GB2602053A - A process of making a lime slurry - Google Patents

Abstract

A process of making a lime slurry, which is suitable for adjusting the pH of water during water treatment, the process comprising the steps of: (a) contacting solid particulate calcium-rich material with water to form a calcium hydroxide slurry, wherein the solid particulate calcium-rich material comprises: (i) from 90wt% to 99.9997wt% calcium material selected from calcium hydroxide and/or calcium oxide; and (ii) from 0.0003wt% to 0.001wt% manganese cations; wherein greater than 95wt% of the solid particulate calcium-rich material has a particle size of less than 100µm, wherein the calcium hydroxide slurry has a solids content of from 1.0wt% to 25wt%, and wherein the calcium hydroxide slurry comprises from 0.90wt% to 25wt% calcium hydroxide; and (b) contacting the calcium hydroxide slurry with an oxidising material; (c) reacting the slurry obtained in step (b) so as to form water-soluble oxidised manganese cations and purified calcium hydroxide solid material; (d) separating the purified calcium hydroxide from the slurry to form purified calcium hydroxide wet cake and a liquid aqueous phase comprising the water-soluble oxidised manganese cations; and (e) contacting water to the purified calcium hydroxide wet cake to form a slurry, wherein the purified calcium hydroxide slurry comprises less than 0.0003wt% manganese cations.

Description

A process of making a lime slurry

Field of the invention

The present invention relates to the processing and purification of calcium hydroxide slurries, referred to as "lime slurries-which are used for water treatment. Lime slurries are very widely used to adjust the pH of water, whether for treatment of water for human consumption or for treatment of wastewater. The present invention relates to a process for the preparation of lime slurries that are suitable for use in treating water.

Background of the invention

The lime slurries used in water treatments are typically prepared from limestone or chalk. Lime slurries are very often added to drinking water treatments so as to raise the pH in a safe way. For example, one of the final steps in a water treatment process for the production of drinking water is a final adjustment of the pH using lime slurry (after the various earlier treatments done previously in the water works).

Typically, calcium oxide and/or calcium hydroxide is mixed with water to form the lime slurry. These calcium rich materials are typically sourced from chalk and limestone For example, the calcium rich material can be formed by the calcination of chalk and/or limestone (both sources of calcium carbonate).

Chalks and limestones contain very high levels of calcium carbonate as a result of the conditions under which the rocks were originally formed. However, any chalk or limestone will inevitably contain other ingredients. Some, such as the silica, are relatively easily removed during milling and processing by techniques such as spectroscopic analysis of materials on moving belts. Larger fragments of materials can be identified and physically removed from the rest of the chalk or limestone.

However, any natural source of calcium carbonate will contain metal species, typically in the form of very small crystals dispersed throughout the calcium carbonate source. These metal species carry over from the original calcium carbonate source during processing and end up in the calcium rich material.

These metal species being present in the calcium rich material, especially manganese cations, can cause problems when the calcium rich material is used to form a lime slurry that is used to treat water, especially when the manganese cations are present at a high level. These manganese cations can restrict the initial sources (e.g. quarries or strata) of chalk and limestone that are suitable for making the lime slurry. It is obviously beneficial if a wider range of sources of chalk and lime, such as different quarries or strata of material, could be used as raw materials to produce lime slurries for water treatment, especially for drinking water.

Manganese is one of the chemical species very often present in water from wells and other sources of raw water in the form of soluble 1\/In211 ions. Most water treatment plants remove it by one technique or another. Common techniques are based on oxidation from Mn(II) to Mn(IV), causing precipitation of solid Mn02, followed by some method of filtration. This is done chemically or by using bacteria to oxidise and precipitate NIn02.

Some manganese is beneficial to health and drinking water can contain low levels without issue. However, a problem can arise if the levels of soluble manganese species in drinking water is high. Soluble manganese can be precipitated by the actions of bacteria on the insides of water pipes. If this sediment is disturbed, it can cause discolouration of the water causing an undesirable phenomenon known as "black water". There is no risk to human health but consumer reaction to "black water" coming from their pipes is strong and negative. Control of manganese levels in the water leaving a treatment works is therefore important.

