Abstract:
A method and composition are disclosed for the preparation of pencil lead from clay, graphite and optionally other material, wherein the clay or the clay/graphite mixture is digested with water, pressed, dried and fired, and at least one alkali phosphate, alkali borate and/or alkali carbonate is added before or as part of the digestion of the clay or the clay/graphite mixture.

Description:
BACKGROUND OF INVENTION 
     This invention is directed to a method and composition for the preparation of fired pencil lead and to the resultant fired pencil lead product. More particularly this invention concerns pencil lead of high strength, prepared from a readily moldable composition which has optimum wet and dry strengths. 
     Known pencil leads consist essentially of graphite and clay; the ratio of graphite to clay determines the hardness of the lead. Usually, the clay, which is obtained in mining operations, is washed, that is, suspended in an excess of water and milled if necessary, mixed with graphite and processed by means of filter presses into filter cakes. The filter cakes are dried and brought to a pressable consistency with water in Z kneaders. So-called pressing aids, such as cellulose ethers or polyglycols, are added as required. These additional components generate appropriate wet and dry strengths, since the pure lead-graphite mixture, when pressed into a lead approximately 2 to 2.5 mm. thick, would be too unstable and too difficult to handle. However, these additional components have the disadvantage of exerting an effect on the wet and dry strengths, but not on the final strength of the fired lead. In some cases, there is even interference with the ceramic bonding due to the development of excessive porosity. 
     It is also disadvantageous that the organic additives burn away during the firing process and lead to considerable contamination of the environment. Moreover, such additives are ineffective with respect to the rapid digestion of the clay. 
     In German Auslegeschrift 1,243,305, a mixture of alkaline earth bentonite, kaolin and graphite is proposed for the preparation of pencil lead. Admittedly, it was possible to do without organic additives here for improving the wet and dry strengths; however, the preparation of the mixture is very time consuming, particularly since the alkaline earth bentonite must be allowed to swell for a long time in order to achieve the desired result. Moreover, relatively high firing temperatures are necessary and compared to conventional clay formulations, the mixture is very expensive. 
     SUMMARY OF THE INVENTION 
     It is an object of the invention to digest mined clay within a few minutes without an excess of water; that is, to swell it and to obtain in admixture with graphite, a readily moldable composition which has optimum wet and dry strengths. 
     It is also an object of the invention to provide pencil lead of high strength at relatively low firing temperatures. 
     Another object of the invention is the provision of a composition for the preparation of pencil lead which is readily moldable, has optimum wet and dry strengths and by means of which, pencil lead of high strength is obtained economically. 
     These objects and others are accomplished by the invention, according to which, at least one alkali phosphate, borate and/or carbonate is added to the clay or to a mixture of clay and graphite during the digestion thereof with water or prior to the addition of water thereto. 
     It has been discovered that a mixture of graphite powder, clay and water containing 0.1 to 20% by weight of at least one alkali phosphate, borate and/or carbonate has very desirable properties for the preparation of pencil lead. 
     DESCRIPTION OF THE INVENTION 
     According to the invention, mined, dried, ground and sifted clay, with a particle size of less than 10 microns, is mixed in a high-speed mixer with graphite powder, an alkali phosphate, alkali borate and/or carbonate and subsequently treated with water. Alternatively, water in which the alkali phosphate, borate and/or carbonate is dissolved is added to the clay or to an intimate mixture of graphite and clay prepared in a high-speed mixer. In this procedure, the clay is digested without an excess of water, that is, directly in the amount of water required for the pressing process. 
     The inventive mixture is composed of: 
     a) 15 to 70% by weight of graphite powder, 
     b) 3 to 60% by weight and more preferably, 3 to 41% by weight of finely ground fire clay, 
     c) 0.1 to 20% by weight of alkali phosphate(s), alkali borate(s) and/or alkali carbonate(s), 
     d) 3 to 50% by weight of water, as well as 
     e) 0 to 35% by weight of other materials such as carbon black. 
     Alkali phosphates, borates or carbonates which are incorporated in the pencil lead composition according to the invention include, for example, sodium polyphosphate, potassium polyphosphate, sodium tetraborate, sodium carbonate and potassium carbonate, lithium carbonate, sodium metaborate, potassium tetraborate and potassium metaborate. 
     By mixing the clay intensively with graphite, large agglomerates are broken up by mechanical forces, so that the clay particles have a large, free, accessible and active surface. Moreover, an optimum, statistical distribution of the graphite and clay particles is achieved. By the addition of alkali phosphates, borates and/or alkali carbonates and their solution in water, the multivalent cations of the clay are inactivated by sequestration and replaced by univalent ions. For example, when alkali phosphates are used, the solution gives off alkali ions, so that the highly charged anion ##STR1## results. The negative charge on this anion becomes greater in proportion to the number of sodium ions that have dissociated; finally the negative charge becomes so high, that all remaining sodium ions are firmly held and lose their mobility. If a multivalent positively charged cation now comes into a contact with such a highly negatively charged anion, it is attracted more strongly, in proportion to its higher charge, than the singly charged sodium ions; that is, the multicharged, positive cations take the place of the displaced sodium ions and are firmly bound in proportion to their higher charge and the clay particles are wetted. 
     If alkali carbonates or borates are used, the multivalent cations of the clay are converted into water insoluble carbonates or borates. The hydroxyl ions formed by the hydrolysis of the carbonate or borate ion are absorbed at the positively charged edges of the clay particle. This leads to partial wetting. 
     In both cases, the clay is digested and swelled within a few minutes. The amount of water for this can be limited to that required for the further processing. 
    
