid was retained in the filter cake.

The filter cake resu lt and the measurement of BA in solution corroborate each other and demonstrate that it is possible to very efficiently separate boric acid from a mixture of boric acid and phosphoric acid by precipitation with Ca(OH)₂ by very simple means, achieving a recovery of 98% of the phosphoric acid as CaH P0₄ leaving only 1.3% of boric acid in the orig inal solution co-precipitated out.

EXAMPLE 6

Precipitation of sulfuric acid

In a 1.0 I beaker with a stir bar, 591.8 g of a sulfuric acid solution (0.0881 g H₂S0₄/g solution), 211.2 g M iliQ water and 9.171 g of boric acid (BA) (>99%) were mixed to produce a homogenous, clear solution at room temperature at 400 rpm. The resultant mixture contained approximately 0.0114 g boric acid/g solution (equivalent to approximately 2.0 g B/L) and app. 65 g H₂S0₄/L. The pH, conductivity, and temperature of the solution were measured continuously by calibrated meters. For pH and conductivity, this was done without temperature compensation. The sulfuric acid was neutralized accord ing to the reaction scheme : H₂S0₄ + Ca(OH)₂→ CaS0₄. 37.317 g calcium hyd roxide was added in one sing le portion to the solution and the pH, conductivity, and temperature measured continuously as in previous examples. The added calcium hydroxide constituted 95% of the equivalent amount of calcium hydroxide for full neutralization, assuming 100% purity of the calcium hyd roxide.

The reaction was stopped after 120 min with pH and conductivity equilibrated . Results are as shown in table 5 below. An approximate conversion percentage has been calculated, being measured as conductivity vs. orig inal conductivity at 0 minutes.

TABLE 5 time conductivity Temp approximate

conversion

(min) PH mS/cm °C %

0 0.34 271.00 20.5 0

15 0.71 39.6 37.6 85

45 0.95 29.4 33.2 89

65 1.01 27.7 31.2 90

105 1.07 25.9 28.8 90

120 1.09 25.2 27.8 91 The formed slurry of CaS0₄ was subsequently filtered in 2 aliquots on Sartorius 392 grade paper using a Buchner funnel, filtration flask and vacuum pump. The slurry filtered generally fast, leaving a totally clear filtrate. The filter cake was subsequently washed with d ifferent amounts of MiliQ water. The filter cake was sucked dry, removed and weig hed . The filter cake was subsequently re-d issolved to a homogenous solution, sampled and d iluted and analysed for boron content by the Azomethine H procedure used in the previous examples.

The procedure for the wet cake re-d issolution was as follows for the first filtration cake sample : all components in the table below were mixed to an initially homogenous slurry at room temperature in a 100 ml beaker at 400 rpm . After 15 min, the slurry is a totally clear and homogenous solution with all precipitated calcium su lphate re-d issolved . The wet filter cake for filtration trial #2, was done using the exact same procedure and material amounts, as shown in table 6. TABLE 6

Mw Approximate Mass

(g/mol) moles (g)

EDTA d isod ium 372.24 0.005 1.861

Sod ium hyd rogen carbonate 84.01 0.02 1.680

calcium sulphate anhyd rous 136.14 0.005 0.681

calcium sulphate, wet cake 1.370

water 50.0 ml

The results of the filtration trials were overall, as in table 7 below.

TABLE 7

Total

rry Wash wet Cake water

Filtration weig ht water cake content cake

1 ial ial ial (% , w) [%]_

1 199.8 37.6 45.3 55% 3.3%

2 222.3 142.5 53.1 57% 1.4%

The results shown were corroborated by measurements of boron content for the filtrate to show mass balance meaning boron added = boron in filtrate + boron in cake (not reported) . The results show the following :

1. Precipitation with su lfu ric acid works equally well as with phosphoric acid and the level of co-precipitation is approximately 1.4% as found in the filter cake under the very thorough washing of the filter cake in filtration #2 (approximately 7 wash volumes at approximately 50% solids content) .

2. Efficient cake washing is attainable with only at little less than 2 wash volumes, as shown in filtration # 1. For the approximately 50 g wet cake and approximately 50% mother liquor content, the BA in the non-washed filter cake is app. 12% of the BA in the slurry.

