Patent Publication Number: US-2007104630-A1

Title: Method of removing fluoride from quicklime and hydrated lime

Description:
BACKGROUND OF THE INVENTION  
      A. Field of the Invention  
      The present invention relates to a method for producing food grade quicklime, CaO, and hydrated lime, Ca(OH) 2 , which meets the standards as required by CODEX without requiring extensive changes in existing equipment or process steps.  
      B. Description of the Prior Art  
      Lime has a variety of uses. It is commonly used in treating waste water and sewage. It is used in agriculture to neutralize acidic soils and to provide nutrients for sustaining plant life. Lime is also used extensively in construction for the stabilization of soils and as a component in a variety of building materials. Lime is also used in a variety of “food grade” products intended for human consumption.  
      The term “lime” is often used in an informal sense to mean both quicklime (calcium oxide) and hydrated lime (calcium hydroxide). Quicklime is produced by heating limestone (calcium carbonate) in a kiln at high temperatures to “calcine” the material and thereby drive off carbon dioxide. Quicklime is usually in the form of lumps or pebbles. In order to further process lime and improve the ease with which it is handled, dry lime is often mixed with water to form a slurry. In the case of quicklime, the water reacts with the quicklime in an exothermic reaction to form hydrated lime. This is often referred to as slaking. During the slaking of quicklime, large amounts of heat are given off which can significantly raise the temperature of the slurry. Water can then be driven off to produce dry, hydrated lime which is usually a powder.  
      Food grade quicklime and hydrated lime are specific materials that are sold to the food processing industry in the United States. The specifications for food grade lime products are defined by CODEX. The CODEX Alimentarius Commission was created in 1962 by two U.N. organizations, the Food and Agricultural Organization (FAO) and the World Health Organization (WHO). CODEX is the major international mechanism for encouraging fair international trade in food while promoting the health and economic interests of consumers. Within the United States, CODEX activities are coordinated by officials from the U.S. Department of Agriculture, the U.S. Food and Drug Administration and the U.S. Environmental Protection Agency. In the United States, there are a number of companies that produce “normal”, i.e., industrial grade, hydrated lime for industrial use. However, there are only two companies, known to Applicant at the present time, that produce “food grade” hydrated lime. This is due, at least in part, to the exacting chemical specifications required by CODEX. Many hydrated limes that are suitable for general industrial use, fail to meet the CODEX standards since they exceed, for example, the limits for trace metals found in the compositions.  
      One of the critical specifications for Food Grade quicklime and hydrated lime is the maximum concentration of fluoride. The Food Chemicals CODEX specification is 50 ppm maximum for hydrated lime and 150 ppm for quicklime. Many commercial limestone deposits which yield material to make commercial quicklime and hydrated lime would meet all the other CODEX specification but are above the 50 ppm, or 150 ppm maximum fluoride limit.  
      As mentioned above, hydrated lime is produced by first heating limestone in a kiln (calciner) to remove carbon dioxide and form quicklime. The quicklime is reacted with an aqueous slaking medium to form calcium hydroxide, commonly referred to as hydrated lime or lime hydrate. To achieve the CODEX chemical specifications has, in the past, required special processing steps or special process conditions which present either economic or practical disadvantages. For example, the production of food grade hydrated lime has generally required the use of the purest stock of limestone as the calciner feed, as well as the use of natural gas as a fuel for the kiln instead of the more inexpensive fuels such as coal or coke.  
      Applicant&#39;s own issued U.S. Pat. No. 6,926,879, to Huege et. al, issued Aug. 9, 2005, discloses a method for producing food grade hydrated lime which involves a special set of classification steps for the hydrated lime after the calcination and hydration steps. A source of raw, hydrated lime is first passed through a classification step which divides the raw hydrated lime into a first fine stream and a first coarse stream. The first coarse stream can be, for example, passed to a grinder which produces a ground coarse product. The first fine stream is separated out from the first coarse stream and, without combining the first fine stream with the first coarse stream or with the ground coarse product, is removed to produce a very fine sized product which meets CODEX chemical specifications. While this method was an improvement to existing technology in, for example, not requiring extremely pure limestone feed, it did involve additional processing steps and equipment and did not work for all sources of limestone feed stock.  
