WO2018052307A1

WO2018052307A1 - Microsilica slurry and method for producing such slurry

Info

Publication number: WO2018052307A1
Application number: PCT/NO2017/050222
Authority: WO
Inventors: Mohamed Al-Bagoury
Original assignee: Elkem As
Priority date: 2016-09-13
Filing date: 2017-09-12
Publication date: 2018-03-22
Also published as: NO20161448A1; MY189848A; NO342672B1

Abstract

The present invention relates to an aqueous microsilica slurry, the microsilica having a particle size less than 1 μm, wherein the slurry contains a polylactic acid as a pH regulator, in an amount of between 0.01 to 10 wt % by the total weight of the slurry. Further, the present invention relates to a method for producing such slurry.

Description

MICROSILICA SLURRY AND METHOD FOR PRODUCING SUCH SLURRY Technical Field

The present invention relates to a microsilica slurry comprising a pH regulator and a method of production of such microsilica slurry.

Background Art

Microsilica (MS) is a co-product of silicon and ferrosilicon alloys. Microsilica slurry as a 50% aqueous dispersion of microsilica particles in water is widely used in applications such as fiber cement, concrete and in oil well cementing. Microsilica acts as pozzolanic material or inorganic binder by interacting with calcium hydroxide, which is a hydration product of Portland cement and water, to improve the compressive strength of the hardened cement matrix. In such applications, microsilica used in slurry form performs much better than the powder form. Most of the commercially available microsilica slurries are normally supplied as 50 wt % microsilica slurries. Microsilica slurry might also be referred to as a dispersion of microsilica.

Microsilica is used in oil well cementing primarily as an anti-gas migration additive. It is used also as an extender to lower the density of the cement slurry. Microsilica reacts with Portland cement quite quickly and forms an impermeable cement matrix.

Therefore, microsilica-containing oil well cement has a very low permeability for gases or liquids and consequently high durability compared to cement without microsilica.

Conventional microsilica slurry suffers from two challenges, namely, settling and gelation.

Settling is a natural phenomenon for many inorganic dispersions such as silica, alumina or titania. According to Stoke' s law, the main factors affecting settling are the low apparent viscosity of the continuous phase (water) and the size of the dispersed particles. The larger the dispersed particles, the faster the settling rate will be.

Generally, a 50 wt % microsilica slurry with a Si0₂ content > 95 wt % has a high potential for settling due to the low apparent viscosity of < 20 mPa.s at a shear rate of 20 s^"1. Settling of coarse particles can be avoided by increasing the viscosity of the slurry by employing thickening agents (viscosifiers) such as xanthan gum, cellulose, clays, polyacrylates or nanosilica. Gelation or network formation by the particles occurs when the particles are attracted to each other due to Van der Waals forces and/or chemical bridging by cations, creating a network structure which can have different forms and strengths. The main factor influencing gelation in the case of a microsilica slurry is contamination with other inorganic metal oxides such as K₂0, Na₂0, CaO, MgO, A1₂0₃ and Fe₂0_3. These oxides occur naturally in microsilica and dissolve to a certain extent into the water, providing the aqueous phase with different types of cations, which tend to make bridges between the particles. The bridging causes the microsilica particles to agglomerate or flocculate which results in a high viscosity and even gel or paste formation in some cases.

Most microsilica slurries are produced with a pH in the range 4-7, to provide good stability. When the pH increases above 7, microsilica slurry is prone to be unstable forming a gel structure. The pH of microsilica slurry is commonly adjusted during the production and post-production using sulphuric acid. However, the pH is subject to change over time depending on the storage conditions. It would be advantageous to be able to keep the pH in the storage containers (for example drum containers or intermediate bulk containers (IBC)) at a stable value below 7 during the whole storage period.

EP 0 246 181A1 relates to microsilica slurries having a microsilica solids content of at least about 50 % by weight, which are stable for extended periods. The slurries have a pH in the range of about 5.0 to 8.5 and are comprised of microsilica, an aqueous carrier medium, an anionic dispersant for the microsilica, and a chelating agent capable of chelating multivalent cationic impurities in the microsilica. A method of preparing the slurries and hydraulic cement compositions containing the same is also described.

Chelating agents such as ethylenediaminetetraacetic acid (EDTA) are not efficient in stabilizing microsilica slurries as they react with the metal ions and form complexes. These metal complexes will remain in the microsilica slurry and can still cause gelation. Dispersants reduce the viscosity of microsilica slurry. However, when the pH increases over time, the dispersant might lose its function and gelation could occur. It is not described use of any in-situ pH regulator/stabilizer.

