Composition and method for simultaneous water softening and silica removal in boiler feed water

Abstract

A slurry product is shown for treating water to both soften the water and to remove silica. The slurry is prepared by blending, in an aqueous medium, hydrated lime under the form of a slurry or of a powder with at least partly hydrated dolime, or magnesium hydroxide or magnesium oxide particles or a combination thereof under the form of a slurry or of a powder, to form an aqueous slurry where the amounts of the dolime, magnesium hydroxide or magnesium oxide particles or the combination thereof are provided such that the solid content of the slurry is up to 60% by weight of the slurry. The slurry also maintains a stable and pumpable viscosity for over a month.

Claims

1. A single slurry product for removing silica from boiler feed water while simultaneously softening the water, the single slurry product comprising: a combination through simple mixing of a hi-cal, calcium hydrate aqueous slurry which has a desired low viscosity, a desired viscostability and high reactivity with a fully hydrated dolomitic hydrate to thereby form a single slurry product, where the single slurry product inherits these desired characteristics of the hi-cal, calcium hydrate aqueous slurry and can be applied to the dual task of water softening and silica removal; wherein the fully hydrated dolomitic hydrate has a particle size range, d.sub.90, of 40-55 μm and a d.sub.50 of 3.0 to 5 μm; wherein the single slurry product is further characterized as having a solid content in the range from about 25% to about 60% by weight in the slurry; wherein the percentage of calcium to magnesium expressed as a percentage of calcium hydroxide to magnesium hydroxide in the single slurry product is in a range from 66-99% Ca(OH).sub.2 to 1-44% Mg(OH).sub.2 by dry weight; wherein the hi-cal, calcium hydrate aqueous slurry is made up, in part, of hydrated lime particles, and wherein the hydrated lime particles have an available lime content of at least 80% in weight of the hydrated lime particles measured according to the ASTM C25 or EN 459-2:2010; wherein the hydrated lime particles have a specific surface area according to the BET method, after degassing in vacuum at 190° C. for at least 2 hours of less than or equal to 10 m.sup.2/g; wherein the hydrated lime particles have a d.sub.90 of 8 to 54 μm and a d.sub.50 of 2 to 3.5 μm; and wherein the boiler feed water has a measurable total hardness and a measurable silica content, and wherein the single slurry product is effective to lower the measurable total hardness of the feed water below 50 mg/dm.sup.3 and to reduce the measurable silica content below 1.5 mg/dm.sup.3, respectively, while maintaining a stable and pumpable viscosity of <1,000 mPa.Math.s for at least one month as measured using a Brookfield viscometer with an RV #3spindle at 100 RPM.

2. The slurry of claim 1, further comprising a polycarboxylate dispersant in an amount comprised between 0.5 and 5 wt %, based upon the total weight of the hydrated lime particles.

3. The slurry of claim 2, further comprising an additive selected from the group consisting of sugars, an anti-scaling agent and/or an additional dispersant compound, present in an amount of up to 2 wt % in weight of the hydrated lime particles.

4. A single slurry product for removing silica from boiler feed water while simultaneously softening the water, the single slurry product comprising: a combination through simple mixing of a hi-cal, calcium hydrate aqueous slurry with a fully hydrated dolomitic hydrate where the single slurry product can be applied to a dual task of water softening and silica removal; wherein the single slurry product is further characterized as having a solid content in the range from about 25% to about 60% by weight in the single slurry product; wherein the percentage of calcium to magnesium expressed as a percentage of calcium hydroxide to magnesium hydroxide in the the slurry product is in a range from 66-99% Ca(OH).sub.2 to 1-44% Mg(OH).sub.2 by dry weight; wherein the hi-cal, calcium hydrate aqueous slurry is made up of hydrated lime particles which have an available lime content of at least 80% in weight of the hydrated lime particles measured according to the ASTM C25 or EN 459-2:2010; wherein the hydrated lime particles have a specific surface area according to the BET method, after degassing in vacuum at 190° C. for at least 2 hours of less than or equal to 10 m.sup.2/g; wherein the hydrated lime particles have a d.sub.90 of 8 to 54 μm and a d.sub.50 of 2 to 3.5 μm; and wherein the boiler feed water has a measurable total hardness and a measurable silica content, and wherein the single slurry product is effective to lower the measurable total hardness of the feed water below 50 mg/dm.sup.3 and to reduce the measurable silica content below 1.5 mg/dm.sup.3, respectively, while maintaining a stable and pumpable viscosity of <1,000 mPa.Math.s for at least one month as measured using a Brookfield viscometer with an RV #3spindle at 100 RPM.