This issue of managing manganese levels in water and avoiding "black water" can be problematic if the lime slurry that treated the water itself contains high levels of manganese. There is little point in removing manganese earlier in the water treatment process only to introduce it back into the water right at the very end of the water treatment process during any lime slurry treatment step. As a result, the lime slurry needs to have a low level of manganese cations.

There is therefore a need for a process that can reduce the level of manganese cations in a lime slurry, prior to the slurry's use in water treatment. It is beneficial if such a process is simple to carry out, environmentally friendly and energy efficient The present invention provides such a process that, instead of aiming to precipitate manganese, aims to oxidise manganese cations to form water-soluble oxidized manganese cations. This is done by careful control of reaction conditions such as pH, temperature and time. The water-soluble manganese dissolved in the aqueous liquid phase which can easily be removed from the calcium rich solid material during the production of the lime slurry. The resulting lime slurry has a low level of manganese and avoids issues such as "black water" when used to treat water.

Summary of the invention

The present invention provides a process of making a lime slurry, wherein the lime slurry is suitable for adjusting the pH of water during water treatment, wherein the process comprises the steps of: (a) contacting solid particulate calcium-rich material with water to form a calcium hydroxide slurry, wherein the solid particulate calcium-rich material comprises: (i) from 90wt% to 99.9997wt% calcium material selected from calcium hydroxide and/or calcium oxide; (ii) from 0.0003wt% to 0.001wt% manganese cations; and (iii) optionally, other material, and wherein greater than 95wt% of the solid particulate calcium-rich material has a particle size of less than 100jtm, wherein the calcium hydroxide slurry has a solids content of from 1.0wt% to 25wt%, and wherein the calcium hydroxide slurry comprises from 0.90wt% to 25wt% calcium hydroxide; and (b) contacting the calcium hydroxide slurry from step (a) with an oxidising material, wherein greater than 0.1gram oxidising material per gram of solids content is contacted to the calcium hydroxide slurry; (c) reacting the slurry obtained in step (b) for more than 5 minutes at an elevated temperature in the range of from greater than 20°C to less than 95°C and at a pH of greater than 12.0 so as to form water-soluble oxidised manganese cations and purified calcium hydroxide solid material; (d) optionally, repeating steps (b) and (c); (e) separating the purified calcium hydroxide solid material from the remainder of the slurry so as to form purified calcium hydroxide wet cake and a liquid aqueous phase comprising the water-soluble oxidised manganese cations; (0 contacting water to the purified calcium hydroxide wet cake to form a purified calcium hydroxide slurry, wherein the purified calcium hydroxide slurry has a solids content of from 18wt% to 50wt%, wherein the purified calcium hydroxide slurry comprises from 16.2wt% to 50wt% calcium hydroxide, and wherein the purified calcium hydroxide wet cake comprises less than 0.0003wt% manganese cations; and (g) optionally, repeating steps (b) to (0; to form the lime slurry.