US 4,321,243 describes a method of stabilizing an aqueous dispersion of silica fume which comprises adding an acid such as acetic acid, hydrochloric acid and sulphuric acid, or ethylenediaminetetraacetic acid and its salts as a chelating agent. US 4,888,058 relates to an aqueous dispersion of silica fume obtained from 70-80 % ferrosilicon metal alloy production and the inclusion of very small amounts of a stabilizing agent selected from a tripolyphosphate, citric acid, hydrofluoric, fluorosilicic acid or their sodium or potassium salts or mixtures thereof.

US 2007/0039733 Al describes the use of a delayed acid-releasing activator for providing a tackifying composition. The tackifying agents are organic polymers, which can form a gel by the presence of acids.

US 2005/0167105 Al describes the use of biodegradable polymers as a coating agent / binder to produce granulates from microfine particles. This is to eliminate handling problems of microfine particles.

It is an object of the present invention to provide an aqueous microsilica slurry, having a stable pH for an extended period of time, preventing problems such as settling and gelation of the slurry during storage.

It is a further object of the present invention to provide an aqueous microsilica slurry which in addition to having a stable pH for an extended period of time is not harmful to the environment.

Short Description of the Invention

In the present invention, it has been found, surprisingly, that by incorporating a polylactic acid (PLA) as a pH stabilizing additive in microsilica slurry, the microsilica slurry remains stable over long storage periods. Polylactic acid slowly releases acid over time, and the acid release may be activated thermally.

According to one aspect of the invention, an aqueous microsilica slurry is provided, the microsilica having a particle size less than 1 μιη, wherein the slurry contains a polylactic acid as a pH regulator, in an amount of between 0.01 to 10 wt % by the total weight of the slurry.

According to an embodiment of the invention, the polylactic acid is present in the range of 0.1 to 5 wt % by the total weight of the slurry. In a further embodiment, the polylactic acid is present in the range of 0.1 to 3 wt % by the total weight of the slurry, in a further embodiment 0.1 to 2 wt % by the total weight of the slurry.

In an embodiment, the polylactic acid is an amorphous polylactic acid. In another embodiment, the polylactic acid is a semi-crystalline polylactic acid. In another embodiment, the polylactic acid is a crystalline polylactic acid. In another embodiment, the polylactic acid is a mixture of amorphous and semi-crystalline polylactic acid. In another embodiment, the polylactic acid is a mixture of amorphous and crystalline polylactic acid. In another embodiment, the polylactic acid is a mixture of semi- crystalline and crystalline polylactic acid. In another embodiment, the polylactic acid is a mixture of amorphous, semi-crystalline and crystalline polylactic acid.

In an embodiment of the present invention, the pH of the slurry is in the range 3 - 7.

In an embodiment, the slurry has a microsilica content in the range 20 to 80 wt % by the total weight of the slurry, in another embodiment 40 to 60 wt % by the total weight of the slurry, in another embodiment 50 wt % by the total weight of the slurry.

In an embodiment, the slurry comprises an acid different from polylactic acid. In an embodiment, the acid is sulphuric acid.

In an embodiment, the slurry further comprises one or more of dispersant(s), viscosifier(s), chelating agent(s) and defoamer(s).

In another aspect, the present invention relates to a method for the production of a microsilica slurry containing water and microsilica, the microsilica having a particle size less than 1 μιη, wherein water, microsilica and a polylactic acid are mixed and the polylactic acid is added in an amount necessary to provide a content of polylactic acid in the final slurry of 0.01 to 10 wt % by the total weight of the slurry.

In an embodiment, the polylactic acid is added to water prior to the addition of the microsilica.

In an embodiment, the polylactic acid is added after microsilica has been mixed with water. In an embodiment, the polylactic acid is added in an amount necessary to provide a content of polylactic acid in the final slurry in the range 0.1 to 5 wt % by the total weight of the slurry, in another embodiment in the range 0.1 to 3 wt % by the total weight of the slurry, in another embodiment in the range 0.1 to 2 wt % by the total weight of the slurry.