As such, this result is very important as it shows that it is possible to wash the filter cake efficiently as desired without d iluting the filtrate to the level of inefficiency in the precipitation of the borate rich filtrate as shown in Example 3. PREAM BLE TO EXAM PLES 7-11

For any industrial process, speed is of importance, as in many cases it is inversely proportional to the volume in-process and cost of equipment. A faster process facilitates a more compact and cheaper installation for the same capacity. A hig her speed can for both stages very likely be obtained by the addition of more calcium hyd roxide than used in the examples. However, for stage A, it is not a robust situation to be add ing calcium hydroxide above the equivalent for neutralization, as the reaction pot will then have the opportunity to go to too hig h pH and start the precipitation of boric acid . To control this will put several timing and control constraints on the process. This is of course possible, but not desired, due to added complexity, hig her cost and lower robustness. Also, the use of more calcium hyd roxide will add cost to the process.

Calcium hyd roxide exhibits slow d issolution kinetics and lower solubility with increasing temperature. It is known in the art that the speed of reaction of calcium hyd roxide increases with smaller particle size due to the larger surface area. The commercial product SLS45 by Lhoist Group is an example of micronized, stabilized calcium hydroxide slurry manufactured with this intent. As such, it is of interest to investigate if the speed of reaction can be increased by working with smaller particles sizes of calcium hydroxide than used in examples 1-6, without any co-precipitation of boric acid The calcium hyd roxide used in the

experiments above was Acros (98% purity), product code 219180025. When subjected to a simple d ry sieve analysis this product d isplayed the following particle d istribution :

Weight in fractions

< 25

micron 25-63 micron 63-200 micron > 200 micron

2% 35% 62% 3%

This g rind size is typical for this type of product, also when supplied industrially.

There are a number of methods available for particle d iminution/micronization. One of the more simple and energy efficient methods that can run continuously is ultrasonication.

Ultrasonication is also used in a range of industrial processes for speed ing up chemical reactions in heterogeneous media by continuous de-agg lomeration and surface activation of particles. EXAMPLE 7

Precipitation of sulfuric acid, with ultrasonication.

To test the impact of using ultrasound in the precipitation of su lfuric acid the following experiment was carried out : In a 300 ml beaker with a stir bar, 148.3 g of a sulfuric acid solution (0.0881 g H₂S0₄/g solution), 52.3 g M iliQ water and 2.3223 g of boric acid (BA) (>99%) were mixed to produce a homogenous, clear solution at room temperature at 400 rpm. The resultant mixture contained approximately 0.0114 g boric acid/g solution (equivalent to approximately 2.0 g B/L) and approximately 65 g H₂S0₄/L. The sulfuric acid was neutralized accord ing to the reaction scheme : H₂S0₄ + Ca(OH)₂→ CaS0₄. 9.508 g calcium hydroxide (Acros, 98%) was added in one single portion to the solution and the reaction pot was then continuously subjected to ultrasonication for 10 minutes (Sonics vibracell, VCX 130, 20 kHz, 13 mm probe, power used : 50W) . Subsequently, the pH, conductivity, and temperature were measured continuously as in the previous examples. The added calcium hyd roxide constituted 97% of the equivalent amount of calcium hyd roxide for full neutralization, assuming 100% purity.

The reaction was stopped after 60 min. Results are as shown in Table 8 below. An

approximate conversion percentage has been calculated, as being measured conductivity vs. orig inal conductivity at 0 mins.

TABLE 8

Time Conductivity Temp approximate (min) PH mS/cm C conversion %

0 0.21 281.00 24.0 0

15 0.81 24.40 55.0 91

25 0.95 22.30 46.5 92

30 0.99 21.80 43.6 92

35 1.02 21.40 41.5 92

40 1.04 21.00 39.8 93

45 1.07 20.70 38.0 93

50 1.09 20.50 36.6 93

55 1.10 20.30 35.5 93

60 1.12 20.20 34.6 93 The formed slurry of CaS0₄ was subsequently filtered on Sartorius 392 grade paper using a Buchner funnel, filtration flask, and vacuum pump. The slurry filtered generally fast, leaving a totally clear filtrate. The filter cake was subsequently washed with 40 ml MiliQ water. The filter cake was sucked dry, removed and weighed . The filter cake was subsequently re- dissolved to a homogenous solution, sampled and diluted and analysed for boron content by the Azomethine H procedure used in the previous examples and d isplayed a Boron content amounting to 0,7% of the orig inally added boron to the reaction solution.