Detailed description of the invention

Process of making a lime slurry. The process of making a lime slurry comprises the steps of: (a) contacting solid particulate calcium-rich material with water to form a calcium hydroxide slurry, wherein the particulate calcium-rich material comprises: (i) from 90wt?/"0 to 99.9997wt% calcium material selected from calcium hydroxide and/or calcium oxide; from 0.0003wt% to 0.001wt% manganese cations; and (Hi) optionally, other material, wherein greater than 95wt% of the solid particulate calcium-rich material has a particle size of less than 100Rm, wherein the calcium hydroxide slurry has a solids content of from 1.0wt% to 25wt%, and wherein the calcium hydroxide slurry comprises from 0.90wt°/0 to 25wt% calcium hydroxide; and (b) contacting the calcium hydroxide slurry from step (a) with an oxidising material, wherein greater than 0.1gram oxidising material per gram of solids content is contacted to the calcium hydroxide slurry; (c) reacting the slurry obtained in step (b) for more than 5 minutes at an elevated temperature in the range of from greater than 20°C to less than 95°C and at a pH of greater than t2.0 so as to form water-soluble oxidised manganese cations and purified calcium hydroxide solid material; (d) optionally, repeating steps (b) and (c), (e) separating the purified calcium hydroxide solid material from the remainder of the slurry so as to form purified calcium hydroxide wet cake and a liquid aqueous phase comprising the water-soluble oxidised manganese cations; (0 contacting water and optionally other, such as viscosity reducing and suspension agents, to the purified calcium hydroxide wet cake to form a purified calcium hydroxide slurry, wherein the purified calcium hydroxide slurry has a solids content of from 8wt% to SOwt%, wherein the purified calcium hydroxide slurry comprises from 16.2wt% to 50wt% calcium hydroxide, and wherein the purified calcium hydroxide comprises less than 0.0003 91) manganese cations; (g) optionally, repeating steps (b) to (0, to form the lime slurry.

It may be preferred to add viscosity reducing agents to the lime slurry. This will reduce the viscosity of the lime slurry and will help to enable higher solids concentrations in the final slurry without excessive viscosities. Suitable viscosity reducing agents include surfactants and/or metal hydroxides. Suitable surfactants include SLS and sodium lignosulphonate. The lime slurry may comprise from 0.1wt% to 4.0wt% viscosity reducing agent, or from 0.2wt% to 4wt%, or from 0.5wt(11) to 3wt% viscosity reducing agent.

It may also be preferred to add suspension aids to the lime slurry. Suitable suspension aids include polycarboxylate polymers. The lime slurry may comprise from 0.1wt% to 3.0wt% suspension aid.

A particularly preferred process uses at least some calcium oxide as the solid particulate calcium rich material such that the heat of hydration of the calcium oxide provides the heat necessary to elevate the temperature of the calcium hydroxide slurry to elevated temperatures.

The process can take place in one reaction vessel or the mix from one reaction vessel can be passed to another reaction vessel in the form of a sequential cascade The conditions in the different vessels can be different. For example, the temperature in sequential vessels can be allowed to decline so as to assist with temperature control of the final slurry.

Step (a) forming a calcium hydroxide slurry. Step (a) contacts solid particulate calcium-rich material with water to form a calcium hydroxide slurry. The solid particulate calcium rich material is described in more detail later. The calcium hydroxide slurry is described in more detail later.

Other ingredients can also be contacted to the solid particulate calcium-rich material and water during step (a). Suitable other materials can include inorganic salts, such as sodium sulphate and/or sodium hydroxide.

Step (b). Step (b) contacts the calcium hydroxide slurry from step (a) with an oxidising material, wherein greater than 0.1gram oxidising material per gram of solids content is contacted to the calcium hydroxide slurry.

It may be preferred that the oxidising material is ozone, and wherein the rate of ozone addition to the calcium hydroxide slurry during step (b) is greater than 0.12kg/hr per kg slurry solids. Ozone is typically continuously generated and used by an ozone generator due to its unstable nature.

When the oxidising material is ozone, it is especially preferable if the water used to form the slurry is tap water or ground water and has a conductivity of greater than 50 mhos/cm. It has been found that if low conductivity water is used, such as distilled water, the rate of solubilisation of the manganese is very significantly reduced.

Step (c). Step (c) reacts the slurry obtained in step (b) for more than 5 minutes at an elevated temperature in the range of from greater than 20°C to less than 95°C and at a pH of greater than 12.0 so as to form water-soluble oxidised manganese cations and purified calcium hydroxide solid material.

Preferably, step (c) takes place at an elevated temperature in the range of from 40°C to 80°C, or from 40°C to 60°C.

