Chemistry of inorganic compounds – Carbon or compound thereof – Oxygen containing
Reexamination Certificate
2000-04-11
2004-07-13
Silverman, Stanley S. (Department: 1754)
Chemistry of inorganic compounds
Carbon or compound thereof
Oxygen containing
Reexamination Certificate
active
06761869
ABSTRACT:
The present invention relates to a process for preparing precipitated calcium carbonate. According to such a method the calcium carbonate is produced by reacting calcium oxide with carbonate ions in a medium.
The invention also relates to an apparatus for preparing precipitated calcium carbonate.
The use of calcium carbonate, particularly precipitated calcium carbonate, is becoming increasingly common within many industries, such as within the paper, the plastics and the pharmaceuticals industry. The aim is to formulate of the precipitated calcium carbonate (PCC) a finely divided, pure pigment which is as white as possible for many purposes.
The production of PCC usually begins with slaked lime or calcium hydroxide which is carbonated with flue gases or subjected to causticizing using sodium carbonate or ammonium carbonate. Very finely divided pigments are often prepared by mixing calcium chloride and ammonium carbonate or soda (sodium carbonate).
The calcium hydroxide, then, is obtained by burning a calcium carbonate-containing starting material, such as limestone, in a vertical or round kiln to form calcium oxide, which is conventionally slaked to obtain calcium hydroxide. It has generally been thought that lime burning must be performed in large units such as are represented by vertical and pipe furnaces. A large unit has been seen as representing an economical process.
What is characteristic of the prior-art solutions is that all operations are carried out in different places because one customer needs burnt lime, another one slaked lime, while a third one requires reprecipitated calcium carbonate. Such operations at different stages and different locations entail transportation and storage which incur significant costs. It is characteristic of the present invention that these operations can be carried out either at the same location or, in certain situations, at different locations.
It is the aim of the present invention to remove the drawbacks hampering the prior art and to obtain an entirely novel type of solution for the preparation of precipitated calcium carbonate.
During the past few years the focus of environmental protection within the sector of power plant technology has been on removing the sulphur dioxide generated during the combustion of fuels. In this connection, much research has been carried out on the use of calcium carbonate as a sorbent which is calcinated in the power plant furnace, and the calcium oxide thus obtained reacts further with sulphur dioxide, in part directly and in part in a so called activation reactor. It has been found during these tests that the conversion of finely ground calcium carbonate into an oxide can occur in <0.5 seconds at a temperature of approximately 1000° C.
The present invention has developed these techniques further. It has namely been found possible to turn the calcium oxide directly into calcium carbonate in one process step without first having to slake it to obtain calcium hydroxide. On the other hand it has also been found that it is both technically and economically advantageous to recombine the calcium oxide and the carbon dioxide released during combustion. In short, the process of the present invention has two important embodiments whereby the first one comprises converting calcium oxide into calcium carbonate without any separate slaking or intermediate storage. In the second embodiment the calcium carbonate containing starting material is decomposed thermally and combined to form a new precipitated calcium carbonate without having to separate the products from each other for storage and without performing separate intermediate steps.
Of the above-described solutions the latter can be carried out by means of an apparatus comprising connected units for grinding limestone and for burning the ground limestone to obtain calcium oxide, a reaction unit for the calcium oxide to obtain carbonate, and a gas recovery and circulation unit. Between the units, finely divided lime or, correspondingly, calcium oxide is transferred along canals in the form of a fluid comprising solids and carrier gas.
In more detail the process according to the invention is mainly characterized by what is stated in the characterizing parts of claim
1
and, correspondingly, claim
2
.
The apparatus according to the invention is characterized by what is stated in the characterizing part of claim
25
.
The invention offers considerable benefits. Thus, from the point of view of thermal economy, a better solution is achieved by means of the method than by conventional lime burning, and all of the heat is recovered, even if at a lower temperature. Even the heat which is removed along with the hot product is of advantage in a northern climate in wintertime because the product can be taken to the user without being frozen.
According to the invention, the calcium carbonate powder is disintegrated into calcium oxide and carbon dioxide at an elevated temperature of approximately 800 to 1400° C., the mixture is cooled and the final temperature is slaked with water. Next, the mixture undergoes intensive stirring, and as the end product, a 10 to 100%, for example, advantageously an approximately 60 to 80% PCC slurry is obtained directly. The solids content of the product can be adapted according to use. A product suited for use as a filler, for instance, can be prepared as an approximately 20% slurry, whereby no dispersing agents are needed. The solids content of a slurry suited for use in coating mixes may be 60 to 80%, but it is also possible to recover the PCC as a dry powder.
It is also possible to recover the calcium oxide and the carbon dioxide separately from the burning of the calcium carbonate powder. The solution is applicable both to carbon dioxide carbonating and to carbonating using known soluble carbonates, whereby, in the latter case, the carbon dioxide can be used in some other process.
One of the key ideas behind the invention is further that gas, liquid and solids are taken forward as a so called fluid, whereby the problems relating to the high viscosities and putty-like composition normally caused by the high solids contents can be controlled. In this context, a fluid is a mixture of solids and gas, possibly containing water which is present as a mist, i.e. as fine water drops. Thus, a fluid comprises a mixture of suspension and aerosol.
The starting material for lime is calcium carbonate which is converted into calcium oxide at a high temperature, the calcium oxide is slaked and calcium carbonate is formed from the calcium oxide in the same process without separating the reaction products from each other. Thus, in the inventive process, all unnecessary separation of gas and solids and water has been eliminated. Hence, according to the invention, new surface and particle shape are created in a process lasting about 3 to 10 seconds in all from the moment the calcium carbonate stone reaches the first grinder and when the same material leaves the last reaction mixer in the form of a completed aqueous slurry.
According to the invention, calcium carbonate is formed of calcium oxide in the presence of water and carbonate ions (or a precursor thereof).
According to a preferred embodiment, the calcium oxide is carbonated with carbon dioxide gas in the presence of water to form PCC. In water, the carbon dioxide forms carbonate ions. It is particularly advantageous to use such carbon dioxide gas for carbonating which has mainly been obtained from the preceding lime burning.
According to a second advantageous embodiment the PCC is prepared by reacting the calcium oxide with an aqueous solution of an alkali metal carbonate. Hereby a separately prepared alkali metal carbonate solution is fed straight into the causticizing reaction. The concentration of the alkali metal carbonate is approximately 5 to 40% by weight, whereby an alkali metal hydroxide solution having a concentration of appr. 10 to 30% by weight is obtained as a by-product of the reaction.
The calcium carbonate-containing limestone used as starting material in the invention is first
Birch & Stewart Kolasch & Birch, LLP
FP-Pigments Oy
Johnson Edward M.
Silverman Stanley S.
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