BACKGROUND OF THE INVENTION

Field of the Invention

This disclosure is related to the field of calcium products manufacturing and more particularly to manufacturing calcium oxide and hydrated lime with a reduced amount of impurities, particularly lead.

Description of the Related Art

Calcium carbonate (CaCO3), more commonly known as lime or limestone, is a naturally occurring sedimentary rock having a number of industrially useful applications usually due to it being a source for calcium. As with most substances, the practical and industrial uses of the substances are related to the chemical composition and structure of the substance. This is because the uses of the substances rely on the chemical composition in reactions.

In its naturally occurring form calcium carbonate is generally a chalky white-ish solid commonly referred to as limestone but there is substantial variation in color and brightness across limestone from different sources. In particular, variations based on the geographical region and the geological environment from which it is mined are common and expected. Similarly, calcium carbonate from mined sources is typically different from that of precipitated or manmade sources. While some limestone is naturally very white, others may have an ashen or gray tone or can be colored yellow or pink depending on mineral impurities present in the limestone. Due to the usefulness of calcium carbonate as a filler in paints, pigments, and paper, brighter white calcium carbonates are usually more commercially desirable and considered higher quality.

In practice, most substances we encounter in our everyday lives are not pure. Even filtered drinking water is not exclusively composed of H2O, and contains some small amount of unwanted inclusions, as well as desired inclusions, such as fluoride. In practice, virtually no substance is completely “pure,” and even artificially purified substances, such as precious minerals and metals, contain at least trace amounts of inclusions and impurities. Chemical purity is assessed on a case-by-case basis depending upon the particular substance in question, and how it is intended to be used. Calcium carbonate, particularly in its naturally occurring form of limestone, frequently contains a large number of inclusions and impurities, in large or small amounts. Common inclusions in limestone include magnesium, clay, silica, iron compounds, and sulfates. Some inclusions, in some quantities, are acceptable for certain uses, but others are problematic. Some of the more problematic materials in limestone can be minerals that are naturally darker, as these can lower the brightness of the limestone, and those that can cause problems for other chemical products made from calcium carbonate.

Two particularly useful chemicals made from naturally occurring calcium carbonate are calcium oxide (CaO), which is commonly known as quicklime and is typically used in making steel, cement, and other industrial products (as well as in some food applications), and calcium hydroxide or hydrated lime (Ca(OH)2), which can be used in food additives, water purification, and pollution mitigation.

Typically both calcium oxide and calcium hydroxide are made from calcium carbonate. Specifically, ground calcium carbonate is heated (or calcined) in a lime kiln to a temperature above 825° C. to form quicklime (calcium oxide) in accordance with the following formula:
CaCO3(s)→CaO(s)+CO2(g)
To form hydrated lime, the quicklime is typically subjected to fine grinding or milling to produce a finer material which can be of varying sizes in different embodiments. 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. Once a certain percentage of all the ground particles meet a desired size (e.g., 95% or smaller than 100 mesh), the ground quicklime meeting the desired size requirements is fed into a hydrator, where the calcium oxide reacts with water (also known as slaking), and is then dried (commonly in an environment relatively free of CO2) to form calcium hydroxide in accordance with the following equation:
CaO+H2O→Ca(OH)2
Finally, the resultant calcium hydroxide is then milled and classified until it meets desired criteria for its end use.

Impurities in the original limestone often are carried into the resultant calcium oxide and calcium hydroxide compositions which are produced above. Depending on the uses of these materials, certain impurities can create substantial problems. One such concerning impurity is lead which, if carried into resultant calcium hydroxide, can create problems in pollution mitigation compounds (since it is a controlled pollutant) and in food applications (as it is highly toxic to humans). However, as a naturally occurring substance, determining the amount of lead in original limestone can be problematic which can also result in concerns in determining the reduction of lead necessary to be performed to provide safe end products efficiently.

Further, while lead is a readily identifiable problematic inclusion, it is not the only one. For example, US Patent Application Publication 2007/0104630 and U.S. Pat. Nos. 2,021,663 and 2,127,626 (the entire disclosure of all of which are herein incorporated by reference) attempt to remove fluoride (another potentially problematic inclusion) from calcium oxide and hydrated lime.

While the above patents and applications provide for methods to reduce fluoride, this is only a single impurity that presents concerns. Therefore, there is a need in the art to purify limestone to reduce the mass of a number of impurities in a given mass of resultant calcium oxide and/or calcium hydroxide and particularly to remove lead.

SUMMARY OF THE INVENTION

The following summary of the invention is provided to give the reader a basic understanding of some aspects of the invention. This summary is not intended to identify key or critical elements of the invention or to delineate the scope of the invention. The sole purpose of this section is to present some concepts of the invention in a simplified form as a prelude to the more detailed description that is presented in a later section.

Because of these and other problems in the art, described herein, among other things, are systems and methods for impurity removal to limestone using modifications to the typical calcination process of turning calcium carbonate into calcium oxide. Traditional calcination of calcium carbonate has not been shown to remove certain heavier elements from the limestone compound, but an alternative process may be employed which does produce reduced content of certain heavy elements. Specifically, substantially increasing the temperature along with increases to soak time can result in reductions in lead and certain other heavier and/or problematic elements.

In an embodiment, there is described a method of calcining limestone comprising: providing limestone including calcium carbonate and impurities, said impurities including lead; and calcining said limestone to produce a calcium oxide composition, said calcining occurring at a temperature of 1100° C. or higher and having a soak time greater than 1 hour; wherein an amount of said lead in said calcium oxide composition is reduced compared to an amount of said lead in said limestone.

In an embodiment of the method, the calcining occurs at a temperature of 1200° C. or greater, 1300° C. or greater, 1400° C. or greater, 1500° C. or greater, or 1600° C. or greater.

In an embodiment of the method the soak time is 2 hours or greater.

In an embodiment of the method, the amount of said lead in said calcium oxide is no more than 75% said amount of said lead in said limestone, no more than 55% said amount of said lead in said limestone, no more than 25% said amount of said lead in said limestone, or undetectable.

There is also described herein, in an embodiment a method of calcining limestone comprising: providing limestone including calcium carbonate and impurities, said impurities including cadmium, cerium, cesium, cobalt, copper, fluoride, lead, lithium, potassium, rubidium, sodium, thorium, and zinc; and calcining said limestone to produce a calcium oxide composition, said calcining occurring at a temperature of 1100° C. or higher and having a soak time greater than 1 hour; wherein an amount of said impurities in said calcium oxide composition is reduced compared to an amount of said impurities in said limestone.

In an embodiment of the method, the calcining occurs at a temperature of 1200° C. or greater, 1300° C. or greater, 1400° C. or greater, 1500° C. or greater, or 1600° C. or greater.

In an embodiment of the method the soak time is 2 hours or greater.

There is still further described herein, a method of calcining limestone comprising: providing limestone including calcium carbonate and impurities, said impurities including fluoride; and calcining said limestone to produce a calcium oxide composition, said calcining occurring at a temperature of 1100° C. or higher and having a soak time of at least 2 hours; wherein an amount of said fluoride in said calcium oxide composition is reduced compared to an amount of said fluoride in said limestone.

In an embodiment of the method, the calcining occurs at a temperature of 1600° C. or greater.