It may be preferred that the particulate calcium-rich material comprises calcium oxide, and wherein at least part of the energy required to raise the calcium hydroxide slurry to the elevated temperature required in step (c) is generated by contacting the calcium oxide with water to form the calcium hydroxide slurry during step (a).

The step (c) oxidises accessible insoluble manganese cations to water-soluble manganese cations, such as permanganate (N4n(VII)) cations. It is preferred if the solids content of the slurry in step (b) and step (c) is not too high as the high solids loading may interfere with the mixing and contacting of the oxidising material.

It is preferred that during step (c) contact of the oxidising material with other oxidisable metal ion sources, such as process equipment like stainless steel impellors is minimised or avoided. Techniques or equipment that achieve this are preferred.

The elevated temperature of step (c) is an important feature of the present inventive due to the highly beneficial effect on the rate of solubilisation of manganese species. Without the use of an elevated temperature the reaction rate would be so slow as to limit the production rate of the slurry or require excessively large reaction vessels.

Step (d). Step (d) is optional and repeats steps (b) and (c) It may be preferred that repeats of steps (b) and (c) are carried out in different reactor vessels.

It may be preferred that repeats of steps (b) and (c) are carried out using the same conditions of solids concentration, time, temperature and amounts of oxidising material used Step (e) forming a purified calcium hydroxide wet cake. Step (e) separates the purified calcium hydroxide solid material from the remainder of the slurry so as to form purified calcium hydroxide wet cake and a liquid aqueous phase comprising the water-soluble oxidised manganese cations.

It may be preferred that step (e) is carried out by filtration.

During step (e), the slurry from step (c) is typically filtered and/or centrifuged to form a purified calcium hydroxide wet cake and a liquid aqueous phase. The purified calcium hydroxide wet cake typically has a manganese content that is lower than the initial manganese concentration of the slurry.

Step (f). Step (f) contacts water to the purified calcium hydroxide wet cake to form a purified calcium hydroxide slurry.

Step (g). Step (g) is optional and repeats steps (b) to (O.

It may be preferred that steps (b) to (f) are repeated until the light absorbance (measured at a wavelength between 510 and 595nm) of the liquid aqueous phase formed in a final step (e) shows an absolute difference of less than 0.05 compared to the liquid aqueous phase formed in the preceding step (e) when measured using the same cell. Suitable spectrometers are supplied by Shimadzu, such as the Shimadzu UV-1900 UV-Vis spectrometer.

Particulate calcium-rich material. The particulate calcium-rich material comprises: (i) from 90wt% to 99.9997wt% calcium material selected from calcium hydroxide and/or calcium oxide; (ii) from 0.0003wt% to 0.001wt% manganese cations; and (iii) optionally, other material.

Greater than 95wt%, preferably all, of the solid particulate calcium-rich material has a particle size of less than 100pm Calcium hydroxide slurry. The calcium hydroxide slurry has a solids content of from lwt% to 25wt%. The calcium hydroxide slurry comprises from 0.90wt% to 25wt% calcium hydroxide.

Typically, the calcium hydroxide slurry is pumpable. Typically, the calcium hydroxide slurry has a viscosity of less than 1000cps.

Oxidising material. Preferably, the oxidising material has an oxidative potential of greater than 0.9V when compared to a standard hydrogen electrode.

Preferably, the oxidising material is selected from sodium hypochlorite, hydrogen peroxide, ozone, or any combination thereof Purified calcium hydroxide slurry. The purified calcium hydroxide slurry has a solids content of from 18wt% to 50wt%. The purified calcium hydroxide slurry comprises from 16.2wt% to 50wt% calcium hydroxide. The purified calcium hydroxide slurry comprises less than 0.0003wt% manganese cations.

Typically, the purified calcium hydroxide slurry is pumpable. By pumpable, it is typically meant that the viscosity is less than 1000 cps. Typically, the purified calcium hydroxide slurry has a viscosity of less than 1000cps.

