Method of manufacturing hydrated lime

Abstract

A method of providing highly reactive hydrated lime and the resultant lime hydrate where an initial lime feed comprising calcium and impurities is first ground to a particle-size distribution with relatively coarse particles. Smaller particles are then removed from this ground lime and the smaller particles are hydrated, allowed to mature in a damp state, and flash dried to form a hydrated lime, which is then milled to a significantly smaller particle size than that of the relatively coarse particles.

Claims

1. A method of producing hydrated lime, the method comprising: providing a lime feed comprising calcium oxide and impurities; milling said lime feed to produce a fine ground lime, wherein said fine ground lime has a particle-size distribution of about 50% to about 60% minus 200 mesh; feeding said fine ground lime into an air classification system; removing a refined fine lime from said air classification system wherein said refined fine lime is finer than said fine ground lime has a particle size distribution of greater than 70% minus 200 mesh; adding water to said refined fine lime to form damp hydrated lime; and drying said damp hydrated lime to form dried hydrated lime.

2. The method of claim 1: wherein said hydrated lime has a BET particle surface area of greater than 30 m.sup.2/g.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 depicts a block diagram of an embodiment of a method of manufacturing hydrated lime.

(2) FIG. 2 depicts a block diagram of an embodiment of a portion of a method of manufacturing hydrated lime showing an integrated milling and classification system.

(3) FIG. 3 depicts a block diagram of an embodiment of a portion of a method of manufacturing hydrated lime showing an integrated milling, classification, and drying system.

(4) FIG. 4 depicts the reactivity of a standard hydrated lime as compared to the reactivity of hydrated lime produced according to an embodiment of a method of manufacturing hydrated lime disclosed herein.

(5) FIG. 5 depicts a graph illustrating the change in resultant pore volume and BET surface area as a damp hydrate putty is allowed to mature over time prior to flash drying.

(6) FIG. 6 depicts a table showing the data values of BET surface area for hydrated lime produced from the damp hydrate putty that is used to generate the graph of FIG. 5.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT(S)

(7) In general, lime feed is subjected to a multi-stage manufacturing process (including grinding, classification, hydration, maturation, and drying) wherein the resultant product is largely calcium hydroxide. The resultant hydrated lime product is highly reactive and has a very high purity and surface area and very small particle size. More specifically, in one embodiment, the hydrated lime has a reactivity in citric acid of 10 seconds or less, a concentration of greater than or equal to about 95% calcium hydroxide, a particle surface area (BET surface area) of greater than 30 m.sup.2/g, and a particle size of less than 44 microns for about 98% of the particles.

(8) It should be noted that values for the reactivity, purity, surface area, and particle size discussed herein are merely exemplary and preferred, and in no way limiting. As would be understood by one of ordinary skill in the art, by altering some or all processing conditions discussed above or by altering the purity of the initial lime feed, it is possible to achieve other reactivity, purity, surface area, and particle size values for the final hydrated lime product in accordance with the below described manufacturing process.

(9) In order to test reactivity of particular lime hydrate compounds, in an embodiment, the reactivity to a weak acid (such as, but not limited to, citric acid) provides for a reactivity time that is measurable with commercial instruments. The problem with determining reaction time to stronger acids is that the reaction can be too quick to effectively measure at laboratory scaling. Thus, it is difficult to predict compositions that will function well without performing large scale pilot testing. In order to deter nine the citric acid reactivity of a particular hydrated lime composition, the amount of time it took 1.7 grams of lime hydrate to neutralize 26 grams of citric acid was measured. As a measurement of effectiveness, it is preferred that this value be less than or equal to 10 seconds in order to have a hydrated lime composition which is classified as being high reactive. However, in an alternative embodiment, it can be less than eight (8) seconds. It is more preferred that this value be four (4) or less, three (3) or less, two (2) or less or one (1) or less. Again, given the practical realities that production of improved material often results in a product having dramatically increased costs of products, utilizing current manufacturing techniques and for current emissions standards, in an embodiment the lime hydrate may be in the two (2) to five (5) second range, or, in another embodiment, in the three (3) to four (4) second range.

(10) The lime feed (also known as quicklime and used synonymously herein) that is utilized in the initial stages of the methods disclosed herein refers to calcium oxide which is generally made by the thermal decomposition of limestone (predominately calcium carbonate) in a lime kiln. This is accomplished by a process called calcination or lime-burning (by heating the material to an extremely high temperature), which liberates the molecule of carbon dioxide (CO.sub.2), leaving quicklime. The quicklime, however, is not pure calcium oxide and includes other impurities (e.g., residual uncalcined stone, dead burned lime, silica nodules, metal oxides, etc.). In some instances, the lime feed may have a chemical purity as low as 90% (i.e., 90% Calcium Oxide (CaO) and 10% impurities). In other instances, the lime feed may have a chemical purity less than 93% but greater than 90%, or less than 90%. In one embodiment, the purity will be about 93%. In still further instances, the lime feed may be greater than 93% CaO. In any event, the lime feed discussed herein is not limited to any particular purity of CaO, unless otherwise noted.