In an embodiment, the added polylactic acid is an amorphous polylactic acid. In another embodiment, the added polylactic acid is a semi-crystalline polylactic acid. In another embodiment, the added polylactic acid is a crystalline polylactic acid. In another embodiment, the added polylactic acid is a mixture of amorphous and semi-crystalline polylactic acid. In another embodiment, the added polylactic acid is a mixture of amorphous and crystalline polylactic acid. . In another embodiment, the added polylactic acid is a mixture of semi-crystalline and crystalline polylactic acid. In another embodiment, the added polylactic acid is a mixture of amorphous, semi- crystalline and crystalline polylactic acid.

In an embodiment, the pH of the slurry is adjusted to 3 - 7 by adding an acid different from polylactic acid. In an embodiment, sulphuric acid is added to the slurry.

In an embodiment, the microsilica is added in an amount necessary to provide a microsilica content in the final slurry in the range of 20 to 80 wt % by the total weight of the slurry, in another embodiment 40 to 60 wt % by the total weight of the slurry, in another embodiment about 50 wt % by the total weight of the slurry.

In an embodiment, one or more of dispersant(s), viscosifier(s), chelating agent(s) and defoamer(s) is (are) added.

These and other features, advantages and benefits and objects will become apparent to one of ordinary skill in the art upon careful consideration of the detailed description of representative embodiments of the invention below and the accompanying drawings.

Brief Description of the Drawings

Embodiments of the invention will now be described with reference to the following drawings. Microblock® slurry is a slurry of water and amorphous silica powder produced by Elkem AS. Fig. 1 shows the particle size distribution of microsilica used in Microblock® slurry measured by the light scattering method.

Fig. 2 shows the pH development of Microblock® slurry in different storage drums (D1-D5) without PLA as a function of storage time.

Fig. 3 shows the evolution of the pH of Microblock® slurry with and without PLA at a temperature of 40 °C as a function of time.

Fig. 4 shows the evolution of the pH of Microblock® slurry with and without PLA at a temperature of 50 °C as a function of time.

Detailed Description of the Invention

When preparing microsilica slurry, it is common to add sulphuric acid to lower the pH. However, during storage, the pH will slowly increase and deteriorate the quality of the slurry. The addition of a chemical compound to regulate the pH within a slurry container in-situ (such as a drum container) during the storage could be a way to eliminate the gelation problem. Such additive should release acid slowly and neutralize metal hydroxides that are generated in the slurry during storage.

It has been found, surprisingly, that polylactic acid (PLA) can act as an in-situ slow-acid release material to regulate the pH of microsilica slurry. PLA as an acidic polymer is readily degradable, releasing lactic acid. The degradation of PLA depends on several parameters such as the temperature, pH, the concentration, and the ionic strength of the medium. It also depends on the polymer micro structure, such as the molecular weight, the D-, L- stereoisomer ratio, the particle size of the powder, etc.

Thus, in the present invention, a microsilica slurry is provided which remains stable over time during storage. The pH is kept in the desired range by the presence of polylactic acid in the slurry; said polylactic acid will degrade to lactic acid over time. Even though the pH of a microsilica slurry is adjusted with, for example sulphuric acid during production or afterwards, the pH in a microsilica slurry will gradually increase over time, which causes problems during storage. With the presence of a polylactic acid which degrades to lactic acid over time, the pH in the slurry will remain stable during storage. The term "microsilica" as used in the specification and claims of this application refers to particulate, amorphous Si0₂ obtained from a process in which silica (quartz) is reduced to SiO-gas and the reduction product is oxidized in the vapor phase to form amorphous silica. Microsilica is preferably obtained as a co-product in the production of silicon alloys in electric reduction furnaces.

Microsilica may contain at least 70 wt % silicon dioxide (Si0₂) by the total weight of dry microsilica and preferably > 95 % by the total weight of dry microsilica and has a specific density of 2.1 - 2.3 g/cm 3 and a surface area of 12 - 40 m 27g, typically 20 m 27g. The primary particles are substantially spherical and have an average particle size below 1 μιη; the average particle size may be of about 0.15 μιη. Figure 1 shows the particle size distribution of microsilica used in Microblock® slurry measured by the light scattering method (Malvern Mastersizer 2000). The size distribution of a well- dispersed microsilica in water using high-energy for dispersion and measured by the light scattering method is the following: 10 % of the particles (dio) has a particle size of < 0.07 μιη, 50 % of the particles (d₅o) has a particle size of < 0.15μιη and 90% of the particles (d₉o) has a particle size of < 0.5μιη (Figure 1). The typical range for the average size (d₅o) of microsilica is 0.14-0.18μιη.