5. A process for manufacturing the single slurry product of claim 1 for removing silica from boiler feed water while simultaneously softening the boiler feed water, the process comprising a step of blending the hi-cal, calcium hydrate aqueous slurry with the fully hydrated dolomitic hydrate.

6. The process of claim 5, including a step of adding a polycarboxylate dispersant in an amount comprised between 0.5 and 5 wt % of the hydrated lime.

7. The process of claim 6, further comprising a step of adding an additive selected from the group consisting of sugars, an anti-scaling agent and an additional dispersant compound added in an amount of up to 2 wt % in weight of the hydrated lime.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 is a graph showing the MgO:SiO.sub.2 ratio before and during a trial treatment of boiler feed water using the compositions of the invention.

(2) FIG. 2 is a graph of silica concentrations in the boiler feed water as a function of time when using calcium hydroxide slurry for water treatment and when using a blended calcium and magnesium hydroxide slurry according to the principles of the invention. In the graph, tC indicates the time of change for water source/quality.

DETAILED DESCRIPTION OF THE INVENTION

(3) The present invention provides a solution to the previously described problems of water treatment and, particularly, treatment of industrial boiler feed water. The compositions of the invention take the form of a slurry product containing calcium hydroxide and magnesium hydroxide, or at least a precursor of it as magnesium oxide and having a solids content of up to about 60 wt % (most preferably 30-40%). Calcined dolomite, magnesium hydroxide or magnesium oxide provides the Mg(OH).sub.2 source for the combined slurry. Calcined limestone provides the source of calcium hydroxide.

(4) Calcium oxide, CaO, is often referred to as “quicklime”, while calcium hydroxide, Ca(OH).sub.2, is referred to as “hydrated lime”, both sometimes being informally referred to as “lime”. Quicklime is usually in the form of lumps or pebbles but it can also be a powder. Dry hydrated lime is usually a powder. In the meaning of the present invention, “powder” means a solid substantially made of particles lower than 2 mm, in particular lower than 1 mm or even lower than 500 μm and notably greater than 0.1 μm, in particular 0.5 μm.

(5) According to present industry practices, in order to further process these compounds and improve the ease with which they are handled, dry CaO or dry Ca(OH).sub.2 is often mixed with water to form an aqueous suspension, i.e., a slurry, sometimes called milk of lime. This fluid suspension of slaked lime, also referred to as hydrated lime (calcium hydroxide—Ca(OH).sub.2), can include impurities, in particular silica, and magnesium oxide to the extent of a few percent. Such a suspension is obtained either by slaking quicklime (calcium oxide—CaO) with a large excess of water, or by mixing hydrated lime with water.

(6) The resulting aqueous suspensions are often characterized by the concentration of the mass of the solid matter (% solids), the chemical reactivity of the slurry, and the distribution of the sizes of the particles in suspension (controlling in part viscosity). These characteristics determine, in part, the properties of the slurry, mainly its viscosity and its reactivity.

(7) The reactivity of an aqueous calcium magnesium suspension is determined by the dissolution rates of the particles. It may be measured by injecting a small amount of the suspension in a large volume of demineralized water. This measurement, based on the recording of the time-dependent change in the conductivity of the resulting liquid phase, was developed for monitoring reactivity of lime milks intended for softening of drinking waters (v. Van Eckeren et al. Improved Milk-of-Lime For Softening of Drinking Water: the Answer to the Carry-Over Problem, In Aqua, 1994, 43 (1), p. 1-10). More details on the procedure for measuring this reactivity of lime milks are available in § 6.11. Determination of solubility index by conductivity of the standard EN 12485: 2010. The reactivity of an aqueous calcium magnesium suspension is also determining for any neutralization or precipitation operation.

(8) In the present discussion, the distribution of particle sizes will be understood to mean the distribution as measured by means of a laser granulometer and the distribution is characterized in terms of, for example, the d.sub.90 interpolated value of the particle size distribution curve, the dimension d.sub.90 corresponding to the dimension for which 90% of the particles are less than the said dimension.