(11) Turning now to FIG. 1, a general block diagram of the method of manufacturing hydrated lime will be described in more detail in accordance with several embodiments. Lime feed (50) is subjected to fine grinding or milling (10) to produce a fine ground lime (51). The fine ground lime can be of varying sizes in different embodiments but generally has a particle-size distribution of less than 80% minus 200 mesh. That is, the lime feed (50) is ground significantly coarser than the desired end product. In a preferred embodiment, the fine ground lime has a particle-size distribution of about 50% to about 60% minus 200 mesh. Any grinding or milling is suitable, including, for example, fine grind cage mill, swing hammer mill, screen mill, etc. where the amount of milling produces the desired particle-size distribution.

(12) After being milled, the fine ground lime (51), including the impurities, then passes through an air classification system (11). The air classification system (11) generally comprises an air classifier (e.g., a standard Raymond or Sturtevant air classifier) or a turbine classifier. The air classification system (11) generally comprises two steps, which may be performed consecutively or concurrently.

(13) As noted above, the lime feed (50)and fine ground lime (51)generally will include various different impurities. Without being limited to a particular particle-size distribution, it is believed that grinding the lime feed (50) to a particle-size distribution of about 50% to about 60% minus 200 mesh grinds a majority of the calcium oxide to the desired particle-size distribution (i.e., passing a 200 mesh screen) but does not grind the harder impurities to a size below 200 mesh. In other words, the harder impurities are larger than 200 mesh while most of the lime particles are smaller than 200 mesh. As a result, removal of the impurities becomes easier and at an earlier stage of the manufacturing process as discussed below.

(14) In the air classification system (11), the harder impurities are then removed and a refined fine lime (53) is produced. As the name implies, the refined fine lime (53) will have a smaller particle-size distribution than both the fine ground lime (51) and the lime feed (50) as the larger particles from the fine ground lime (51) are removed. In a preferred embodiment, the refined fine lime (53) will have a particle-size distribution of greater than about 80% minus 200 mesh. In another embodiment, it will be greater than 93.5% minus 200 mesh. In some embodiments, though, the particle-size distribution will be less than about 80% minus 200 mesh (e.g., greater than 70% minus 200 mesh, with the fine ground lime (51) having a particle-size distribution of less than 70% minus 200 mesh). The coarse fraction (52) (i.e., the portion of the fine ground lime (51) from the air classifier system (11) larger than 200 mesh and which is believed to include a higher percentage of the impurities than the lime feed (50)) is then removed and can be used, for example, as a de-dusted fine granular lime. In some embodiments, the coarse fraction (52) may be fed into the mill (10) for re-grinding to the desired particle-size distribution discussed above and then again fed into the air classifier system (11). This additional re-grinding of the coarse fraction (52) can continue as many times as desired or necessary, but is by no means required. In some instances, however, such re-grinding ensures that the maximum amount of calcium oxide from the lime feed (50) can be utilized in generating the desired hydrated lime, as discussed below.

(15) As can be seen from Table 1 below, the combination of the mill (10) and air classification system (11) results in refined fine lime (53) with a higher purity (i.e., less impurities and higher concentration of CaO).

(16) TABLE-US-00001 TABLE 1 Sample Feed Composition Description CaO CaCO.sub.3 MgO SiO.sub.2 Al.sub.2O.sub.3 Fe.sub.2O.sub.3 Sulfur MnO Feed to 93.95 1.42 1.040 0.800 0.101 0.099 0.024 46 Mill (50)/ Classifier (51) Refined Fine 95.32 0.63 0.825 0.730 0.083 0.095 0.026 45 Lime (53)

(17) It should also be noted that, although the above description refers to particles passing through a mesh, this description and use of mesh merely refers to the common use of the term mesh as it relates to particle-size distribution. A mesh screen or sieve need not be used to measure or classify the particle-size. Instead, the air classification system described above is used, which generally separates particles less than 200 mesh from particles greater than 200 mesh.