The impact of ultrasonication is clearly evident. By the nature of the process, it was not possible to continuously measure the pH and conductivity during the sonication process, so no data exists for the first 15 minutes. Another interesting observation is the significantly lower BA content in the cake, measured to be 0,7% of the orig inally added BA to the reaction pot.

EXAMPLE 8

Precipitation of sulfuric acid, with micronized calcium hydroxide

To test the impact of calcium hydroxide particle downsizing by itself and addition of such particles to the Stage A process, as demonstrated in the examples above, the following experiment was performed :

A paste of app 45 w% micronized calcium hydroxide was prepared as follows : 30.1 g Calcium hyd roxide and 37.3 g MiliQ water was mixed with a spatu la to a homogenous paste. This paste was subjected to ultrasonication for 3 minutes (Sonics vibracell, VCX 130, 20 kHz, actual power: app. 50 W) . The effects of sonication were clear - a hig hly thixotropic paste was produced, clearly ind icating particle downsizing .

In a 300 ml beaker with a stir bar, 150.0 g of a sulfuric acid solution (0.0881 g H₂S0₄/g solution), 53,5 g M iliQ water and 2.3045 g of boric acid (BA) (>99%) were mixed to produce a homogenous, clear solution at room temperature at 400 rpm. The resultant mixture contained approximately 0.0114 g boric acid/g solution (equivalent to approximately 2.0 g B/L) and app. 65 g H₂S0₄/L . The sulfuric acid was neutralized accord ing to the reaction scheme : H₂S0₄ + Ca(OH)₂→ CaS0₄. 21.1 g of the produced micronized calcium hyd roxide paste above was added in one single portion to the solution. Subsequently, the pH, conductivity, and temperature were measured continuously as in previous examples. The added calcium hyd roxide constituted 94.5% of the equivalent amount of calcium hyd roxide for full neutralization, assuming 100% purity. The reaction was stopped after 60 minutes. Results are as shown in table 9 below. An approximate conversion percentage has been calculated, as being measured conductivity vs. orig inal conductivity at 0 minutes.

TABLE 9

Time Conductivity Temp Approximate,

conversion

(min) PH mS/cm C %

0 0.19 284.00 23.3 0

2 0.26 43.00 41.0 85

6 0.49 28.00 40.3 90

10 0.65 22.10 39.1 92

15 0.85 19.27 37.3 93

20 0.96 17.52 36.2 94

25 1.02 16.32 35.2 94

30 1.06 15.75 34.2 94

35 1.09 15.41 33.4 95

40 1.12 15.04 32.8 95

45 1.14 14.75 32.3 95

50 1.16 14.57 31.8 95

55 1.18 14.38 31.4 95

60 1.19 14.00 31.0 95 The formed slurry of CaS0₄ was subsequently filtered on Sartorius 392 grade paper using a Buchner funnel, filtration flask and vacuum pump. The slurry filtered generally fast, leaving a totally clear filtrate. The filter cake was subsequently washed with 40 ml MiliQ water. The filter cake was sucked dry, removed and weighed . The filter cake was subsequently re- dissolved to a homogenous solution, sampled and diluted and analysed for boron content by the Azomethine H procedure used in the previous examples and d isplayed a Boron content amounting to 2.5% of the orig inally added boron to the reaction solution.

The better time resolution above shows a very fast reaction without any effects on the co- precipitation of BA. Equ ilibrium is obtained after app. 20 mins, and app. 97% of the equilibrium conversion point is obtained after 10 minutes (92%/95% = 97%) . EXAMPLE 9

Precipitation of sulfuric acid, close to full neutralization.

To investigate how close to equivalence of the reaction components it is possible to go without co-precipitating BA, the following experiment was conducted : The same calcium hyd roxide paste as used in the example 8 was used for this experiment.

In a 300 ml beaker with a stir bar, 150.2 g of a sulfuric acid solution (0.0881 g H₂S0₄/g solution), 52.7 g M iliQ water and 2.3339 g of boric acid (BA) (>99%) were mixed to produce a homogenous, clear solution at room temperature at 400 rpm. The resultant mixture contained approximately 0.0114 g boric acid/g solution (equivalent to approximately 2.0 g B/L) and app. 65 g H₂S0₄/L . The sulfuric acid was neutralized accord ing to the reaction scheme : H₂S0₄ + Ca(OH)₂→ CaS0₄. 22.2 g of the produced micronized calcium hyd roxide paste above was added in one single portion to the solution. Subsequently, the pH, conductivity, and temperature were measured continuously as in previous examples. The added calcium hyd roxide constituted 99.3% of the equivalent amount of calcium hyd roxide for full neutralization, assuming 100% purity.