As shown in Table 1, microsilica consists mainly of silicon dioxide (Si0₂) with an average content of > 90 % by the weight of dry matter. The remaining associated metals are in a form of oxides such as K₂0, Na₂0, CaO, MgO, A1₂0₃ and Fe₂0₃. The content of these impurities not only depends mainly on the raw materials such as quartz, coke, charcoal and wood chips used for production of silicon alloys but also on the method of operating the furnace such as the silicon yield. The higher the silicon yield, the lower the Si0₂ content in the produced fume.

Table 1 : Typical XRF analysis of microsilica

The presence of such associated metals influences the stability of the aqueous slurry (dispersion) of microsilica to a great extent. Most of these metal oxides are basic oxides. This means that when they dissolve or hydrate into water, they form the corresponding metal hydroxides as the illustrative examples below:

K₂0 + H₂0→ 2 KOH (Equation 1)

CaO + H₂0→ Ca(0H)₂ (Equation 2)

Al₂0₃ + 3H₂0→ 2Al₂ (OH)₃ (Equation 3)

The dissolution or hydration of metal oxides present in microsilica is a quite complex process and is relatively slow at ambient temperature. Figure 2 shows the increase in pH of Microblock® slurry without PLA in 5 drum tanks, Dl - D5, as a function of time. pH was adjusted with sulphuric acid during the slurry production. The composition of the slurries in the drums was almost similar. The drums were stored indoor in the

Netherlands for 12 months, with storage temperatures in the range 15 - 30 °C. From Figure 2, it is seen that the pH slowly increases during the whole storage time. The dissolution rate of the metal oxides depends on the surrounding temperature, the solids content in the slurry, and the total metal oxides in the microsilica. At elevated temperature >40 °C, the rate of dissolution, and consequently, the increase of pH are quite fast.

Some of the formed hydroxides, such as Al₂(OH)3, polymerize and form bulky cationic oligomers, which may cause flocculation of the silica particles. The association of microsilica particles by the chemical bridging / electrostatic interactions through the cations creates a network structure, which can have different forms and strengths. The gel strength depends on the pH and the total cations present in the slurry. The gel structure can be quite firm, which causes the slurry to become unpourable or unusable. Thus, the generated metal hydroxides as described in equations 1-3 should be neutralized.

In the present invention, an aqueous slurry of amorphous silica is provided, with an in- situ pH regulator/stabilizer for keeping the pH at a desired range, particularly a pH range 3 to 7 or 3.5 to 6.5 or 4 to 6. The pH regulator is a polylactic acid, in an amount of between 0.01 to 10 wt % by the total weight of the slurry, or 0.1 to 5 wt % by the total weight of the slurry, or 0.1 to 3 wt % by the total weight of the slurry, or 0.1 to 2 wt % by the total weight of the slurry. The slurry can have a microsilica content in the range 20 to 80 wt % by the total weight of the slurry, or 40 to 60 wt % by the total weight of the slurry, or about 50 wt % by the total weight of the slurry. The slurry might also comprise sulphuric acid in an amount sufficient to lower the pH to a desired value.

Further, a method for the production of an aqueous microsilica slurry is provided, the microsilica having a particle size less than 1 μιη, wherein water, microsilica and a polylactic acid are mixed and wherein the polylactic acid is added in an amount necessary to provide a content of polylactic acid in the final slurry of 0.01 to 10 wt % by the total weight of the slurry. The microsilica slurry may be produced using a high shear mixer.

Polylactic acid (PLA) is an aliphatic polyester produced from renewable resources. There are two main different polymerization methods to produce PLA:

1) ring opening polymerization of L-lactide, and

2) polycondensation of lactic acid.

The lactic acid feedstock used for the production of PLA is produced industrially either by chemical or by fermentation processes. Fermentation with lactic acid bacteria produces ca. 100.000 tons of lactic acid per year. PLA is not only commercially available as a homopolymer but also as copolymers with other monomeric materials such as ε-caprolactone, glycolide, σ- valerolactone, and trimethylene carbonate, poly(ethylene oxide )(PEO) and poly(ethylene glycol) (PEG).