Manganese cations. The magnesium cations comprised by the particulate calcium-rich material are typically in the oxidative states (II), (III) and/or (IV), most typically (II) and/or (IV). The manganese cations are oxidised during step (c) to form water-soluble oxidised manganese cations. The water-soluble oxidised manganese cations are typically in the oxidative states (V), (VI) and/or (VH), most typically (VII).

Liquid aqueous phase from step (e). It may be preferred that at least part of the liquid aqueous phase from step (e) is re-used in step (a) of subsequent productions.

Method of measuring the particle size. The particle size distribution of the slurry can be measured using laser diffraction techniques with the slurry dispersed in a non-aqueous liquid Suitable equipment is a Mastersizer 3000 coupled to a Hydro MV liquid dispersion unit. The Mastersizer 3000 is widely available and meets standards for size measurement using laser diffraction techniques.

A suitable dispersed mixture for measuring the particle size is 1 g of slurry added to 100g of methanol with gentle stirring which is then placed in the Hydro MV unit. All required set-points and analysis software are pre-loaded into the Mastersizer 3000 and results such as the d95 are automatically and the unit is capable of handling a very wide range of particle sizes without needing any operator controlled changes, such as lens changes The unit should be operated as per the manufacturers' guidelines.

Method of measuring the viscosity of a slurry. The viscosity of a slurry can be measured using a Brookfield RV viscometer operating at 5 rpm and using Spindle 2 Method of measuring the conductivity of water. This can be measured by use of conductivity cells designed for analysing water quality and operated as per manufacturer's instructions. Suitable equipment includes the TetraCon 325 Conductivity Cell from Xylem Inc. Method of measuring the purity of a sample of calcium oxide or hydroxide in a solid sample. Analysis of the elemental composition of lime slurry solids is preferably done by X-ray Fluorescence (XRF) techniques. This measures the levels of elemental calcium which is then reported as the equivalent level of oxide or hydroxide Suitable XRF equipment includes the Rigaku Supermini 200, equipped with the EZ Analysis software and the EZ Scan software module, using the fusion method for sample preparation. The sample to be analysed is dispersed/dissolved in a sample disc of molten flux.

The sample disc is typically prepared by mixing 1.000 g of sample with 8.000g of di-lithium tetraborate flux and heating to over 920 °C so as to melt the flux/ The test material dissolves into the flux and gives highly accurate results due to the high sample homogeneity.

After cooling, the disc is inserted into the Supermini 200 for analysis. "Omani Analysis" simply needs to be specified in the EZ Scan set-up after which the operation and analysis is automatic.

Method of measuring the absorbance of solutions. The absorbance of solutions can be measured using a suitable UV-Vis spectrophotometer such as a Perkin-Elmer Lambda-25-Scan-UV-VIS or Shimadzu UV-1900i or UV-1295 at a wavelength between 510 and 590 nm and operated as per manufacturer's instructions. Mn(VII) ions show a strong absorbance to light at this wavelength. The dimensions of the cells used are standardised at lcm. For dual-beam spectrophotometers such as the UV-1900i and UV-1295, absorbance is first measured by filling a cell with distilled water and using this as the reference with an absorbance defined as zero. Then the solution to be tested is placed in another call and placed in the unit as well and the absorbance measured. What is important is the absolute difference in the absorbance values between sequential samples. Once this difference is small, most of the accessible Mn ions will have been removed.

Measuring Standard Oxidative Potential. The oxidative potential of a species refers to its potential at standard conditions (298 K, 1 atmosphere, 1 mol/L solution) in respect to the standard hydrogen electrode or SHE. The oxidative potentials of very many chemical species are already listed in the literature, for example the 92"d Edition of the CRC Handbook of Chemistry and Physics, Volume 5, pages 89 -95.