(9) As used in the discussion which follows, the following terms will be understood by those skilled in the relevant industries to have the following meanings: Limestone (calcium carbonate—CaCO.sub.3 with impurities) is present in large quantities in natural rock around the world. Quicklime (calcium oxide—CaO with impurities) is an alkali and the result of the chemical transformation of limestone by heating it typically above 900° C., which requires energy (typically 3.2 GJ/tCaO). Given its rapid reaction with water, calcium oxide, also called burnt lime, is often referred to as quick lime. Hydrated lime or Slaked lime [calcium (di-)hydroxide—Ca(OH).sub.2 with impurities] is a strong alkali formed when calcium oxide reacts with water. This reaction generates heat. Depending on the amount of water used, calcium hydroxide can either be a dry hydrate (dry powder), a paste (putty lime) or a liquid milk of lime also called lime slurry (dry suspension in water). High Calcium Hydrate or Hydrated calcium lime or “hical”—hydrated lime containing mainly calcium hydroxide thus containing a low amount of magnesium compound as impurity, i.e. when expressing magnesium as MgO, having less than 5% MgO typically a MgO content lower than 3%, in particular lower than 2% in weight. Dolomite (double carbonate of calcium and magnesium—CaCO.sub.2.MgCO.sub.2) is the result of a partial or full dolomitization of calcium carbonate. Dolime or dolomitic lime (calcium & magnesium oxide—CaO.MgO) is the result of the chemical transformation of double carbonate of calcium and magnesium by heating it typically above 900° C., which requires energy (typically 2.935 GJ/t CaO.MgO). Like quicklime, dolime reacts with water. CaO's affinity for water is higher than that of MgO. Hydrated dolime (calcium & magnesium (tetra-)hydroxide—Ca(OH).sub.2.Mg(OH).sub.2) represents the completion of the hydration reaction carried out in pressurized reactors at temperatures of around 150° C.

(10) The lime slurries preferred for purposes of the present invention are fine milk of lime slurries with high solids content and relatively low viscosity so as to be easily pumpable. Those skilled in the relevant arts will appreciate that it is sometimes difficult to achieve the desired balance between viscosity, solids content and reactivity in the resulting lime slurries. Variables that generally affect the quality of slaked lime are disclosed in J. A. H. Oates—“Lime and Limestone” (pages 229-248) as well as in Boynton—“Chemistry and Technology of Lime and Limestone” (pages 328-337).

(11) Some of the known commercial technologies for producing lime slurries having high solids contents include the following:

(12) For example, it is known to increase the solids content of the milk of lime by adding a dispersing agent, in the presence of a small quantity of an alkaline metal hydroxide (U.S. Pat. Nos. 5,616,283, 4,849,128, and 4,610,801). This method of preparation makes it possible to achieve concentrations of dry matter greater than 40 wt % based on the total weight of the milk of lime, with a viscosity less than 1200 mPa.Math.s.

(13) It is also known to increase the solids content in the suspension, while limiting the increase in viscosity, by incorporating hydrated lime having a coarser particle size or by slaking quicklime tinder conditions favorable to the growth of the grains; for example, by limiting the increase in temperature during slaking and by adding additives such as sulfates etc. (U.S. Pat. No. 4,464,353).

(14) One high solids content calcium hydroxide slurry useful for purposes of the present invention can be prepared according to the teachings of U.S. Pat. No. 8,206,680, issued Jun. 26, 2012, to Diaz Chavez, et al. and assigned to the assignee of the present invention. That reference describes a calcium magnesium aqueous suspension having particles of solid matter with (before being put into suspension) a specific surface area, calculated according to the BET nitrogen absorption method, which is less than or equal to 10 m.sup.2/g. Such an aqueous suspension of calcium magnesium solid matter can achieve a very low viscosity, making it possible to greatly increase the solid matter concentration of the suspension, or again to reduce the size of the particles in suspension, thus obtaining a concentrated and reactive milk of lime.

(15) In the discussion which follows, the term “BET” nitrogen absorption method will be understood to mean the determination of the specific surface area of the slaked lime as measured by nitrogen adsorption manometry and calculated according to the BET method, after degassing in vacuum at 190° C. for at least 2 hours.

(16) Preferably, the particles of solid matter of the high solids content calcium hydroxide slurry have a specific surface area according to the BET method of less man or equal to 25 m.sup.2/g, preferably less than or equal to 10 m.sup.2/g. The suspensions thus prepared advantageously have a dynamic viscosity less than or equal to 1000 mPa.Math.s, preferably less than or equal to 600 mPa.Math.s. Under these conditions it is possible to obtain a suspension having solid matter contents greater than 25 wt %, and advantageously at or greater than 40 wt %, and/or d.sub.98 granulometric dimensions of less than 20 microns, preferably equal to or less than 5 microns.

(17) One “hical” lime slurry products that can be used for manufacturing the slurry product of the invention is a 45 wt % solids slurry, with a viscosity of less than 600 mPa.Math.s and a particle size distribution with a d.sub.50 value of 2.5-3.5 μm and d.sub.98 value of less than 90 μm.