PLA is a biodegradable polymer which readily degrades into lactic acid. The IUPAC name of lactic acid is 2-hydroxypropanoic acid. Lactic acid has the chemical formula C₃H₆0₃ (CH₃CH(OH)C0₂H), and its molar mass is 90.08 g/mol. PLA degrades thermally upon heating either in dry form or in liquid mixture. The degradation of PLA depends on the temperature and the pH of the medium. The higher the temperature, the faster the degradation rate becomes. At a temperature of 20°C, the degradation is relatively slow and can take years to be completely degraded. However, at a

temperature > 40 °C certain PLA grades can degrade within a few months. At a temperature above 100 °C, most of PLA degrades within a few days. The degradation of PLA at low pH is rather slow. However, at pH 7 the hydration and the degradation of PLA are faster than in an acidic medium. Thus, PLA present in microsilica slurry will degrade when the temperature and the pH in the slurry increases. When the pH increases above 7, gelation and stability problems of the slurry occur. Microsilica slurry forms a gel when the temperature and pH increase. If microsilica slurry is stored at temperatures above 30 °C, such as in the range 35 to 60 °, or in the range 35 to 55 °C, or in the range 40 to 50 °C, the presence of PLA degrading into lactic acid will prevent the pH of the slurry from increasing, and prevent the slurry from forming a gel.

The degradation of polylactic acid depends among others on its internal microstructure. Lactic acid (C₃H₆0₃) as an alpha hydroxy (a-hydroxy) acid is a chiral molecule, which is an optically active compound. It is found as two different isomer forms by means of L- and D-lactic acid. D-lactic acid (dextro isomer) rotates the plane of polarized light clockwise. L-lactic acid (levo isomer) rotates the plane of polarized light

counterclockwise. The ratio of D- and L- isomers in the PLA structure and the thermal history during the processing control the degree of crystallinity and consequently the degree of thermal degradation of the compound. Pure D- or L-PLA is a highly crystalline compound and degrades very slowly. However, PLA made from a mixture of D- and L-isomers is a semi-crystalline or amorphous compound and degrades rather readily at low temperature. Generally, PLA containing >10 mol.-% of D-isomer is considered to be an amorphous material. Different types of PLA with different degrees of crystallinity can be used to regulate the pH of microsilica slurry. In the present invention, PLA 1 and PLA 2 are amorphous L-PLAs with a high percentage of D- isomer (> 10 mol.-%). PLA 3 is a semi-crystalline L-PLA with a low percentage of D- isomer (< 10 mol.-%). Polylactic acids are commercially available products by various suppliers. Examples of such amorphous and crystalline PLA polymers are Ecorene PLA from A. Schulman or Ingeo grade PLA polymer from NatureWorks LLC. Examples of crystalline grade PLA is Ecorene 31 and examples of amorphous PLAs are Ecorene 61 and Ecorene 80 from A. Schulman.

Lactic acid reacts with metal hydroxides in the following manner to stabilize microsilica slurry:

KOH + CH₃CH(OH)C0₂H→ CH₃ CH(OH)C0₂K + H₂0 (Equation 4)

CaO + 2CH₃CH(OH)C0₂H→ (CH₃ CH(OH) C0₂)₂Ca + 2H₂0 (Equation 5)

Al₂ (OH)₃ + 3CH₃CH(OH)C0₂H→ (CH₃ CH(OH)C0₂)₃Al + 3H₂0 (Equation 6)

PLA is available in powder form with particle sizes in the range 1-500μιη. PLA powder can be introduced in the microsilica slurry before or after mixing the water and microsilica: a) PLA can be added first into water and homogenized using a mixing device. Then microsillica and optionally other additives are added afterwards. b) The microsilica slurry can be prepared first by adding dry microsilica into water and optionally other additives and then let it stabilize for days/weeks. At the end, dry PLA can be added to the stabilized slurry.

Preferably, a high shear mixer is used to prepare the slurry initially, and to mix in any additional components.

Polylactic acid is added in an amount necessary to provide a content of polylactic acid in the final slurry in the range 0.01 to 10.0 wt % by the total weight of the slurry, or 0.1 to 5 wt % by the total weight of the slurry, or 0.1 to 3 wt % by the total weight of the slurry, or 0.1 to 2 wt % by the total weight of the slurry. The amount of water and the amount of solids are arranged so that the solids content of the slurry is in the range 20 to 80 wt % by the total weight of the slurry, 40 to 60 wt % by the total weight of the slurry, or about 50 wt % by the total weight of the slurry.

The pH of the slurry is adjusted to a value in the range 3 to 7, or 3.5 to 6.5, or 4 to 6, by adding an acid different from polylactic acid to the slurry.

Sulphuric acid might be added during the production of the microsilica slurry.