A Standard Hydrogen Electrode consists of a platinum electrode immersed in an acidic solution having a 1 mol/L concentration of 1-1-ions with hydrogen gas at 1 atm being bubble through the solution with everything at 298 K This forms a half-cell The SHE electrode is connected to a potentiometer which is connected to an electrode in a 1 molar solution of the material to be tested at 298 K. A salt bridge completes the circuit and the potential of the material being tested can now be read from the potentiometer.

Method of measuring the total manganese levels in lime slurry solids via inductively coupled plasma -optical emission spectroscopy.

Sample preparation via acid digestion. The sample of calcium hydroxide solid is prepared by drying lOg of slurry at 100°C until there is no further weight loss. The dried sample is allowed to cool and a test portion, of between 0.1g and 0.5g, is weighed to four decimal place and transferred into a PTFE digestion vessel. A solution of 3 ml of concentrated nitric acid, 1 ml of concentrated hydrochloric acid (i.e. aqua regia) and 1 ml of 40% hydrofluoric acid is added to the digestion vessel so to fully dissolve the sample, including any silica present in the solid sample. Romil super-purity spectroscopic grade acids are suitable for use.

The digestion vessel is then closed and inserted into a carousel holder and placed in a CEM Mars 5 microwave-assisted digester. In the digester unit, the vessel temperature is heated to 200°C over a 15 minute period and then held constant at this temperature for a further 20 minutes. Pressures of 200 -250 psi are typical at the higher temperatures.

After cooling, the vessel is opened and 4m1 of 4% spectroscopic grade boric acid is injected The vessel is then placed back into the microwave and heated again to 170°C for 15 minutes and held at this temperature for 15 minutes. The addition of the boric acid "mops up" any remaining HE' by forming BEI This second stage is required because samples containing free HE will damage the internals of the ICP-OES (Inductively Coupled Plasma -Optical Emission Spectroscopy) analytical equipment due to its internal glass components.

After further cooling, the digested liquid is then transferred to a pre-weighed tube, such as a Sarstedt tube. The digestion vessel itself should then be rinsed three times with ultra-high quality (UHQ) water, the washings added to the tube and the whole solution made up to 40m1 with UHQ water. The tube should then be re-weighed to four decimal places giving the weight and concentration of the digest. This is now suitable for analysis for the manganese level by Inductively Coupled Plasma Optical Emission Spectroscopy (ICP-OES). Suitable equipment is a Perkin Elmer Optima 5300 DV, operated according to USEPA SW 846 Method 6010B. Key points of the method are given below.

This method describes multi-elemental determinations by ICP-AES using simultaneous optical systems and axial or radial viewing of the plasma. The instrument measures characteristic emission spectra by optical spectrometry. Samples are nebulized and the resulting aerosol is transported to the plasma torch. Element-specific emission spectra are produced by radio-frequency inductively coupled plasma. The spectra are dispersed by a grating spectrometer, and the intensities of the emission lines are monitored by photosensitive devices. Background correction is required for trace element determination. Background must be measured adjacent to analyte lines on samples during analysis. The position selected for the background-intensity measurement, on either or both sides of the analytical line, will be determined by the complexity of the spectrum adjacent to the analyte line. In one mode of analysis, the position used should be as free as possible from spectral interference and should reflect the same change in background intensity as occurs at the analyte wavelength measured. Background correction is not required in cases of line broadening where a background correction measurement would actually degrade the analytical result.

Instrument ICP-OES Optima 5300, 3300DV and 4300DV, or equivalent Decomposition Prior to analysis, samples must be acidified or digested using appropriate Sample Preparation Methods.

Calibration 0.01 ppm -60 ppm plus matrix specific calibrations Sample Intro Peristaltic pump, cross flow nebulizer, gemcone nebulizer, scott ryton spray chamber and quartz cylonic spray chamber Determination Atomic emission by radio frequently inductively coupled plasma of element-specific emission spectra through a grating spectrometer monitored by photosensitive devices.

