A METHOD TO PREPARE CALCIUM CARBONATE

Abstract: In the invention a method of manufacturing paper has been developed, which considerably reduces the carbon dioxide content of the atmosphere. Carbon dioxide and, for example, waste gypsum from the fertilized industry are combined as calcium carbonate and sulphuric acid by means of chemical reactions using water as an additional substance. The sulphuric acid is returned to the fertilizer factory and the calcium carbonate is used in the manufacture of paper as a filler and coating pigment. In this way paper that contains a great deal of filler and coating may contain as much as half of its weight as calcium carbonate, which means that as much as 100 000 tons of carbon dioxide is removed from the atmosphere by a modern full-scale paper machine.

Description: A METHOD TO PREPARE CALCIUM CARBONATE. The operating sphere of the invention is the manufacturing methods of paper and board products. By means of the manufac¬ turing method described, paper or board filled and/or coated with calcium carbonate is obtained. In previously known methods, in which paper industry calcium carbonate filler or coating pigments have been used, the calcium carbonate has been manufactured from flue gas carbon dioxide and from calcium hydroxide sludge deposited in water. Thus pure calcium carbonate with good lightening properties for paper filling and coating is obtained by precipitation in water. The lightening properties of pigments depend considerably on the size and size distribution of the particles. These can be easily controlled by concentration in water precipitation and by temperature. The method does not, however, reduce atmospheric carbon dioxide, because slaked lime, i.e. calcium hydroxide, is made from limestone, i.e. calcium carbonate, by releasing carbon dioxide from into the atmosphere in a kiln. Because all proces¬ ses never achieve one hundred percent efficiency, this kind of manufacturing process for calcium carbonate in principle in¬ creases the amount of carbon dioxide in the atmosphere. By grinding limestone directly, without water precipitation, a sufficiently high quality calcium carbonate cannot be obtained for the paper industry. When limestone is burnt, and calcium pigment is made afresh for the paper industry by means of the slaked lime obtained and carbon dioxide, its price typically increases ten- fold. The method developed has the intention of removing the defect, in which in the manufacturing process for calcium carbonate carbon dioxide is released at an earlier stage of manufacture, and of creating a papermaking process which reduces, as a net phenomenon, the carbon dioxide content of the atmosphere. The objective of the invention is achieved by means of a method, the characteristics of which are presented in the Patent Claims. Figure 1 shows a block diagram of the method, in which the various stages of papermaking can be seen. During the growth of a forest (1) the trees take, through assimilation, atmospheric carbon dioxide as a building material, which is released back into the atmosphere in the natural cycle through decay. In the method, timber is harvested (2) from the forest and converted into wood-chips (3) , after which the chips are used for the production of electrical and heat energy in a combination power plant (4) , as well as for the production (14) of cellulose in a paper mill (16) . In the power plant (4) the chips burn and provide electricity and steam for the paper mill, as well as often district heating for a community in the neigh¬ bourhood. The flue gases that form are cleaned of particles as well as often of sulphur and nitrogen oxides (5) . After this, carbon dioxide is separated from the flue gases and cleaned of other flue gases. On the other hand, waste gypsum (CaS0 4 ) , which is deposited as sludge in water (9) , is brought into use. According to the following net reaction calcium carbonate and sulphuric acid are obtained from gypsum, water, and carbon dioxide: CaS0 4 + H 2 0 + C0 2 = CaC0 3 + H 2 S0 4 . Calcium carbonate is precipitated from the process with the aid of a drum filter (11) and the sulphuric acid is collected and sold, for example to the fertilizer industry for re-use in the manufacture of phosphoric fertilizers, from which waste gypsum is also obtained. There is reason to note that the basic mate¬ rials, waste gypsum, water, and flue gas carbon dioxide are all free materials. The calcium carbonate thus made is used for filling base paper and as a coating pigment at the paper mill (16) . The paper produced can contain about 20 % calcium car- bonate in the base paper (60 g/m 2 ) and coatings on both sides 15 + 15 g/m 2 about 90 % calcium carbonate. Calcium carbonate is then 43 % of the total weight of the paper (90 g/m 2 ) . If the paper is made on a modern machine 8 m wide, the annual produc- tion is about 400 000 tonnes. Calcium carbonate contains 44 % carbon dioxide, so that a single machine manufacturing paper by this technique reduces atmospheric carbon dioxide by roughly 80 000 tonnes annually. This has already a considerable signi- ficance in principle in environmental protection from the point of view of the development of the greenhouse effect. As an example from the papermaking process: per tonne of ori¬ ginal substances calcium carbonate is required, according to the above net reaction CaS0 4 1,06 tonnes H 2 0 0,18 tonnes C0 2 0,44 tonnes from which is obtained CaC0 3 1,00 tonnes H 2 S0 4 0,68 tonnes 449 kg of carbon dioxide is compressed into a liquid from flue gases, filtered as a liquid, and allowed to vapourize back into pure gas. The compressor is driven by a turbine, which in turn is driven with the aid of the expanding compressed flue gas and carbon dioxide. The process consumes practically no additional energy and in the compression stage the compressed gas is cooled with lake or sea water (about 5 °C) and the expanding gas is heated with waste heat from the paper mill (about 70 °C) . This heat pump principle makes good the friction losses of the devices in the process (about 20 - 30 %) and achieves a very economical means of separating and cleaning carbon dioxide (6) . Gypsum is deposited as sludge in water and transferred as a water/sludge mixture by pumping it to an ammonium sulphate reactor, to which is also led 440 kg of carbon dioxide and, in addition, 344 kg of ammonia as a reusable chemical. In the reactor, the substances brought to it are circulated next to one another with the aid of pumping and the water containing the reaction products is collected in a drum filter, which filters out undissolved calcium carbonate, while dissolved ammonium sulphate remains in the filtrate. The reaction is as follows: CaS0 4 + 2 NH 3 + C0 2 + H 2 0 = CaC0 3 + (NH 4 ) 2 S0 4 . The technique has been published in detail in: The British Sulphur Corp. Ltd. : Direct production of ammonium sulphate from gypsum. Sulphur No. 85, Nov/Dec. 1969 p. 29. The reaction is 100 % for all original substances and products except ammonium, 1,3 % is lost in filtration. Thus the desired fine-grain calcium carbonate is obtained for paper manufacture. In the next stage ammonium is separated again in a sulphuric acid reactor and a second final product, sulphuric acid, is formed. This takes place in two stages. First of all the diam- monium sulphate that has been dissolved in the previous stage is crystallized by evaporation and raised to a temperature of about 400 °C, which is most advantageously achieved by superheated steam, when the following generally known chemical reaction takes place: (NH 4 ) 2 SO = NH 3 + NH 4 HS0 4 . The ammonium obtained from this is released into the first reactor. In the second stage, ammonium hydrosulphate is dis- solved in a small amount of water at a temperature of about 80 "C, when the following reaction takes place: 2 NH 4 HS0 4 = (NH A ) 2 S0 4 + H 2 S0 4 . The sulphuric acid is extracted with the aid of mineral turpen¬ tine and the crystallized ammonium sulphate is transferred to the beginning of this stage. The mineral turpentine is washed from the sulphuric acid which is in a 30 % water solution and is available for use in a very clean form. This process, which has been retained as it is, is described in great detail in European Patent 008331 Bl. Now the aforementioned net reaction has been achieved with no detrimental by-products. The washed and dried calcium carbonate is taken to the paper mill, in which it is added as a filler before the head box of the paper machine, for example in such a way that it forms 20 - 22 % of the dry weight of the base paper. If the paper is, for example, 60 g/m 2 , the amount of filler in it is 12,0 - 13,2 g/m 2 . The paper is coated after this with, for example a double coating on both sides, the basis weights of which are 2 - (10 +10) g/m 2 = 40 g/m 2 . Thus in this example a paper with a total weight of 100 g/m 2 is obtained. When the typical composition of the coating material is calcium 88 %, latex 10 %, and other substances 2 %, then the paper receives 0,88 • 40 g/m 2 + 13,2 g/m 2 , i.e. 48,4 g/m 2 , i.e. 48,4 % calcium carbonate. If the paper machine is 8 wide and the running speed is 1000 m/min its annual production is in the region of 400 000 tonnes, or, as the previous calculation demonstrates, carbon dioxide is reduced in the atmosphere by about 100 000 tonnes annually. The invention is not limited to the example presented above, but may be adapted within the limits of the Patent Claims.

