Precipitated silica paste
The present invention relates to a precipitated silica paste, to a method for producing it and to a method of using the precipitated silica paste to impart hydrophobic characteristics to precipitated silicas. 
     BACKGROUND OF THE INVENTION 
     It is known that precipitated silicas can be rendered waterproof with silicon oils in a dry state (see published German Patent Application DE-OS 26 28 975--U.S. Pat. Nos. 4,273,589 and 4,308,074). Such waterproofed precipitated silicas are used as defoaming silicas in detergents. 
     The precipitated silicas waterproofed according to the known method have the disadvantage that the silicon oil used as hydrophobing agent is not distributed uniformly on the surface of the silica. 
     There has been, therefore, a need for a method of producing precipitated silicas, waterproofed with silicon oil, which have a uniform distribution of the silicon oil. 
     SUMMARY OF THE INVENTION 
     The present invention provides a precipitated silica paste consisting of 5 to 15% by weight precipitated silica and 85 to 95% by weight silicon oil. The precipitated silica paste of the invention has sufficient stability for several days. 
     The invention also provides a method of producing a precipitated silica paste consisting of 5 to 15% precipitated silica and 85 to 95% by weight silicon oil in which silicon oil is placed in a receiver, precipitated silica is worked in using shearing forces and this dispersing process is continued until the precipitated silica paste obtained exhibits sufficient stability. 
     During the dispersing, the temperature of the precipitated silica can be elevated from room temperature to 50° C. 
     The invention further provides a method of waterproofing precipitated silicas using the precipitated silica paste of the invention consisting of 5 to 15% by weight precipitated silica and 85 to 95% by weight silicon oil in which a precipitated silica filter cake is liquefied in a known manner, the pH is adjusted to neutral or alkaline, preferably to pH=9, the amount of precipitated silica paste necessary for the waterproofing is added using a shearing device, the mixture is subsequently sheared, the suspension is dried, preferably spray-dried, the dried product is tempered and it is subsequently ground with an air jet mill. 
     The liquefaction of the precipitated silica filter cake can be carried out preferably according to Published German Patent Application DE-OS 24 14 478 (c.f. U.S. Pat. No. 4,150,101). 
     The precipitated silica suspension can have a solid content of 150 to 180 g/l. An Ultra-Turrax or a Kothoff mixing siren can be used as shearing device. 
     The waterproofed precipitated silica produced by the foregoing methods has a uniform coating of silicon oil. It can be used in detergents as a defoaming silica. 
     The method of the invention has the advantage that the tempering can be carried out with the non-ground product. This raises the capacity of the tempering step by a factor of 2 to 3. 
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     The invention is illustrated by the following examples. 
     The suspension hydrophobing of the invention is carried out in several test series. In particular, the influence of the pH of the suspension was investigated in detail. To this end, a VN3 membrane filter cake (approximately 23% solid) is liquefied by means of shearing forces and the addition of dilute hydrochloric acid or sulfuric acid (test series A), demineralized water (test series B) and dilute sodium hydroxide solution (test series C) and adjusted to the desired pH&#39;es (3, 6 and 9). In the next step, the calculated amounts of the waterproofing paste are charged into the suspensions obtained. A silicon oil content of 13.5%, relative to the silica component (TS content), is selected as the base for the suspension. The waterproofing paste is worked in by means of the mixing siren at its highest speed. 
     The total shearing time is 45 minutes. Thereafter, the suspension is transferred without further filtration step into a porcelain dish and dried at 105° C. in a drying cupboard. The dried product is then ground on a UPZ mill (mechanical beater mill of the Alpine company) and tempered under standard conditions (1 hour/350° C.) in a Naber oven. Table 1 summarizes representative tests (test parameters/analytical data) from test series A-C. It can be seen that the suspension waterproofing, in both the alkaline and also in the neutral range provides in good hydrophobic properties--methanol wettability &gt;60%--but that in the acidic range only products of lesser quality are obtained. 
     Further test series are carried out thereafter only in the &#34;neutral&#34; or &#34;alkaline liquefaction&#34; range. In test series D and E, the influence of the sequence of the method steps tempering and grinding on the product quality is investigated. The tests are carried out as described thereby; however, the further processing is divided after the method step of &#34;drying&#34;. In the first instance, the &#34;grinding&#34; was carried out with subsequent &#34;tempering&#34;. In the other instance, the series of the last method steps is reversed. 
     Table 2 summarizes representative tests (test parameters/analytical data) from test series D and E. It was surprisingly found that the sequence of the method steps &#34;tempering&#34; and &#34;grinding&#34; has no influence on the product quality as regards hydrophobic properties. Consequently, it is possible to utilize the higher bulk weight of the non-ground product in the &#34;tempering&#34; method step and, thereby, to achieve a higher space-time yield in this method step. 