      A need exists, therefore, for a method for producing food grade hydrated lime which meets CODEX specifications without drastic changes in the equipment or without adding additional process steps to those presently employed in calcining limestone to produce quicklime, or in slaking the quicklime to produce hydrated lime.  
      A need exists for a method for producing food grade quicklime and hydrated lime which would allow the use of normal limestone as feed to the calciner, without requiring the purest of limestone as feed.  
      A need also exists for such a process which would allow the use of solid fuel sources in the calciner, rather than requiring the use of more expensive natural gas as a fuel source.  
     SUMMARY OF THE INVENTION  
      The present invention provides a method for removing fluoride from quicklime/hydrated lime which method makes what would otherwise be an unacceptable material instead a material which is capable of meeting CODEX fluoride specifications. In practicing the improved method, it is not generally necessary to add additional process equipment or process steps. Rather, the inventive method involves the calcination temperature at which the calcining kiln is operated. It is well recognized that the decomposition of calcium carbonate to calcium oxide occurs at approximately 900-1000° C., and the typical commercial kiln is operated at a kiln discharge temperature in the range from 1000° C. to 1100° C. Operating a lime kiln at a higher discharge temperature than that necessary to adequately effect the decomposition of the calcium carbonate would not normally be considered desirable, since such higher operating temperature would adversely affect (lower) the reactivity of the quicklime and also increase the production cost because of the greater fuel requirement.  
      The present invention involves the discovery that, by increasing the calcination temperature a selected amount, the concentration of fluoride can be dramatically reduced to a level below what is required for food grade hydrated lime. By increasing the calcination temperature to 1200° C., the concentration of fluoride in the quicklime can be reduced below the 50 ppm requirement for the CODEX specifications for food grade hydrated lime and below the 150 ppm requirement for quicklime.  
      Additional objects, features and advantages will be apparent in the written description which follows.  
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       FIG. 1  is a simplified, perspective view of a lime calcining plant showing raw limestone being fed to a horizontal kiln;  
       FIG. 2  is a simplified flow diagram showing the steps involved in a process for producing hydrated lime.  
    
    
     DETAILED DESCRIPTION OF THE INVENTION  
      In the discussion which follows, the term “quicklime” will be taken to mean calcium oxide and should not be confused with limestone (calcium carbonate). As briefly outlined in Applicant&#39;s background discussion, quicklime is manufactured from limestone by heating to remove carbon dioxide. Quicklime can be converted to Ca(OH) 2  by a slaking process where water and CaO are mixed under agitation and temperature to produce Ca(OH) 2 , known in the industry as slaked lime or lime hydrate.  
       FIG. 1  is a somewhat simplified overall view of one type of lime calcining plant. Raw limestone is first fed, in this case on a conveyor belt  19  to a calciner  11  which in this case is a horizontal kiln. The kiln  11  is fired by burners which typically utilize pulverized coal as a fuel and are typically operated in a calcining discharge temperature range of about 1000 to 1100° C. The load of raw limestone is introduced at an inlet end of the kiln. The kiln is a substantially cylindrical, horizontally oriented elongate chamber having a drive means for rotating the chamber. The longitudinal axis of the rotatable chamber is given a slight slope to the horizontal so that the load within the chamber travels downwardly toward a discharge end by gravity. The load is located in a lower portion of the approximately circular cross-section of the kiln as the kiln rotates. One or more burners are located adjacent the discharge end of the rotatable chamber so that the temperature in the kiln becomes progressively hotter as the load approaches the discharge end. All kilns are refractory lined in order to tolerate the extreme temperatures employed.  
      Conventional rotary kilns  11  of the type shown in  FIG. 1  typically have a diameter to length ratio of about 1:30-40 with lengths of 75-500 feet and diameters of 4-11 feet being typical. See Boynton, “Chemistry and Technology of Lime and Limestone”, Wiley-Interscience, 2 Ed., pages 254-255. The incline angle for such kilns is typically in the range from about 3-5° on a series of foundation piers  12 , allowing the kiln to rotate on trunnions  17  at each pier. The rotation speed of the kiln is adjustable through the use of variable speed drives, or similar mechanisms, with the typical kiln revolving at a rate of about 35-80 revolutions per hour.  