The microsilica slurry might also contain additives such as dispersants, viscosifiers, chelating agents and defoamers. For example, a dispersant may be employed to eliminate any undesirable interactions between the silica particles that might be caused by the various metal oxides that exist in the slurry. A viscosifier for aqueous dispersion can be water-soluble polymers, inorganic clays or nanomaterials such as nanosilica. Examples of water-soluble polymers are xanthan and guar gum, cellulose and cellulose derivatives like methyl cellulose (MC), ethyl cellulose (EC), hydroxyethyl cellulose (HEC), hydroxypropyl cellulose (HPC), hydroxypropylmethyl cellulose (HPMC), carboxymethyl cellulose (CMC) and sodium carboxymethyl cellulose (NaCMC), synthetic polymers such as acrylamides, polyamines, polyethyleneimines, and quaternary ammonium polymers, polyvinyl alcohol (PVOH), polyacrylic acid and copolymers, Polyvinylpyrrolidone (PVP). Examples of inorganic clay viscosifiers are sepiolite, bentonite and attapulgite.

There are several acids and chemicals which can be used in combination with PLA. Any inorganic acids such as sulphuric acid and phosphoric acid may be used. Any organic acids such as carboxylic acids or sulphonic acids may be used. Examples of such organic acids are formic acid, acetic acid, citric acid, tartaric acid, oxalic acid, benzoic acid, sulphonic acid and alkyl sulphonic acids. In addition, any dispersants such as polyacrylic based dispersant, lignosulphonate, etc., may be used.

In an embodiment, the pH value of the slurry is in the range 3.0 to 7, in another embodiment in the range 3.5 to 6.5, in another embodiment in the range 4 to 6.

The concentration of polylactic acid needed in order to stabilize microsilica slurry depends on the composition of microsilica used in slurry production. A typical PLA content would be less than 10 wt % by the total weight of microsilica slurry. Preferably, the polylactic acid content is less than 2 wt % by the total weight of the slurry.

Polylactic acid and its derivatives can be added directly into water prior to the addition of microsilica. It can also be added after the microsilica is added.

Examples

Unless stated otherwise, the microsilica slurry used in the examples was Microblock ® slurry, which is a slurry of water and amorphous silica powder produced by Elkem AS.

PLA 1 used in the examples was Ecorene 61 from A Schulman. PLA 2 used in the examples was Ecorene 80 from A Shulman. PLA 3 used in the examples was Ecorene 31 from A. Schulman.

Example 1

500 g microsilica were mixed with 500 g water in a Warring blender for 5 min at a rotation speed of 12000 rpm. The slurry was then transferred into a container and the viscosity, solid content and pH were measured. Subsequently, the slurry was divided into two parts. Then, 500 g of microsilica slurry were taken into a new container and treated with diluted sulphuric acid to adjust the pH to 4. To the other 500 g of microsilica slurry, 5 g of PLA 2, where PLA 2 is as defined above, were added while mixing in a Warring blender for 2 min at a rotation speed of 12000 rpm and pH was adjusted to 4. The slurries were stored in an oven at a temperature of 40 °C and the pH of the slurry with and without PLA 2 was monitored over time. After 6 months of storage, the pH of the slurry without PLA 2 was 8.35 while the pH of the slurry with PLA 2 was 3.45.

Example 2

Two different types of polylactic acid; PLA 2 and PLA 3, as defined above, were used.

5 g of PLA 2 were added to 500 g water and mixed for 5 min at high speed 12000 rpm. Then, 500 g microsilica were added to the PLA 2 solution. Sulphuric acid was added to lower the pH to 4. The preparation was the same for slurries containing PLA 3. The pH and the viscosity were monitored over time. A polylactic acid concentration of lwt % by the weight of microsilica slurry was used for both slurries (PLA 2 and PLA 3). The slurries were kept in a heating oven at a temperature of 40 °C. Figure 3 shows the pH evolution as a function of time for the microsilica slurry without polylactic acid as a reference, and with PLA 2 and with PLA 3. PLA 3 did not show any decomposition at a temperature of 40 °C even after 12 weeks, while PLA 2 started to decompose and release lactic acid after week 7 causing a reduction in pH. PLA 3 has a high degree of crystallinity compared to PLA 2 and therefore it degrades slower than PLA 2. This means that if the storage temperature is expected to be around 40 °C then amourphous PLA (PLA 2) is a good candidate.