     ENGINEERING TESTS 
     The knowledge gathered in the laboratory tests is checked in engineering tests. For the sake of better examination and better comparability, the tests were carried out only in the alkaline range. Instead of the &#34;drying cupboard&#34; drying unit, a spray drier is used. 
     The filter cake is liquefied thereby, with a dissolver disk and dilute sodium hydroxide solution, and adjusted to pH 9. Then the coating with silicon oil is carried out by adding the appropriate amount of waterproofing paste (13.5% silicon oil relative to the dry substance) in a liquefaction container. After an additional dispersing time of approximately 30 min., the suspension is spray-dried on the spray drier (centrifugal atomizer). 
     The spray-drying method step is followed by the tempering of the non-ground intermediate product at 350° C. The tempered product is subjected finally to an air-jet grinding. 
     
                       TABLE 1
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Test series A-C/ test parameters; analytical data
Test            A         B       C
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Suspension
type           VN3        VN3     VN3
pH             3.0        5.8     9.0
TS content [%] 16.2       16.5    14.0
Hydrophobing agent
silicon oil    TG 50      TG 50   TG 50
amount [%]     13.5       13.5    13.5
Drying
aggregate       drying cupboard
time [h]        12        12      12
temperature [°X]
                105       105     105
Tempering - aggregate
                Naber oven
time [h]        1         1       1
temperature [°C.]
                350       350     350
Sequence
tempering       2         2       2
grinding        1         1       1
Analytical data
pH              3.35      7.52    10.3
Sears number
V1 [ml]         5.9       0.2     1.3
V2 [ml]         4.2       0.8     0.8
methanol wettability [%]
                48.8      63.7    63.7
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                                           TABLE 2
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Test series D and E; test parameters; analytical data
Test         D1  D2   E1  E2   E3  E4
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Suspension
type         VN3 VN3  VN3 VN3  VN3 VN3
pH           5.1 5.1  8.5 8.5  8.5 8.5
TS content [%]
             18  18   13.3
                          13.3 15.3
                                   15.3
Hydrophobing agent
silicon oil  TG 50
                 TG 50
                      TG 50
                          TG 50
                               TG 50
                                   TG 50
amount [%]   13.5
                 13.5 13.5
                          13.5 13.5
                                   13.5
Drying
aggregate    drying cupboard
time [h]     12.0
                 12.0 12.0
                          12.0 12.0
                                   12.0
temperature [°C.]
             105 105  105 105  105 105
Tempering
aggregate    Naber oven
time [h]     1   1    1   1    1   1
temperature [°C.]
             350 350  350 350  350 350
Sequence
tempering    1   2    2   1    1   2
grinding     2   1    1   2    2   1
Analytical data
pH           7.1 7.1  9.85
                          9.8  9.8 9.95
Sears number
V1 [ml]      0.18
                 0.18 0.85
                          1.0  1.3 1.1
V2 [ml]      0.8 0.6  0.8 0.8  0.9 0.7
methanol wettability [%]
             63.8
                 66.2 64.1
                          63.6 63.8
                                   64.5
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                       TABLE 3
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Engineering tests SPT 180/200; test parameters; analytical data
Test                SPT 180  SPT 200
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Suspension
type                VN3      VN3
pH                  9.0      9.1
TS content [%]      16.1     18.6 (16.2)
Hydrophobing agent
silicon oil         Cp 50    Cp 50
amount [%]          13.5     13.5
Drying
aggregate           VTC-A test spray drier
entry temperature [°C.]
                    420      400
exit temperature [°C.]
                    80       80
Intermediate product (spray-dried)
water content [%]   5.2      4.4
pH                  9.3      9.4
conductivity [μS]
                    --       650
sieve tailings:
&gt;45 μm [%]       65.0     61.0
&gt;63 μm [%]       35.0     34.0
C content [%]       4.2      4.37
stamping weight [g/l]
                    330      333
Tempering
aggregate           D 10 pilot reactor
temperature [°C.]
                    350      330
time [min]          60       50
Grinding
aggregate           Engineering LS mill
grinding pressure [bar]
                    3.5-4.0   3.5
injector pressure [bar]
                    5.5      5.5
throughput [kg/h]   20       20
Final product
water content [%]
pH                  10.7     10.5
average particle size [m]
                    3.5      5.1
C content [%]       3.6      3.2
stamping weight [g/l]
                    110      110
Sears number:
V1 [ml]             --       4.1
V2 [ml]             2.9      1.6
methanol wettability [%]
                    65       67
BET surface [m.sup.2 /g]
                    106      109
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