The reaction was stopped after 60 minutes. Results are as shown in table 10 below. An approximate conversion percentage has been calculated, as being measured conductivity vs. orig inal conductivity at 0 mins.

TABLE 10 time end conductivity temp approximate

conversion

(min) PH mS/cm C %

0 0.23 287.00 24.7 0%

2 1.89 46.70 41.1 84%

6 2.22 20.70 41.1 93%

10 2.48 13.02 40.1 95%

15 2.63 9.50 38.6 97%

20 2.72 8.11 37.3 97%

25 2.79 7.27 36.3 97%

30 2.84 6.73 35.4 98%

35 3.33 4.92 34.6 98%

40 3.45 4.44 34.1 98%

45 3.55 4.15 33.4 99% time end conductivity temp approximate

conversion

(min) PH mS/cm C %

50 3.61 3.93 32.8 99%

55 3.74 3.75 32.4 99%

60 3.80 3.63 31.9 99%

The formed slurry of CaS0₄ was subsequently filtered on Sartorius 392 grade paper using a Buchner funnel, filtration flask and vacuum pump. The slurry filtered generally fast, leaving a totally clear filtrate. The filter cake was subsequently washed with 40 ml MiliQ water. The filter cake was sucked dry, removed and weighed . The filter cake was subsequently re- dissolved to a homogenous solution, sampled and diluted and analysed for boron content by the Azomethine H procedure used in the previous examples and d isplayed a Boron content amounting to 3.4 % of the orig inally added boron to the reaction solution.

The above shows that it is possible to go very close to full equilibration without any detrimental effects from co-precipitation of BA. The higher concentration of calcium hyd roxide also makes for a slightly faster process.

EXAMPLE 10

Precipitation of sulfuric acid, close to equivalence. Importance of time in relation to BA in filter cake

A micronized calcium hyd roxide paste was produced as in example 8 with 44.8% (w/w) calcium hyd roxide.

In a 300 ml beaker with a stir bar, 147.7 g of a sulfuric acid solution (0.0881 g H₂S0₄/g solution), 52.4 g M iliQ water and 2.3006 g of boric acid (BA) (>99%) were mixed to produce a homogenous, clear solution at room temperature at 400 rpm. The resultant mixture contained approximately 0.0114 g boric acid/g solution (equivalent to approximately 2.0 g B/L) and approximately 65 g H₂S0₄/L . The sulfuric acid was neutralized accord ing to the reaction scheme : H₂S0₄ + Ca(OH)₂→ CaS0₄. 22.2 g of the produced micronized calcium hyd roxide paste above was added in one sing le portion to the solution. Subsequently, the pH, conductivity, and temperature were measured continuously as in previous examples. The added calcium hyd roxide constituted 100.1 % of the equivalent amount of calcium hyd roxide for full neutralization, assuming 100% purity. The reaction was stopped after 20 min. Results are as shown in table 11 below. An approximate conversion percentage has been calculated, as being measured conductivity orig inal conductivity at 0 mins.

TABLE 11 time conductivity temp approximate

conversion

(min) PH mS/cm C %

0 0.24 286.00 24.5

2 1.85 42.40 41.5 85

6 2.14 17.76 40.9 94

10 2.39 11.97 39.4 96

15 2.55 9.68 37.7 97

20 2.62 8.65 36.4 97 The formed slurry of CaS0₄ was subsequently filtered on Sartorius 392 grade paper using a Buchner funnel, filtration flask and vacuum pump. The slurry filtered generally fast, leaving a totally clear filtrate. The filter cake was subsequently washed with 40 ml MiliQ water. The filter cake was sucked dry, removed and weighed . The filter cake was subsequently re- dissolved to a homogenous solution, sampled and diluted and analysed for boron content by the Azomethine H procedure used in the previous examples and d isplayed a Boron content amounting to 4.0 % of the orig inally added boron to the reaction solution.