      Kilns are typically lined with about 6 to 10 inches of refractory brick, plus some insulation and are encased in a shell  14  of heavy steel boiler plate that has been welded in sections. Limestone is charged into the kiln at the elevated, inlet end  21  from a storage silo or conveyor feed and quicklime is discharged at an outlet or lower end  23 , moving countercurrent to the flow of combustion gases, derived from fuel injected at the lower end. Such kilns are typically charged with only a maximum of about 10% limestone so that about 90% of the interior kiln space is confined to the flame and hot gases. The term “calcining discharge temperature” will be taken in this discussion to mean the temperature at the outlet or lower end  23  of the kiln  11  as viewed in  FIG. 1 .  
      Any type of fuel can be employed with rotary kilns, such as petroleum coke, coal tar from coke ovens, and waste gaseous carbon monoxide from steel and chemical plants. Pulverized coal is perhaps the leading fuel for rotary kilns in the United States. All coal burning rotary plants generally maintain their own pulverization equipment attached to each kiln. Finally divided pulverized coal of about 75% passing a number 200 mesh screen is typically used as the fuel source. As has been mentioned, the typical horizontal kiln of the type shown is operated at a calcining discharge temperature in the range from about 1000-1100° C. This temperature range will be referred to in this discussion as the calcining discharge temperature range “customarily used in the industry.” 
      The intense heat causes a chemical reaction as follows: 
 
CaCO 3 +heat=CaO(quicklime)+CO 2  
 
      This is referred to the step of calcining or the calcining reaction. The calcined quicklime can then be converted to hydrated lime in a slaking operation by mixing with an aqueous slaking medium in hydrator.  
       FIG. 2  is a simplified flow diagram showing the further processing steps in producing a typical hydrated lime product. The calcined quicklime from the calciner  11  passes to the hydrator  13 . The hydrator  13  may be, for example, a tank or other chamber having an internal paddle agitator. Contact between the calcined quicklime and the water in the hydrator  13  results in an exothermic reaction generating heat and calcium hydroxide: 
 CaO+H 2 O═Ca(OH) 2 +heat+steam  
      The size and quality of slaked lime particles in the resulting slurry are dependent on a number of variables. These include the reactivity, particle size and gradation of the quicklime used. Other variables include the amount of water used, the quality of the water, and the amount and type of water impurities. Further, the temperature of the water and the amount of agitation can affect slaked lime quality and particle size.  
      The excess water not converted to calcium hydroxide is heated to steam and the steam is volatized from the solid calcium hydroxide particles. The solid calcium hydroxide leaving the reactor is composed of individual calcium hydroxide particles, agglomerated calcium hydroxide particles, individual impurity particles, and impurities associated with the individual and agglomerated calcium hydroxide particles. These materials are represented as “dry agglomerate” in the step  15  shown in  FIG. 1  and comprise a broad distribution of calcium hydroxide particles.  
      This broad distribution of particles is then normally screened or air classified in a step  16  which divides the raw hydrated lime into a first fine stream  18  and a first coarse stream  20 . Because a large percentage of the calcium hydroxide is present in the coarse fraction of stream  20 , this fraction is normally ground in a dry grinding step  22  and then returned to the fine calcium hydroxide particles in a stream  24 , the particles being mixed in a step labeled as  26  in  FIG. 2 . During this process all the impurities present in the quicklime, calcium oxide feed are also present in the final calcium hydroxide product which passes out stream  28 .  
      The process described in  FIGS. 1 and 2  is capable of producing a quicklime or hydrated lime product which is acceptable for many industrial and commercial applications. These would include the previously mentioned treatment of waste water and sewage, agricultural uses, construction uses for the stabilization of soils and as a component of various building materials. However, lime products of the above type are not generally acceptable for use in “food grade” products intended for human consumption since they would not comply with CODEX specifications, particularly the specifications for acid insoluble material.  
      The inventive method provides a relatively simple mechanism for removing fluoride from limestone/quicklime that would make otherwise unacceptable material capable of meeting CODEX fluoride specifications. Applicants have found that the fluoride contained in a raw limestone feed does not become volatile, i.e. is not removed, during the normal process of producing quicklime or hydrated lime. Thus, a limestone that contains approximately 50 ppm fluoride as a raw feed will, when calcined, contain approximately 100 ppm fluoride. This is primarily the result of the 44% loss in weight which occurs in the transformation of calcium carbonate to calcium oxide as explained by the above described chemical reactions.  