Claims: PATENT CLAIMS 1. A method for manufacturing paper that reduces atmospheric carbon dioxide characterized in that the calcium carbonate used as a filler and/or coating pigment in the paper is manufactured from carbon dioxide that is separated from the flue gases when wood or other organic material, which extracts its carbon from the carbon dioxide of the atmosphere, is burnt, and from calcium from the manufacturing process of which carbon dioxide is not released, such as waste gypsum from the fertilizer industry. 2 . A method in accordance with Patent Claim 1, characterized in that the net quantities of the manufacture of the calcium carbonate used as a raw material in papermaking are in per- centages by weight 63,1 % gypsum i.e. calcium sulphate, 10,7 % water, and 26,2 % carbon dioxide, and that from these original substances a corresponding quantity in mass of final products are obtained: calcium carbonate 59,5 % and sulphuric acid 40,5 %. 3. A manufacturing method in accordance with Patent Claims 1 and 2, characterized in that the sulphuric acid formed is returned to the process, from which the original substance comes which contains calcium that binds carbon dioxide. 4. A paper manufacturing process in accordance with Patent Claims 1 - 3, characterized in that in the manufacture of calcium carbonate a catalyst such as ammonium is used, which binds temporarily to the intermediate chemical stages and is released from them before the final product, thus forming its own circulation in the process. 5. A method in accordance with Patent Claims 1 - 4, charac¬ terized in that the mill making the paper obtains its energy (electricity, steam) from a power plant burning wood- chips, from the flue gases of which the carbon dioxide required by the process is taken.

Inventors: PUUMALAINEN PERTTI, PUUMALAINEN, PERTTI
Applicants: PUUMALAINEN PERTTI, SUPER PAPER OY, PUUMALAINEN, PERTTI
Application year: 1995
Main classification: C01F11
IPC codes: C01B17, C01F11