Chemistry of inorganic compounds – Carbon or compound thereof – Oxygen containing
Reexamination Certificate
2000-03-06
2004-02-03
Bos, Steven (Department: 1754)
Chemistry of inorganic compounds
Carbon or compound thereof
Oxygen containing
C423S430000
Reexamination Certificate
active
06685908
ABSTRACT:
FIELD AND BACKGROUND OF THE INVENTION
The present invention relates to a novel form of precipitated aragonite and to a novel process for producing it.
Various routes are known for the production of calcium carbonate, which finds use as a thickening material, as a filler, as an extender, and most of all as a pigment, in a variety of industries such as pharmaceuticals, plastics, adhesives, printing, coating (paint), paper, rubber and in filtration. For such purposes, there may be used ground calcium carbonate (GCC) or precipitated calcium carbonate (PCC). PCC in general possesses advantages over GCC, in that it is economical to produce and its precise composition, or purity, can be more strictly controlled.
The most frequently used chemical process for producing PCC is based on the carbonation of aqueous suspensions of calcium hydroxide (also known as “milk of lime” or “slaked lime”) with carbon dioxide gas, or with a carbon dioxide containing gas. This process gives rise to relatively pure precipitated calcium carbonate and is a preferred process, because there are no serious problems of contamination of the product with undesired salts, and moreover it can be controlled in order to adjust the properties of the final product. Thus, the process is based essentially on four stages: firstly, calcination of raw limestone to produce calcium oxide or “quicklime” and carbon dioxide gas or a carbon dioxide containing gas; secondly, “slaking” of the quicklime with water to produce an aqueous suspension of calcium hydroxide; thirdly, carbonation of the calcium hydroxide with carbon dioxide gas or a carbon dioxide containing gas; and finally, downstream operations such as dewatering, drying, deagglomeration, grinding, surface treatment, surface coating, mixing with other minerals (e.g. titanium dioxide, talc, kaolin, GCC, PCC—including aragonite PCC) and dyeing, which allow optimization of the properties of the precipitated calcium carbonate particles in order to be adapted to their intended uses.
Calcium carbonate can be precipitated from aqueous calcium hydroxide slurries or solutions in three different crystallographic forms (polymorphs): the vaterite form which is thermodynamically unstable, the aragonite form which is metastable under normal ambient conditions of temperature and pressure, and the calcite form which is the most stable and the most abundant in nature. These forms of calcium carbonate can be prepared by carbonation of slaked lime by suitable variations of the process conditions.
The calcite form is easy to produce on industrial scales, as precipitated calcium carbonate particles. It exists in several different shapes, of which the most common are the rhombohedral shape and the scalenohedral shape.
Aragonite forms crystals having a length/width ratio (hereinafter—“aspect ratio”) in the range between >1:1 and 100:1 of which a typical aspect ratio is 10, in which case the aragonite forms long, thin needles. Therefore, aragonite having a high aspect ratio may be denoted hereinafter—“acicular aragonite” or “needle-shaped aragonite”. The production of aragonite is a slow process and is very difficult to control on an industrial scale.
PCC particles are used as thickening materials, fillers, extenders and, most of all, as inexpensive pigments. The latter use implies that a particularly desirable property of this material is its light scattering characteristics, in order to be able impart opacity and brightness to the products containing it. Such characteristics are optimized, when the pigment particles are very effectively dispersed and are apart by an average distance in the range between 0.2 &mgr;m and 0.4 &mgr;m in their final products, and their size distribution is in the range between 0.2 &mgr;m and 0.4 &mgr;m, namely, in the range of half a wavelength of the visible light. That means that either the production of the PCC should be adjusted to produce small particles in order to avoid expensive downstream particle size reduction operations and to cope with the expensive problems of dewatering and drying the product, or, alternatively, the process should be adjusted to produce large particles, and subsequently effect the downstream dewatering and grinding operations. In both cases, the production costs of precipitated calcium carbonate of pigment grades may be doubled or tripled just because of these unavoidable downstream steps.
High light scattering pigments currently available to the above-mentioned industries include titanium dioxide particles, which are very effective to scatter the light due to their relatively high refractive index (2.76; for the rutile form) and their meticulously controlled particle size distribution of which median is in the range between 0.2 &mgr;m and 0.4 &mgr;m. However, this product is of a high specific gravity (~4.0 g/cm
3
), of a high surface area due to its small particles, and most of all, is quite expensive. Fine kaolin particles are also being used as pigments, but this product has a much lower refractive index (1.56), is of limited whiteness and is still relatively expensive. Particulate calcium carbonate is the ideal least expensive pigment and could replace much more of the titanium dioxide and kaolin pigments in their respective present applications, if it could be prepared in a form having improved light scattering properties.
Calcium carbonate pigments are produced in part by grinding coarse natural rocks and in part by precipitation processes. Of the precipitated calcium carbonate particles, a particulate precipitated aragonite is considered to be the most effective light scattering calcium carbonate pigment, of which refractive indices are 1.530, 1.681 and 1.685, depending on its crystallographic surfaces, its specific gravity is above 2.5 g/cm
3
, and is the most suitable for same applications. However, its production rate is characteristically very slow and its production conditions are very difficult to control, industrially.
While the majority of references, cited hereinafter, relate to the technology for producing a particulate precipitated aragonite, some of the references are included in order to better present the state of the art for the production of PCC more generally, including the downstream operations, which may be common to all these processes and also to the present invention.
1. U.S. Pat. No. 2,081,112 (N. Statham et al.) describes a process for producing precipitated calcium carbonate by carbonating milk of lime with carbon dioxide containing gas, where the temperature in the gas absorber is maintained at 50-60° C., preferably around 55° C. It is recognized that the more violent the agitation in the gas absorber, the finer will be the product; the aim being to create a fine mist of calcium hydroxide slurry.
2 U.S. Pat. No. 2,964,382 (G. E. Hall, Jr.) describes production of. precipitated calcium carbonate by various chemical routes, in which calcium ions are contacted with carbonate ions in a precipitation zone, the process including also carbonation of milk of lime with carbon dioxide gas. A high shear stator/rotor agitator is used to provide turbulence by rotating at a peripheral speed of at least 1160 feet per minute (589 cm per second) in the precipitation zone. Also, this patent teaches that it is desirable to operate the process at pH values of at least 8.5 and that at temperatures above 60° C., needle-shaped precipitated aragonite particles are formed, which however produce an adverse flow property effect.
3. U.S. Pat. No. 3,320,026 (W. F. Waldeck) describes the production of various forms of precipitated calcium carbonate.
4. GB Patent No. 941,900 (assigned to Kaiser Aluminium & Chemical corporation) describes the production of precipitated aragonite particles, for use as a filter aid, by reacting continuously sodium carbonate solution and aqueous calcium hydroxide slurry at temperatures higher than 60° C. in a multistage system. The product and the solution are withdrawn at the third stage from the bottom of the reactor, the product is then separated from the solution and part of the crystals are
3P Technologies Ltd.
Bos Steven
Browdy and Neimark , P.L.L.C.
LandOfFree
Precipitated aragonite and a process for producing it does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Precipitated aragonite and a process for producing it, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Precipitated aragonite and a process for producing it will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-3280112