The results show that there is no discernible effect on the percentage of BA in the filter cake as a function of time, and as such a process time for stage A can depend ing on purpose be as low as 10 minutes. EXAMPLE 11

Precipitation of borate with micronized calcium hydroxide.

The same considerations of increased speed as d iscussed above can be made for the Stage B, borate precipitation process. The following experiment was made :

A micronized calcium hyd roxide paste was produced as in the example 8 with 43.1% (w/w) calcium hyd roxide. In a 300 ml beaker with a stir bar, 200.6 g of M iliQ water and 3.49 g of boric acid (BA) (>99%) were mixed to provide a homogenous clear solution at 80°C. 20.2 g micronized calcium hyd roxide paste was added to the hot solution at 80°C and the solution was left to stir for 2 hours. A sample of 4 ml was taken from the solution at the intervals shown below in table 12 and filtered throug h a 0.45 micron Teflon membrane, properly d iluted and analysed for boron content by the Azomethine H method as described in Example 1. In table 12 below is shown the % BA retained in the solution of the orig inally added BA to the reaction pot as a function of time.

TABLE 12

Time % BA

M in retained

0 100.0

10 11.0

20 4.3

30 4.0

40 3.9

50 3.6

60 3.5

120 3.4 The results show that values very close to equilibrium are attained very fast on the order of 20 mins reaction time at 80°C. This is substantially faster than anything previously reported and supports that the processing with advantage is being run with calcium hydroxide with as small particles as possible.

CONCLUSION FROM EXAM PLES It will be appreciated that when the mineral salt is separated out as shown in example 5 and 6, the level of the boron content is so low in the precipitate that the mineral salt

advantageously can be used d irectly for a range of purposes, e.g . as a fertilizer or a soil improver. This has value not only in its use, but also in the economics of the log istics of hand ling the waste. If the mineral acid in the regeneration brine is phosphoric acid and CaHP0₄ 2H₂0 is produced as a solid by-product, this product can be used either directly as a phosphate fertilizer or be further processed to a more soluble phosphate fertilizer. If the mineral acid in the regeneration brine is sulfuric acid, CaS0₄ 2H₂0 (gypsum) is produced as a solid by-product, which can be used for a variety of purposes (soil improver, gypsum wall board etc.) - Due to its high purity, which is on the level of flue-gas derived gypsum, this is a hig hly suitable raw material for the manufacture of gypsum wall board .

It will also be appreciated, that a borate salt produced as in the above examples can be used as a raw material that substitutes mined colemanite. The borate salt may also be used directly as is for the manufacture of a range of borate products currently employed in fertilizers, g lass manufacture, g lass fibre manufacture, detergents, fire retarders, and biocides.

It will also be appreciated that the hig h level of removed boron in the process as described in the 3^rd aspect of the invention is such that the filtrate without any detrimental effects in a water purification application can be slowly bled back into a product water stream or alternatively be fed back to resin capture, hereby efficiently facilitating a Zero Liqu id

Discharge process.

It will also be appreciated, that it has been demonstrated that the processes of Stage A and B can be performed even more effectively by using ultrasonication d irectly in the reaction mixture or by using calcium hyd roxide in as small particles as possible to facilitate a fast process.

Claims

1. A process for preparing a boric acid (BA) containing aqueous solution and/or preparing a mineral acid salt containing precipitate from an initial acidic aqueous solution, wherein the initial acid ic aqueous solution comprises boric acid (BA) and a mineral acid (MA) and has a pH < 5, the method comprising precipitating the MA by add ition of a strong base (StBa) in an amount, which does not exceed the amount of MA acid ic equ ivalents necessary to fully neutralize the solution, to achieve a pH < 7, so as to produce a mineral acid salt precipitate and a BA containing aqueous solution, and subsequently recovering

- said mineral acid salt precipitate, which comprises less than 10% of the boron present in the initial acidic solution, and/or

- said BA containing aqueous solution, which has a BA: MA ratio of at least 10 times the BA: MA ratio of the initial acid ic aqueous solution and/or which has a BA content of at least 90% of the BA content of the initial aqueous solution .

2. The process accord ing to claim 1, wherein the mineral acid salt precipitate comprises less than 9%, 8%, 7%, 6%, 5%, 4% of the BA in the initial acid ic solution, preferably less than 3.5%, and more preferably less than 3.3% .

