Method for processing finely divided solids during production of chlorosilanes

Abstract

The invention provides a method for the processing of finely divided solids during the production of chlorosilanes, which is characterized in that the finely divided solids are hydraulically pressed to give bodies of increased density. Moreover, also provided is the compact obtained by the process according to the invention which is characterized by a filling factor of the finely divided solids to be hydraulically pressed of 3.9 to 4.5.

Claims

1. A process for processing finely divided solids during production of chlorosilanes, the process comprising hydraulically pressing the finely divided solids to obtain at least one compact of increased density by applying a pressing pressure initiated at zero at a pressing rate of from 0.1 to 1 kN/cm.sup.2s until a final pressing pressure of at most 14 kN/cm.sup.2 is reached, then holding the final pressing pressure for a period of from 0.5 to 1.5 s, and subsequently reducing the pressing pressure to zero over a period of from 0.5 to 1.5 s.

2. The process as claimed in claim 1, wherein said pressing occurs in a pressing vacancy which is a cylindrical sheath comprising ceramic.

3. The process as claimed in claim 2, further comprising ejecting the compact from the pressing vacancy with an ejection force of 7 to 8 kN.

4. The process as claimed in claim 2, wherein the pressing vacancy is a cylindrical sheath comprising high-strength silicon nitride ceramic.

5. The process as claimed in claim 1, further comprising carrying out at least one deaeration stroke, wherein for each deaeration stroke, an initial pressure p.sub.1i, which is at most as great as the pressing pressure, is reduced over a period .sub.1 from 0.5 to 1.5 s, to a value p.sub.1f, p.sub.1f is held for a period .sub.1 from 0 to 1 s, and then the pressing pressure is initiated until the final pressing pressure is reached.

6. The process as claimed in claim 5, wherein two deaeration strokes a and b are carried out, and a is carried out at an initial pressure p.sub.1a, with the periods .sub.1a and .sub.1a, b is carried out at an initial pressure p.sub.1b, with the periods .sub.1b and .sub.1b, p.sub.1a and p.sub.1b are identical or different, the periods .sub.1a and .sub.1b are identical or different, and the periods .sub.1a and .sub.1b are identical or different.

7. The process as claimed in claim 5, wherein for each deaeration stroke, the period .sub.1 is 1 s, p.sub.1f=0, and p.sub.1f is held for a period .sub.1 of 0 s.

8. The process as claimed in claim 5, wherein two deaeration strokes a and b are carried out, and a is carried out at an initial pressure p.sub.1a, with the periods .sub.1a and .sub.1a, b is carried out at an initial pressure p.sub.1b, with the periods .sub.1b and .sub.1b, p.sub.1a and p.sub.1b are different, the periods .sub.1a and .sub.1b are different, and the periods .sub.1a and .sub.1b are different.

9. The process as claimed in claim 5, wherein each deaeration stroke is carried out with vacuum.

10. The process as claimed in claim 5, wherein each deaeration stroke is carried out without vacuum.

11. The process as claimed in claim 5, wherein the initial pressure p.sub.1i for each deaeration stroke ranges from 0.2 to 1 kN/cm.sup.2.

12. The process as claimed in claim 1, where the final pressing pressure is from 10 kN/cm.sup.2 to 12 kN/cm.sup.2.

13. The process as claimed in claim 1, wherein the pressing rate during said hydraulically pressing is from 0.1 to 0.5 kN/cm.sup.2s, the final pressing pressure is held for a period of 1 s, and said reducing is carried out over a period of 1 s.

14. A compact, obtained according to the process as claimed in claim 1, wherein a filling factor of the finely divided solids to be hydraulically pressed is from 3.9 to 4.5.

Description

BRIEF DESCRIPTION OF THE DRAWING

(1) FIG. 1 shows the change of the density and the filling factor of the obtained compact with the pressing pressure.

(2) The invention is illustrated in more detail below.

(3) In the process according to the invention, it can be advantageous to use a cylindrical sheath made of ceramic, preferably of high-strength silicon nitride ceramic, as pressing vacancy. Here, the upper and lower punch of the hydraulic press can be selected from hardened steel. Preferably, punches can be used in cylindrical form, although any other form is also possible, for example angular punches, which produce cube- or square-shaped compacts, or semispherical shapes.