Example 3

In this example, microsilica with low Si0₂ content of 92.2 wt % is used to demonstrate the effect of polylactic acid in controlling the slurry properties. Table 2 shows the composition of the three slurries prepared without polylactic acid (slurry A), with a polycarboxylate ether dispersant (slurry B) and with PLA 2, where PLA 2 was as defined above, and a polycarboxylate ether dispersant (slurry C). The slurries were stored in the lab at 50°C for 60 days.

Table 2: Composition and properties of microsilica slurry prepared without polylactic acid and dispersant (slurry A), without PLA but with dispersant (slurry B) and with a combination of PLA 2 and a dis ersant (slurr C).

*) The viscosity was measured using Physica Rheometer MCR - Anton Paar with Couette geometry CC27 at shear rate of 20s^"1 and a temperature of 20 °C.

As shown in Table 2, the viscosity and pH of the slurries A and B increased after storing in the lab at 50 °C for 60 days. Slurry C with PLA 2 maintained the viscosity significantly low to about 32 mPa.s and the pH dropped from 4.3 to 3.3. This example shows the usefulness of polylactic acid in controlling the slurry properties.

Example 4

Two different types of polylactic acid; PLA 1 and PLA 2, as defined above, were used.

5 g of PLA 1 were added to 500 g water and mixed for 5 min at high speed 12000 rpm. Then, 500 g microsilica were added to the PLA 1 solution. Sulphuric acid was added to lower the pH to 4. The preparation was the same for slurries containing PLA 2. The pH and the viscosity were monitored over time.

Figure 4 shows the pH evolution as a function of time for microsilica slurry samples prepared with PLA 1, PLA 2 and without polylactic acid. In addition, two different concentrations of PLA 1 and PLA 2 were tested; 0.5 wt % and lwt % by the weight of microsilica slurry. The slurries were stored in a heating oven at a temperature of 50 °C. The decompositions of PLA 1 and PLA 2 were faster at 50 °C, compared to the decompositions at 40 °C. PLA 2 degraded faster than PLA 1. PLA 2 started to decompose after 3 weeks while PLA 1 started to decompose after 6 weeks. In addition, a polylactic acid concentration of lwt % produces more acid compared to a lower concentration of 0.5 wt %. In use, when a large temperature fluctuation during storage is expected, a combination of various grades of polylactic acid can be used. As can be seen from Figure 4, samples comprising polylactic acid tested for 10 weeks showed good effect on stability.

Example 5

A microsilica slurry containing PLA 1, where PLA 1 was as defined above, prepared using the procedure described above was tested in oil well cementing. Microsilica slurry is used for oil well cement in a content in the range of 10-30 % by weight of cement (BWOC). The addition of chemicals such as polylactic acid to microsilica slurry might have an impact on the properties of cement slurry such as viscosity, fluid loss, compressive strength, and thickening time. To assess such effects, microsilica slurry comprising PLA 1 with a dosage of lwt % by the weight of microsilica slurry was tested in cement slurry and compared with microsilica slurry without polylacti acid. A cement test using microsilica slurry with PLA 1 according to the invention was conducted according to the API 10 standard. The cement formulation shown in Table 3 was used to prepare cement slurry with a density of 1.89 g/ml.

Table 3: Composition of oil well cement containing Microblock® slurry and oil well cement containing Microblock® slurry comprising PLA 1.

The additives such as dispersant, fluid loss additive, retarder and defoamer are common chemicals for the formulation of oil well cement.

The following equipment was used to prepare and characterize the cement slurry:

Chandler Fann 35 rheometer with thermo-cup, consistometer, equipment for measuring fluid loss (HTHP), ultrasonic compressive strength analyzer (UCA) from Chandler, constant-speed Warring mixer, 200 - 250 ml measuring cylinder and precision balance.

The results set out in Table 4 show that the viscosity properties of the two cement slurries measured at 20 and 85 °C are quite similar. The fluid losses are also similar. This indicates that the addition of PLA 1 as a pH regulator has no or negligible effect on the cement slurry properties. The compressive strength for the two cement slurries after 12 and 24 hours is quite similar. Table 4: Viscosity and fluid loss of oil well cement.

The experimental work showed that polylactic acid degrades slowly in aqueous microsilica slurries/dispersions, generating lactic acid that contributes to stabilizing the pH of the slurry/dispersion and improves the slurry/dispersion stability over time. The degradation at temperatures such as 30-50 °C makes polylactic acid a suitable material to stabilize microsilica slurry. The test of microsilica slurry containing polylactic acid in oil well cement showed that the microsilica slurry containing polylactic acid has no detrimental effect on the cement slurry properties.