3. The process accord ing to claim 2, wherein the mineral acid salt precipitate comprises 0.5% - 3%, such as 0.5%-2%, 0.5%-1.5%, and 0.5%-l% of the BA in the initial acid ic solution.

4. The process accord ing to any one of the preced ing claims, wherein the BA containing aqueous solution

- has a BA content of at least 91%, such as at least 92%, at least 93%, at least 94%, and at least 95% of the BA content of the initial aqueous solution and/or

- has a BA: MA ratio, which is at least 15 times, such as at least 20, at least 25, at least 30, at least 35, at least 40, at least 45, and at least 50 times the BA: MA ratio of the initial acid ic aqueous solution.

5. A process for preparing an aqueous solution having a low boron content and/or preparing a boron salt from an initial acid ic aqueous solution, the method comprising 1) preparing a BA containing aqueous solution according to the method of any one of the preced ing claims, and 2) subsequently adding to the BA containing aqueous solution at a temperature of at least 60°C, a strong base (StBa), to precipitate boron from the BA containing aqueous solution, and subsequently

- recovering said aqueous solution having a low boron content, and/or

- recovering said boron salt from the solution.

6. A process for preparing an aqueous solution of low boron content and/or preparing a precipitate containing a boron salt, comprising add ing to a BA containing aqueous solution, which comprises at least 0.90 g/L boron in the form of boric acid, a strong base (StBa) at a temperature of at least 60°C so as to precipitate boron in the form of a salt, and

subsequently

- recovering said aqueous solution of low boron content and/or

- recovering the boron containing precipitate.

7. The method accord ing to claim 6, wherein the BA containing aqueous solution is substantially free from mineral acid .

8. The process accord ing to claim 6 or 7, wherein the pH of the BA containing aqueous solution is at least 4.5, such as at least 5.0.

9. The process accord ing to any one of claims 6-8, wherein the temperature of at least 60°C is at least or about 65°C, such as at least or about 70°C, at least or about 75°C, and at least or about 80°C.

10. The process accord ing to any one of the preced ing claims, wherein

- add ition of the StBa to the initial acidic aqueous solution in any one of claims 1-5 is controlled by not adding further StBa when the pH is at most 7, such as at most 6, preferably at most 5, or

- wherein pH prior to recovery of the mineral acid salt precipitate or the BA containing aqueous solution does not exceed 6.9 or 6.8 or 6.7 or 6.6 or 6.5 or 6.4 or 6.3 or 6.2 or 6.1 or 6.0 or 5.9 or 5.8 or 5.7 or 5.6 or 5.5 or 5.4 or 5.3 or 5.2 or 5.1, and where the pH preferably does not exceed 5 after addition of the StBa to the initial aqueous solution and prior to recovery of the mineral acid salt precipitate or the BA containing aqueous solution.

11. The process accord ing to any one of the preced ing claims, wherein the pH of the initial acid ic aqueous solution is <4, such as <3.5, < 3.0, < 2.5, <2.0, and < 1.5.

12. The process accord ing to claim any one of the preceding claims, wherein the amount of StBa added does not exceed the amount needed to neutralize the MA.

13. The process accord ing to any one of the preced ing claims wherein removing or recovering precipitate comprises filtration or centrifugation.

14. The process accord ing to any one of the preced ing claims, wherein the mineral acid in the initial acidic solution forms a hig hly insoluble salt when reacted with calcium hydroxide or oxide.

15. The process accord ing to claim 14, wherein the mineral acid is phosphoric acid or sulphuric acid .

16. The method accord ing to any one of the preceding claims, wherein the StBa is an alkaline metal oxide.

17. The method accord ing to claim 16, wherein the alkaline metal oxide is selected from the group consisting of calcium hydroxide, calcium oxide, mag nesium oxide, mag nesium hyd roxide, zinc oxide, and zinc hydroxide, Strontium hyd roxide, strontium oxide, barium hyd roxide, barium oxide, Fe(II) hyd roxide, Fe(II) oxide, aluminium hyd roxide, and aluminium oxide.

18. The method accord ing to any one of the preceding claims, wherein the BA containing aqueous solution comprises at least 1.0 g/L boron, such as at least 1.5 g/L, at least 2.0, at least 2.5, and at least 3.0 g/L.

19. The process accord ing to any one of the preced ing claims, wherein the StBa is either

20. The process accord ing to claim 19, where the micronizing influence is u ltrasound treatment.