(18) As mentioned, the slurry product of the invention has a source of calcium as one component and a source of magnesium as a second component. Particularly preferred sources of the calcium for the slurries of the invention are from calcium hydroxide such as a hical slurry, as described, or from products as described in the previously cited U.S. Pat. No. 8,206,680 B2. As also mentioned, the preferred calcium hydroxide slurries have an average particle size distribution d.sub.90 of 8-145 μm; a d.sub.50 of 2-17 μm; and an available lime as measured by the ASTM C25 or EN 459-2:2010 standard of greater than or equal to about 80%.

(19) The slurry of the invention typically comprises up to 44 wt % Mg(OH).sub.2 as the second component of the slurry formulation. The Mg(OH).sub.2 can conveniently be sourced from dolomitic hydrate or magnesium hydroxide or magnesium oxide. One preferred source of magnesium for the magnesium hydroxide slurries can be from dolomitic hydrate which has an average particle size distribution d.sub.90 of about 40-55 μm; a d.sub.50 size distribution from about 3.0 to 5 microns. The source of magnesium can also be from any commercially available magnesium hydroxide or magnesium oxide.

(20) The slurries of the invention can also contain other conventional additives, such as an optional dispersing agent. The dispersing agent can be one of those previously described, including the use of a conventional polycarboxylate or polyacrylate and/or polyphosphonate dispersant in an amount comprised between about 0.2 and 5.0 wt %, preferably between about 0.5 and 5 wt %, of the hydrated lime. Other conventional additives may also be present such as an additive selected from the group consisting of sugars, such as sucrose, or preferably sorbitol, and present in an amount of up to 2 wt %; and/or an additive selected from the group consisting of anti-scaling agents present up to about 2 wt %, and/or other dispersants, all weights being based upon the weight of hydrated lime used.

(21) The water used to suspend the hydroxides can be used from multiple sources; however, softened water or low hardness tap water (total hardness of <100 ppm) is preferred to maintain the product's reactivity and effectiveness.

(22) The manufacturing process of the slurry product is created by blending in an aqueous medium a hydrated lime product with an at least partially hydrated dolime product or magnesium oxide or magnesium hydroxide or a combination thereof in predetermined ratios (optionally with a dispersant or other additive of the type described) and wherein the hydrated lime product is under the form of a slurry or a powder and the at least partially hydrated dolime product or magnesium oxide or magnesium hydroxide or a combination thereof is under the form of a slurry or a powder.

(23) In an embodiment of the process of the invention, a hical (standard hydrate at 1-2% moisture) calcium hydroxide slurry or a slurry according to U.S. Pat. No. 8,206,680 B2 is blended with a dolomitic hydrate (fully hydrated dolime) or magnesium hydroxide slurry.

(24) In another embodiment of the process of the invention, a hical calcium hydroxide under the form of powder is blended with a dolomitic hydroxide or magnesium hydroxide under the form of a powder in presence of water.

(25) In another embodiment of the process of the invention, a hical calcium hydroxide slurry or an aqueous suspension as described in U.S. Pat. No. 8,206,680B2 is blended with a dolomitic hydroxide or magnesium hydroxide under the form of powder.

(26) In another embodiment of the invention, a hical calcium hydroxide under the form of a powder is blended with a dolomitic hydroxide slurry or magnesium hydroxide slurry.

(27) The ratio of Ca(OH).sub.2 to Mg(OH).sub.2 employed in the slurries of the invention varies depending upon raw water chemistry. For example, a low silica concentration removal (20 ppm) was found to be effective using a dry ratio of approximately 9:2 calcium hydroxide to dolomitic hydrate [92% Ca(OH).sub.2 to 8.0% Mg(OH).sub.2] or high silica concentration removal (100 ppm) a dry ratio of approximately 3:10 calcium hydroxide to dolomitic hydrate [66.2% Ca(OH).sub.2 to 33.8% Mg(OH).sub.2] was found to be effective.

(28) The slurries of the invention are also characterized as having a stable viscosity over 30 days of <1,000 mPa.Math.s measured using a Brooksfield viscometer with an RV #3 spindle at 100 RPM, thereby remaining pumpable. The slurries are easily resuspendable without hard packing.