    
     The following examples further illustrate the best mode currently contemplated for carrying out the invention. However the examples must not be construed as limiting the invention in any manner. 
     EXAMPLE 1 
     Graphite powder (66.0 kg.), with a carbon content of 96% and a particle size of less than 10 microns, and 34 kg. of finely ground fire clay, with a particle size of less than 10 microns, are intensively mixed in a high-speed mixer. Subsequently, while continuing the mixing process, 22 liters of an aqueous solution of 0.8 kg. of sodium polyphosphate with an average chain length and 1.6 kg. of sodium tetraborate.10H 2  O are added. After a few minutes, the clay digestion commences with the formation of granulate. The clay/graphite mixture has a moisture content of 18% and is subsequently pressed in a conventional commercial extrusion press into 2.3 mm. thick pencil leads. The leads have a very good wet breaking strength. The leads, cut to the appropriate length, are dried at a temperature between 100° and 150° C. and subsequently fired at 800° C. in an inert or reduced atmosphere. 
     EXAMPLE 2 
     Graphite powder (60 kg.), with a carbon content of 96% and a particle size of less than 10 microns, and 40 kg. of finely ground fire clay, with a particle size of less than 10 microns, are premixed dry as in Example 1. During the mixing process, 20 liters of an aqueous solution of 0.8 kg. of sodium polyphosphate are added. The further processing is as in Example 1. 
     EXAMPLE 3 
     Graphite powder (60 kg.), with a carbon content of 96% and a particle size of less than 10 microns, and 40 kg. of finely ground fire clay, with a particle size of less than 10 microns, are premixed dry as in Example 1. During the mixing process, 20 liters of an aqueous solution of 1.2 kg. of sodium polyphosphate with an average chain length are added. The further processing is as in Example 1. 
     EXAMPLE 4 
     Graphite powder (50 kg.), with a carbon content of 96% and a particle size of less than 10 microns, and 50 kg. of finely ground fire clay, with a particle size of less than 10 microns, are mixed dry with 0.8 kg. of sodium polyphosphate (average chain length) and 1.3 kg. of sodium tetraborate.10H 2  O in a high-speed mixer. While continuing the mixing process, 20 liters of water are added. The further processing is as in Example 1. 
     EXAMPLE 5 
     Graphite powder (66 kg.) with a carbon content of 96% and a particle size of less than 10 microns, and 34 kg. of finely ground fire clay, with a particle size of less than 10 microns, are mixed dry with 1.0 kg. of sodium polyphosphate (average chain length) and 1.5 kg. of lithium carbonate in a high-speed mixer. The further processing is as in Example 4. 
     It has been ascertained that sodium polyphosphate with an average to long chain structure is particularly suitable for digesting the clay. Likewise, it has been ascertained that sodium tetraborate distinctly improves the clay plasticity and thus the wet breaking strength and reduces the sintering temperatures of the clay. This can be explained on the basis of a flux action and with that, on the basis of the formation of a melt phase. It is thus possible to produce lead using distinctly lower firing temperatures without lowering its strength. 
     The advantages of the invention are made even clearer by comparison with the conventional methods for preparing pencil lead. 
     COMPARISON EXAMPLE A 
     Thirty-five parts by weight of a clay mixture of calcium bentonite and kaolin in the ratio of 6:4 parts by weight is mixed with 65 parts by weight of powdered graphite, made into a paste with water, concentrated, dried and fired. 
     COMPARISON EXAMPLE B 
     With Organic Additives 
     In a basin with stirrer, 20% by weight of clay are washed in 80% by weight of water. After this cleaning process and an optional fractionation process (hydrocyclone), the clay content is determined. By stirring graphite powder into the aqueous clay slip, the ratio of graphite to clay is adjusted to a value of 1:0.5. The mixture is concentrated in filter presses to filter cakes. The dried filter cakes are comminuted in a Z kneader, adjusted to a pressing consistency with 16% by weight of water and plasticized with 2% polyglycol having a molecular weight of 6,000. If necessary, the composition is rolled and then pressed and fired into pencil lead in the known manner. 
     
                       TABLE 1______________________________________Comparison of Illustrative and Comparative Examples                     Comp.    Illustrative Examples                     Examples    1    2      3      4    5    A    B______________________________________Water excess      no     no     no   no   no   yes  yesOrganic additives      no     no     no   no   no   no   yesOpen porosity of       9      8      8    5    8    10   15the lead in %Wet breaking      high   high   high high high aver-                                        slightstrength (sample                        agebroken by hand)Firing tempera-      800    800    800  800  800  1000 1000ture needed °C.Final strength      105    110    115  140  104   75   68(N/mm..sup.2)Total mixing       10     10     10   10   10   120 2-3time (min.)                                  daysHardness   HB     F      F    3H   HB   HB   HB______________________________________