(18) Without being limiting to any particular theory of operation, it is believed that the removal of potential impurities from the lime feed (50) (and fine ground lime (51)) improves the reactivity of the final hydrated lime product. As noted above, the impurities are often harder than the calcium oxide and thus more difficult to grind. As a result, efficiencies are wasted during this milling step if the lime feed is continuously milled until the batch of lime feed meets the desired size requirements. Additionally, the continuous milling step of the prior art leads to ultrafine calcium oxide particles. It is believed these ultrafine calcium oxide particles results in a less reactive final hydrated lime product as the smaller particles results in the formation of more agglomerates, and thus less reactivity of the final hydrated lime product. Finally, the refined fine lime (53) without impurities helps resolve a major problem of localized overheating of the lime during the subsequent hydration that was commonly seen in the prior art. The end result, and the remaining processing steps below, is the production of a small average diameter particle and a more reactive resultant lime hydrate.

(19) As quicklime is generally not stable and, when cooled, will spontaneously react with CO.sub.2 from the air until, after enough time, it is completely converted back to calcium carbonate, all the milling and classification of the quicklime should preferably be produced entirely in a closed-circuit system to prevent air slaking and recarbonation (i.e., CaO to CaCO.sub.3) from occurring, although a closed-circuit system is by no means required. In some embodiments, additional measures are employed to prevent recarbonation. For example, conditioned low CO.sub.2 air (18) can be injected (66) into these systems to replace any air being drawn in and around process equipment bearings and seals. This conditioned air (18) is also very useful if the quicklime needs to be pneumatically conveyed. The process for conditioning this air is discussed more fully below and is also described in U.S. Pat. No. 6,200,543 (the entire disclosure of which is incorporated herein by reference).

(20) As noted above, the refined fine lime (53) from the air classification system (11) then undergoes a hydration or slaking process (12). The hydration process (12) generally comprises a hydrator (12), and in a preferred embodiment, the refined fine lime (53) is combined with an excess of water (54) and is rapidly mixed which allows the calcium oxide to react with the water to form a wet or damp calcium hydroxide (hydrated lime). The water (54) for hydration is generally fed at a reasonably high temperature but low enough that the refined fine lime (53) is not overheated (burned). In this regard, the water feed (54) and hydrator (12) temperature should be maintained below the boiling point of water, and more preferably, at a temperature equal to or below 180 F. Utilizing an excess of water (i.e., more than necessary to react with the CaO) also helps prevent overheating and burning and helps separate the individual particles. In any event, the damp hydrated lime (56) that leaves the hydrator (12) should have residual moisture of at least 5%. Generally, the water level will be about 5-35% or about 10-25%.

(21) In addition to the water, other additives (55) may be included in the water feed (54) that is utilized in the hydration process (12). These additives can help improve the reaction rate of the final hydrated lime product. These additives are generally accelerators or retarders, which, as their names suggest, accelerate or retard the reaction of calcium oxide to calcium hydroxide. Any known accelerators or retarders can be utilized, including, alkaline-earth chlorides (e.g., barium chloride, calcium chloride, sodium chloride, potassium chloride, aluminum chloride), other salts (e.g., aluminum nitrate, sodium carbonate, sodium borate, potassium permanganate, potassium chlorate, table salt, Rochelle salt), acids (e.g., hydrochloric acid, sulfuric acid, oxalic acid, nitric acid, acetic acid, lactic acid), alkanols (e.g., mono-, di-, and tri-ethanolamine, dimethylethanolamine, methyl diethanolamine, triisopropanolamine), and sugars. Further examples of such accelerators and retarders, and their use in hydrated lime production can be found in U.S. Pat. Nos. 1,583,759; 1,649,602; 1,664,598; 2,193,391; 2,423,335; 2,437,842; 3,120,440; 4,626,418; 4,786,485; 5,173,279; 5,306,475; 5,308,534; 5,332,436; 5,502,021; 5,618,508; 5,705,141; 6,322,769; and 7,744,678 (the entire disclosures of which are incorporated herein by reference).

(22) Once the water addition is complete, excess water is removed from a wet hydrated lime to make a damp hydrated lime (56) but the material is not completely dried. Instead, the damp calcium hydroxide (56) is allowed to mature (22) prior to flash drying. It has been found that improved results of both pore volume and surface area can be obtained by utilizing aging (maturation) of a damp calcium hydroxide (56) prior to flash drying (14) and milling (40). Traditionally, after formation of a damp hydrate (slaking), the damp hydrate (56) will generally be flash dried quite quickly (less than 30 minutes) after the water is supplied. This can still be performed in an embodiment if time is not available for the maturation.