      As has been discussed, the fuels which are used to fire either horizontal or vertical kilns are typically natural gas, coal, coke, fuel oil, or waste material with some heat value. In many cases, perhaps with the exception of natural gas, the fuel itself may introduce additional fluoride into the quicklime during the calcination process. For these and other reasons, the typical calcining operation used to convert limestone to quicklime, which is converted ultimately to hydrate lime, produces a product which fails to meet CODEX standards in terms of fluorine content for hydrated lime.  
      Applicant&#39;s inventive method is directed toward improvements in the calcining process as described above which allow the production of food grade hydrated lime or quicklime from typical raw limestone feedstock, rather than requiring feed stock of unusual purity. The normal types of fuel sources can be utilized, rather than having to use natural gas exclusively. Also, it is not generally necessary to perform drastic modifications to the existing calcining plant equipment or to add additional process steps.  
      Rather, Applicant&#39;s inventive method involves the calcination temperature at which the calcining kiln is operated. It is well recognized that the decomposition of calcium carbonate to calcium oxide occurs at approximately 900-1000° C., and the typical commercial kiln is operated at a kiln discharge temperature in the range from 1000° C. to 1100° C., as was discussed in Applicant&#39;s Background portion of the specification. Operating a lime kiln at a higher discharge temperature than that necessary to adequately effect the decomposition of the calcium carbonate would not normally be considered desirable, since such higher operating temperature would lower the reactivity of the quicklime and also increase the production cost because of the greater fuel requirement.  
      The present invention involves the discovery that, by increasing the calcination discharge temperature a selected amount, the concentration of fluoride in the quicklime can be dramatically reduced to a level below what is required for food grade quicklime (150 ppm), or for hydrated lime (50 ppm). Applicant has discovered that, by increasing the calcination temperature above the current operating temperature ranges, i.e., to 1200° C., the concentration of fluoride in the quicklime can be reduced below the 50 ppm requirement for the CODEX specifications for food grade hydrated lime. As used in this discussion, the “selected amount” by which the kiln discharge temperature is increased is defined to be the amount of temperature increase, above the normal operating ranges of 1000-1100° C., which is necessary to produce a calcined quicklime product having a fluoride content of 50 ppm or below for hydrated lime, or 150 ppm in the case of quicklime. The particularly preferred operating temperature, according to the principles of Applicant&#39;s invention, is approximately 1200° C.  
      This is best achieved in commercial kilns such as rotary kilns or vertical kilns where the hot combustion gases are not passed over the quicklime to improve energy efficiency, as would be true in regenerative vertical kilns. In a regenerative vertical kiln, the volatilized fluoride could be re-deposited back onto the quicklime.  
      The data contained in Table I below demonstrates the beneficial effect of a selected discharge operating temperature increase (increasing the discharge temperature to at least 1200° C.) on the fluoride concentration of commercial quicklimes.  
               TABLE I                          Volatilization of Fluoride from Quicklime                         PPM Fluoride                                 AS IS (1)   1100*C. (2)   1200*C. (2)                                                 Quicklime # 1   144   96   30           Quicklime # 2   92   109   50           Quicklime # 3   135   112   52           Quicklime # 4   63   48   34           Quicklime # 5   82   76   46                         (1) Commercial pebble QL                (2) Calcined at temperature for 2 hours in Muffle furnace             
 
      By increasing the kiln discharge temperature by 100° C., i.e., from 1100° C. to 1200° C., the concentration of fluoride was decreased in all of the commercial quicklime samples to a level that would produce food grade hydrated lime, meeting the CODEX fluoride limits of 50 ppm maximum. Applicant&#39;s additionally observe that further increasing the kiln discharge temperature to, for example, 1300° C., did not yield any additional lowering of fluoride concentration, and thus served no useful purpose. Applicant&#39;s have also observed that, regardless of the starting fluoride concentration of the commercial quicklimes, the final fluoride concentrations were in the range of 30-50 ppm which would achieve CODEX food grade hydrated lime.  
      An invention has been provided with several advantages. The method of the invention produces food grade hydrated lime from ordinary raw limestone feed stock. It is not necessary to use more expensive, high purity feedstock. Ordinary fuel sources, such as powdered coal, can be utilized in the calcining operation without requiring the use of more expensive natural gas. It is not necessary to add process steps or additional processing equipment such as might be involved in material classification and sorting systems.  
      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.