Polylactic acid may be used as a pH regulator in microsilica slurry in different forms, such as in crystalline form, semi-crystalline form, amorphous form, in one of the forms alone or in mixtures of said forms, such as a mixture of crystalline and semi-crystalline forms, a mixture of crystalline and amorphous forms, a mixture of semi-crystalline and amorphous forms or a mixture of crystalline, semi-crystalline and amorphous forms.

Having described preferred embodiments of the invention it will be apparent to those skilled in the art that other embodiments incorporating the concepts may be used. These and other examples of the invention illustrated above are intended by way of example only and the actual scope of the invention is to be determined from the following claims.

Claims

Claims 1.

An aqueous microsilica slurry, the microsilica having a particle size less than 1 μιη, c h a r a c t e r i z e d i n that the slurry contains a polylactic acid as a pH regulator, in an amount of between 0.01 to 10 wt % by the total weight of the slurry.

2.

Microsilica slurry according to claim 1, wherein the polylactic acid is present in the range of 0.1 to 5 wt % by the total weight of the slurry.

3.

Microsilica slurry according to claim 2, wherein the polylactic acid is present in the range of 0.1 to 3 wt % by the total weight of the slurry, or 0.1 to 2 wt % by the total weight of the slurry.

4.

Microsilica slurry according to any of the preceding claims, wherein the polylactic acid is an amorphous polylactic acid, or a semi-crystalline polylactic acid, or a crystalline polylactic acid, or a mixture of amorphous and semi-crystalline polylactic acid, or a mixture of amorphous and crystalline polylactic acid, or a mixture of semi-crystalline and crystalline polylactic acid, or a mixture of amorphous, semi-crystalline and crystalline polylactic acid.

5.

Microsilica slurry according to any of the preceding claims, wherein the pH of the slurry is in the range 3 - 7.

6.

Microsilica slurry according to any of the preceding claims, wherein the slurry has a microsilica content in the range 20 to 80 wt % by the the total weight of the slurry, or 40 to 60 wt % by the total weight of the slurry, or about 50 wt % by the total weight of the slurry.

7.

Microsilica slurry according to according to any of the preceding claims, wherein the slurry comprises an acid different from polylactic acid.

8.

Microsilica slurry according to claim 7, wherein the acid is sulphuric acid.

9.

Microsilica slurry according to according to any of the preceding claims, wherein the slurry further comprises one or more of dispersant(s), viscosifier(s), chelating agent(s) and defoamer(s).

10.

Method for the production of an aqueous microsilica slurry, the microsilica having a particle size less than 1 μιη, c h a r a c t e r i z e d i n that it comprises mixing of water, microsilica and a polylactic acid, and that the polylactic acid is added in an amount necessary to provide a content of polylactic acid in the final slurry of 0.01 to 10 wt % by the total weight of the slurry.

11.

Method according to claim 10, wherein the polylactic acid is added to water prior to the addition of microsilica.

12.

Method according to claim 10, wherein the polylactic acid is added after microsilica has been mixed with water.

13.

Method according to any of the claims 10 to 12, wherein the polylactic acid is added in an amount necessary to provide a content of polylactic acid in the final slurry in the range 0.1 to 5 wt % by the total weight of the slurry, or in an amount in the range 0.1 to 3 wt % by the total weight of the slurry, or in the range 0.1 to 2 wt % by the total weight of the slurry.

14.

Method according to any of the claims 10 to 13, wherein the added polylactic acid is an amorphous polylacic acid, or a semi-crystalline polylactic acid, or a crystalline polylactic acid, or a mixture of amorphous and semi-crystalline polylactic acid, or a mixture of amorphous and crystalline polylactic acid, or a mixture of semi-crystalline and crystalline polylactic acid, or a mixture of amorphous, semi-crystalline and crystalline polylactic acid.

15.

Method according to any of the claims 10 to 14, wherein microsilica is added in an amount necessary to provide a microsilica content in the final slurry in the range of 20 to 80 wt% by the total weight of the slurry, or 40 to 60 wt % by the total weight of the slurry, or about 50 wt % by the total weight of the slurry.

16.

Method according to any of the claims 10 to 15, wherein the pH of the slurry is adjusted to 3 - 7 by adding an acid different from polylactic acid.

17.

Method according to claim 16, wherein sulphuric acid is added to the microsilica slurry.

18.

Method according to any of the claims 10 to 17, wherein one or more of dispersant(s), viscosifier(s), chelating agent(s) and defoamer(s) is (are) added.