(29) Example of the Practice of the Invention:

(30) An oil refinery was using lime softening for boiler feed water preparation from a blend of ground water and municipal tap water. The average quality composition of the feed water is 175 mg/dm.sup.3 total hardness and 13.5 mg/dm.sup.3 SiO.sub.2. Target concentrations for hardness and silica after lime softening are <50 mg/dm.sup.3 and <1.5 mg/dm.sup.3, respectively. The water quality composition fluctuates in terms of total hardness and ratio MgO:SiO.sub.2 (1:1 to 5:1). For the lower ratio of 1:1 to 3:1, silica levels in the boiler feed water increased from <0.5 mg/dm.sup.3 to 2.1 mg/dm.sup.3, exceeding the target concentration of <1.5 mg/dm.sup.3. This was attributed to two factors: (1) the low MgO:SiO.sub.2 ratio (1:1 to 3:1) in the raw water is insufficient to remove silica through precipitation of a magnesium hydroxide silicate compound, and (2) the lower total hardness of the raw water (120 mg/dm.sup.3) results in reduced co-precipitation of silica. The MgO:SiO.sub.2 ratio needed at this site, taking into account the incoming and target silica concentrations, was calculated at ≥3:1.

(31) The solution proposed for this plant was to change the dosing reagent from a solely calcium-based product to the slurry product of the invention with a solid content typically greater than 40 wt %. The blend was optimized based on operating parameters and treatment targets for softening and silica removal at this refinery. A Ca(OH).sub.2 to Mg(OH).sub.2 ratio of 92:8 was engineered to provide sufficient magnesium content for removal of silica to the required <1.5 mg/dm.sup.3 in the boiler feed water while providing simultaneous softening. Silica concentrations in the boiler feed water immediately and significantly decreased. A reduction from 2.1 mg/dm.sup.3 silica to less than 0.6 mg/dm.sup.3 silica was measured after two days from the start of dosing of the new composition. Silica continued to decrease as the slurry took full effect in the system and the boiler feed water returned consistent silica concentrations of 0.2 to 0.5 mg/dm.sup.3 in the months that followed. In addition, the ratio of hydroxide consumed per hardness removed decreased by 11%, indicating further optimization of the softening process with the new composition.

(32) FIG. 1 of the drawings is a graph showing the MgO:SiO.sub.2 ratio before and during a trial treatment of boiler feed water using the compositions of the invention. The data on the left of the graph represents the historical data and the data on the right of the graph represents the data taken during the trial. The required ratio of MgO:SiO.sub.2 is typically optimized between about 1-5.

(33) FIG. 2 of the drawings is a graph of silica concentrations in the boiler feed water in function of the time when using calcium hydroxide slurry for water treatment and when using a blended calcium and magnesium hydroxide slurry according to the principles of the invention. It will be observed that the source of water changed after at a certain time (Tc), thus necessitating a change in the water treatment protocol at the plant. The triangle data points represent a slurry of calcium hydroxide alone. The circle data points represent a treatment with a slurry of the invention containing a combined calcium hydroxide slurry and magnesium hydroxide.

(34) In summary, after the raw water change it was found that supplemental magnesium was necessary to reach the silica concentration targets in the boiler feed water. Once switching to the new composition of the invention as described herein, the silica targets were easily met due to the fine particle size (d.sub.50≤2.5 μm) and high reactivity of the engineered calcium hydroxide slurry paired with the fine dolomitic hydrate. This stable viscosity engineered slurry promoted quick and efficient hardness removal and silica precipitation. The refinery was able to avoid any additional treatment/chemicals and their associated equipment costs to achieve the necessary final water quality. The solution is flexible and the chemistry of the composition with both calcium and magnesium can easily be tailored to address any future raw water changes.

(35) An invention has been provided with several advantages. The combined slurry product of the invention provides a single product for simultaneous control of hardness and silica in boiler feed water meeting the abovementioned requirements as well as offering the following additional advantages: Minimum number and volume of chemical agents to be inventoried and handled, and preferably a single storage stable product; Rapid process to match intake of make-up water; Production of easily settleable or filterable flocs; Able to be installed in-line with a small spatial footprint and energy demand; pH compatible with other components and processes or discharge regulation.

(36) The invention allows achievement of water treatment targets using one single product that has a stable and pumpable viscosity over greater than a one month period of storage; it contains magnesium hydroxide, which targets silica removal, and calcium hydroxide, which rapidly increases pH and promotes water softening. The amount of magnesium in the ultimate slurry blends of the invention is sufficient to encompass fluctuations of naturally occurring silica and magnesium components in the raw water. The invention thus improves technical performance, eliminates handling of multiple products, and reduces overall treatment costs. In addition, this combined product provides odor control, as the readily available calcium hydroxide quickly neutralizes the source and the magnesium hydroxide provides continuous treatment. Lastly, the product is a source of alkalinity, as both the calcium and magnesium source provides alkalinity for water treatment.

(37) While the invention has been shown in several of its forms, it is not thus limited but is susceptible to various changes and modifications without departing from the spirit thereof.