(23) In the present case, the damp hydrate (referred to herein as a damp hydrate putty) (56) is maintained, or allowed to mature, as a lime putty (lime hydrate putty) for a period of time (22) prior to flash drying (14) which is generally greater than 30 minutes and often significantly greater than 30 minutes. It is generally preferred that this time be at least 30 minutes. In an embodiment, the amount of time will be at least 4 hours. In another embodiment, it will be at least 6 hours, and in a still further embodiment, it may be measured as a multiple of days, weeks, months, or even years. It is generally preferred that the damp hydrate putty (56) be matured (maintained) for between 4 hours and 14 days. To provide for more reasonable commercial manufacturing times, the maturation period may be between 4 and 6 hours.

(24) For purposes of this disclosure, a damp hydrate putty (56) will generally be considered any hydrate including at least 5% residual moisture, and will generally have at least 10% residual moisture. However, it may be less in certain embodiments. In an embodiment, a damp lime hydrate (56) will be one which will generally have about 10% to about 30% residual moisture through the maturation process. However, the amount of moisture at any instant may vary freely within that range, and may, at times, go outside the range so long as it is maintained in the range a majority of the time. For effective commercial manufacturing and handling, the putty (56) may, in various embodiments, comprise between 10% and 30% residual moisture, 30% or more residual moisture, 40% or more residual moisture, or 40%-60% residual moisture.

(25) Aging of high calcium slaked lime putty has been recognized as a means of improving the quality of calcium hydroxide used in lime mortars and plasters which is effectively what the damp hydrated lime putty (56) is. Putty, as the name implies, is a wet, plastic paste form of hydrated lime that has a moldable body. At 40-60% excess water by weight, it is characterized by some investigators as a thixotropic colloidal gel. The amount of excess water content determines whether it is a stiff or sloppy putty. Such putties are common as a building material and are also often use in building restoration to mimic original materials since they dry into an effective mortar. However, such putties are naturally hardened (often in the sun) and are allowed exposure to atmospheric CO.sub.2.

(26) Aging or maturation of the present putty (56) is preferably performed without the putty (56) being exposed to CO.sub.2 such as in ambient air. During maturation, hydrate crystals in the putty (56) are known to undergo morphological and dimensional changes, developing sub-micrometer, plate-like crystals. Without being limited to any theory of operation, these microstructure changes that occur during this aging process, commonly referred to as maturation, seems to induce the development of submicron tabular-shaped Ca(OH).sub.2 particles and a decrease in crystallinity (as determined per XRD analysis). Nitrogen adsorption measurements (BET methods) also show an increase in BET surface area of aged lime putties. These changes in crystalline structure may explain why an aged calcium hydroxide putty (56) sees improvement in plasticity, workability, and the water retention properties.

(27) Lime putties, however, are known to behave as an irreversible colloid that, once dried, lose the reactivity and rheological properties that had existed as a lime putty. This irreversible colloidal behavior would appear to imply that any benefits that were to be gained by maintaining the hydrate as a lime putty (56) would be lost upon later flash drying (14). However, that does not appear to be the case with pore volume and BET surface area when the lime putty (56) is later flash dried in a flash dryer (14).

(28) While the micro-structural changes associated with the maturation of a high calcium lime putty (40-60% excess water by weight) (56) can occur after months and/or years of storage, FIGS. 5 and 6 show that the aging of a damp calcium hydroxide putty (56) for as little as 4-6 hours has a very positive effect on improving both the BET surface area and pore volume of a later flash dried hydrate (57) as illustrated in FIGS. 5 and 6. As can be seen from FIGS. 5 and 6, additional time provides ever increasing benefits.

(29) Once matured, the damp hydrated lime (56) is then dried (14). In a preferred embodiment, the damp hydrated lime (56) is flash dried using air (64) from an indirect heat source (17) with a temperature of about 550 F. to about 850 F. Using indirect heat prevents the hydrate from contacting the combustion gas, which can occur if a direct heat source were to be used. This contact would result in the loss of some of the available calcium hydroxide. In any event, the dried hydrated lime (57) generally will have a residual moisture content of about 1% or less.

(30) As noted above, the presence of CO.sub.2 in air which then comes in contact with the hydrated lime can compromise the chemical integrity of the hydrated lime. While hydrated lime has greater moisture stability than calcium oxide, hydrated lime is perishable unless adequately protected from CO.sub.2 absorption and the introduction of CO.sub.2 into the hydrated lime can result in recarbonation (i.e., Ca(OH).sub.2 to CaCO.sub.3). Thus, in some embodiments, the chemical purity can be further improved if the indirect heater is supplied with conditioned air (63) that has a reduced CO.sub.2 content. Examples of apparatuses and methods for such air conditioning (i.e., reduction of CO.sub.2 content in the air stream) are disclosed, for example, in U.S. Pat. Nos. 5,678,959 and 6,200,543 (the entire disclosures of which are incorporated herein by reference). In one preferred embodiment, ambient air (65) (e.g., about 300 ppm CO.sub.2) is fed into an air conditioner (18), resulting in conditioned air (63) with a CO.sub.2 concentration of less than 100 ppm CO.sub.2. The benefits of this low CO.sub.2 conditioned air can be seen in Table 2 below, which shows various compositions and BET surface area dried hydrated lime depending on the amount of CO.sub.2 in the air used to dry the damp hydrated lime.