Quant tation Limit Element and calibration specific ranging from 0.01-2 ppm Precision & Accuracy ± 10% RSD Interferences Spectral, chemical, physical, memory Calculations wt = (fc v/10 x D)/spl ppm = (t'c v D)/SPL Where fc=final concentration in [ig/mL, v = sample volume in mL; D = dilution factor; spl = sample mass in mg; SPL = sample mass in g Method of measuring the solids content of a slurry. The solids content of a slurry can be measured by drying a 10.0g sample of the slurry at 100 °C until there is no further weight loss. The solids content in % is then the remaining weight of the sample multiplied by 10.

Examples

Example showing the beneficial effect of temperature and water conductivity on the rate of solubilisation of Mn. This is shown by the absorbance of the liquid phase of the slurry. The absorbance of the liquid is directly proportional to the concentration of permanganate in the solution.

Effect of Temperature 1: lg of calcium hydroxide of 96% purity, having a d95 of 25pm and a manganese level of 477 ppm, was dispersed in 80 ml of tap water having a conductivity of 246 riS/cm (microSiemens/cm) at 3°C to give a slurry of 1.23vvt9'o. The pH of the mix was 12.4.

An Ozonia Triogen Lab2B ozone generator with an oxygen flowrate of 4 litres/min and an amplification setting of 4 was connected to bubble oxygen and ozone through the agitated slurry. Under these conditions the rate of ozone generation was 3 5g/hr.

After 5 minutes, a faint pink colour could be seen. After 20 minutes of ozonation, the liquid was sampled and found to have an absorbance of 0.083 at 533nm using a Perkin-Elmer Lambda-25-Scan-UV-VIS unit. This corresponds to a permanganate concentration of 25 RM/1. Further ozonation did not have much benefit and after 60 minutes of continuous ozonation the absorbance was only 0.089 such that the absolute difference between the readings was 0.006.

2: The above test was repeated using identical conditions, materials and procedures except that the temperature of the tap water was now 21°C. The absorbance of the liquid was now 0.264, corresponding to a permanganate concentration of 150)1M/1. The increase in temperature of 18°C resulted in the amount of manganese being solubilised by over three times The amount of oxidising material (ozone) contacted with the calcium hydroxide slurry during each of these 20 minute tests was 1.2g for 1g of solids.

3: Test 2 was repeated but now using distilled water with a conductivity of 2.4 RS/cm. After 20 minutes ozonation, the absorbance was 0.067. This means that using tap water having a higher conductivity increased the rate of Mn solubilisation by a factor of 3.9 times.

4: 18.75g of calcium oxide of 94% purity (ML90 from Singleton Birch Ltd) was mixed with 131.25g of tap water as above. The calcium oxide had a manganese level of 453ppm (0.000453%). The calcium oxide was allowed to hydrate to form a slurry of calcium hydroxide of 16.5% solids (24.8g Ca(OH)2 solids). This material had a dss of 7.8m. The slurry was allowed to cool to 22°C. The slurry was placed in an ultrasound bath to provide agitation and the Ozonia Triogen Lab2B ozone generator with an oxygen flowrate of 4 litres/min and an amplification setting of 10 was connected to bubble oxygen and ozone through the agitated slurry for 1 hour. Under these conditions the rate of ozone generation was 9.3 g/hr so the amount of ozone per gram of solids was 9.3/24.8 or 0.375grams per gram of solid.

The ozonated slurry was then vacuum filtered and washed with water. A sample of wet cake was dried in the oven at 100°C for 10 hours and the manganese level tested by ICP-OES. When tested this material was found to have a manganese level of 228 ppm (0.000228%). The wet cake was then redispersed in water to give a calcium hydroxide slurry having a solids content of 25% and a viscosity of 254 cps.

5: 1 g of the calcium hydroxide used in Test 1 was added to 80 ml of 8wt% sodium hypochlorite solution at 90°C and agitated for 10 minutes. The sample was then filtered, and a portion of the filtrate was dried and tested in the same manner as in Test 4 and found to have a manganese level of 266.6 ppm. The residual wet cake was red spersed in water to give a slurry of 25% solids content.