(31) TABLE-US-00002 TABLE 2 Effect of CO.sub.2 in Drying Air Available % CO.sub.2 in B.E.T. Ca(OH).sub.2 Hydrate % CaCO.sub.3 Surface Area 11-12% 80-84% 6.4-7.2% 15.34% 32-33 m.sup.2/g CO.sub.2 (82.54% avg.) (6.75% avg.) (32.57 avg.) 560 ppm 93.5-94.5% 1-1.2% 2.29% 35-38 m.sup.2/g (94.06% avg.) (1.01% avg.) (37.77 avg.) 120 ppm 94-95% 0.6-0.7% 1.41% 37-39 m.sup.2/g (94.64% avg.) (0.62% avg.) (38.81 avg.)

(32) As noted above, conditioned air (66) with the same or different CO2 concentrations as the conditioned air (63) for drying can also be fed into the mill (10), air classification system (11), and/or hydrator (12) to help prevent recarbonation. Additionally, conditioned air (61) can be fed into any additional classifiers (15) and/or mills (16), if present and as discussed more fully below.

(33) After being dried in a flash dryer (14), the dried hydrated lime (57) is then classified (15) and milled (16). The dried hydrated lime (57) is first fed into a classifier (15). If it meets the desired properties (e.g., those discussed herein, including purity reactivity, BET surface area, and particle size), dried hydrated lime (57) is utilized as the final hydrated lime product (60). Some of the dried hydrated lime (57), however, may not meet the desired properties. This non-final hydrated lime (58) is then fed into the mill (16) to be grinded, with the grinded hydrated lime (59) being fed back into the classifier (15) to determine if the material can be utilized as the final hydrated lime product (60). This process of milling (16) and classifying (15) can continue for as long as is necessary.

(34) Again, in a preferred embodiment, the milling (15) and classification system (16) are conducted in a closed circuit system to prevent air carbonation from occurring. Conditioned air (61) (i.e., low CO2) can further be injected into the milling (15) and classification system (16) to replace any transient air being drawn into the process and prevent recarbonation.

(35) The above process of manufacturing describes a process in which the drying (14), classifying (15), and milling (16) of the damp hydrated lime (56) are conducted independently. As would be understood by one of ordinary skill in the art, milling and classification system can be, and commonly are, integrated into one system. An example of an integrated milling and classification system is depicted in FIG. 2, wherein dried hydrated lime (57) is fed into the milling/classification system (40) with injected conditioned air (61) and the resultant final hydrated lime product (60) has the desired properties as discussed above. Similarly, an integrated milling and classification system can be further integrated into a dryer. An example of such a fully integrated system is depicted in FIG. 3, wherein the damp hydrated lime (56) is fed into the milling/classification/dryer system (41) with an indirect heat source (64) and the resultant final hydrated lime product (60) has the desired properties as discussed above.

(36) In any event, embodiments of the manufacturing process described herein result in a final hydrated lime product (60) with a high purity and high reactivity. In particular embodiments, the final hydrated lime product (60) has a purity of 95% calcium hydroxide or greater, a reactivity in citric acid of less than 10 seconds, a BET surface area of greater than 30 m.sup.2/g, and a particle size of less than about 44 microns (325 mesh) for about 98% of the particles. As can be seen from FIG. 4, lime produced by the manufacturing processes described herein has a much faster reactivity over lime produced by conventional methods.

(37) While the invention has been disclosed in conjunction with a description of certain embodiments, including those that are currently believed to be the preferred embodiments, the detailed description is intended to be illustrative and should not be understood to limit the scope of the present disclosure. As would be understood by one of ordinary skill in the art, embodiments other than those described in detail herein are encompassed by the present invention. Modifications and variations of the described embodiments may be made without departing from the spirit and scope of the invention.

(38) It will further be understood that any of the ranges, values, properties, or characteristics given for any single component of the present disclosure can be used interchangeably with any ranges, values, properties, or characteristics given for any of the other components of the disclosure, where compatible, to form an embodiment having defined values for each of the components, as given herein throughout. Further, ranges provided for a genus or a category can also be applied to species within the genus or members of the category unless otherwise noted.