A similar comparative test was done as above but with the sodium hypochlorite solution at 15°C After 20 hours contact with the bleach solution, the residual manganese level was found to be 249 ppm

Claims (1)

1. Claims A process of making a lime slurry, wherein the lime slurry is suitable for adjusting the pH of water during water treatment, wherein the process comprises the steps of: (a) contacting solid particulate calcium-rich material with water to form a calcium hydroxide slurry, wherein the solid particulate calcium-rich material comprises: (i) from 90wt% to 99.9997wt% calcium material selected from calcium hydroxide and/or calcium oxide; (ii) from 0.0003wt% to 0.001wt% manganese cations; and (Hi) optionally, other material, and wherein greater than 95wt% of the solid particulate calcium-rich material has a particle size of less than 100itm, wherein the calcium hydroxide slurry has a solids content of from 1.0wt% to 25wt%, and wherein the calcium hydroxide slurry comprises from 0.90wt% to 25wt% calcium hydroxide; and (b) contacting the calcium hydroxide slurry from step (a) with an oxidising material, wherein greater than 0.1gram oxidising material per gram of solids content is contacted to the calcium hydroxide slurry, (c) reacting the slurry obtained in step (b) for more than 5 minutes at an elevated temperature in the range of from greater than 20°C to less than 95°C and at a pH of greater than 12.0 so as to form water-soluble oxidised manganese cations and purified calcium hydroxide solid material; (d) optionally, repeating steps (b) and (c); (9) separating the purified calcium hydroxide solid material from the remainder of the slurry so as to form purified calcium hydroxide wet cake and a liquid aqueous phase comprising the water-soluble oxidised manganese cations; (f) contacting water to the purified calcium hydroxide wet cake to form a purified calcium hydroxide slurry, wherein the purified calcium hydroxide slurry has a solids content of from 18wt91:. to 50wt%, wherein the purified calcium hydroxide slurry comprises from 16.2wt% to 50wt% calcium hydroxide, and wherein the purified calcium hydroxide slurry comprises less than 0.0003wt% manganese cations; and (g) optionally, repeating steps (b) to (0; to form the lime slurry.A process according to claim 1, wherein step (c) takes place at an elevated temperature in the range of from 40°C to 80°C A process according to any preceding claim, wherein the particulate calcium-rich material comprises calcium oxide, and wherein at least part of the energy required to raise the calcium hydroxide slurry to the elevated temperature required in step (c) is generated by contacting the calcium oxide with water to form the calcium hydroxide slurry during step (a).A process according to any preceding claim, wherein the oxidising material has an oxidative potential of greater than 0.9V when compared to a standard hydrogen electrode.A process according to any preceding claim, wherein the oxidising material is selected from sodium hypochlorite, hydrogen peroxide, ozone, or any combination thereof A process according to claim 5, wherein the oxidising material is ozone, and wherein the rate of ozone addition to the calcium hydroxide slurry during step (b) is greater than 0 12kg/hr per kg slurry solids A process according to claim 6, wherein the water used in step (a) has a conductivity of greater than 50 p.S/cm.A process according to claim 5, wherein the oxidising material is sodium hypochlonte.A process according to claim 5, wherein the oxidising material is hydrogen peroxide A process according to any preceding claim, wherein the absorbance, measured at a wavelength between 510 nm and 595 nm, of the liquid aqueous phase formed in a step (e) shows an absolute difference of less than 0.05 compared to the liquid aqueous phase formed in an immediately preceding step (e) when measured using the same cell.11 A process according to any preceding claim, wherein repeats of steps (b) and (c) are carried out in different reactor vessels.12. A process according to any preceding claim, wherein step (e) is carried out by filtration.13 A process according to any preceding claim, wherein at least part of the liquid aqueous phase from step (e) is re-used in step (a) of subsequent productions