(4) Since the finely divided solids contain chlorine compounds, the compacts have hydrochloric acid on their surface upon contact with water or atmospheric moisture, and said acid corrodes both inadequately alloyed steel as well as unpainted constituents of the hydraulic press. Consequently, the use of a ceramic sheath made of silicon nitride, Si.sub.3N.sub.4, as pressing vacancy is advantageous.

(5) Hydrogen chloride likewise attacks mucosa, skin and eyes of people. Consequently, it must be strictly observed that in the event of carrying out the process manually, personal protective equipment is worn which prevents direct contact between the finely divided solids used and the compacts obtained according to the invention and skin, eyes and mucosa.

(6) Preferably, inorganic binders, preferably silicas, aluminates, zirconates, calcium oxide, calcium hydroxide, cement, calcium sulfate, binders containing organic compounds, for example silicic acid esters, or a mixture of these binders can be used. If organic binders are used, the organic fraction has to be removed in a calcination step prior to use for the chlorosilane production. The use of a binder or of a mixture of aforementioned binders has the advantage of an alkaline pH. This is because the solids from the chlorosilane synthesis contain HCl and hydrolyzable silicon-halogen bonds on account of the prevailing reaction conditions in the chlorosilane production process, and these adhere to the solids which thus have an acidic pH. This is neutralized by the binder or binders. The unpleasant property of the compacts of releasing hydrogen chloride can thus be at least partially overcome.

(7) It may be advantageous to use a pressing pressure in the process according to the invention which is at most 14 kN/cm.sup.2, preferably from 10 kN/cm.sup.2 to 12 kN/cm.sup.2.

(8) In the context of the invention, pressure and pressing pressure are understood as meaning the superatmospheric pressure above the ambient pressure of 1013 hPa at 20 C. which is exerted upon the finely divided solids in the hydraulic press. At a pressing pressure of zero, just the ambient pressure is thus exerted and therefore no compaction is achieved.

(9) At a pressing pressure of at most 14 kN/cm.sup.2, surprisingly stable compacts are obtained since these can be handled, for example conveyed into a reactor for the production of chlorosilanes by a conveyor belt, without crumbling or breaking into lumps.

(10) If the pressing pressure is selected from the interval from 10 kN/cm.sup.2 to 12 kN/cm.sup.2, those compacts obtained according to the invention even withstand a drop from a height of up to 2 m without breaking. This is because these compacts do not have an inhomogeneous density distribution synonymous with the fact that the compacts do not have zones with increased density gradients, so-called layers. The layers can be expanded crossways, longitudinally or in any other direction within the volume of the compact and based on the direction of action of the pressing pressure and have a fleece or lens-like configuration. The formation of such layers, the so-called layer formation, arises to a particularly increased extent above a pressing pressure of 14 kN/cm.sup.2. If the layer formation is avoided, the compacts can be returned to the reactor in a particularly simple manner.

(11) In a further embodiment of the process according to the invention, the pressing pressure can be applied by initiating the pressure starting at zero with a selected pressing rate. In the context of the invention, the pressing rate is the quotient of pressing pressure and the duration during which the pressure is increased to the pressing pressure with monotonic increase, synonymous with the fact that the pressing pressure is initiated during this period.

(12) Preferably, in the process according to the invention, the pressing pressure can be initiated with a pressing rate from 0.1 to 1 kN/cm.sup.2s, preferably of 0.5 kN/cm.sup.2s, until the pressing pressure is achieved, and then held for a period from 0.5 to 1.5 s, preferably of 1 s, and then reduced to zero over a period from 0.5 to 1.5 s, preferably of 1 s.

(13) In the process according to the invention, at least one deaeration stroke is particularly preferably carried out, in which case each deaeration stroke is characterized in that at at least one initial pressure p.sub.1i, which is at most as great as the pressing pressure, this pressure is reduced over a period .sub.1 from 0.5 to 1.5 s, preferably of 1 s, to a value p.sub.1f, preferably to p.sub.1f=0, and p.sub.1f is for a period .sub.1 held from 0 to 1 s, preferably for 0 s, and then the pressure is initiated until the pressing pressure is reached.

(14) Preferably, the initial pressure is selected to be less than the pressing pressure. This is because it has been found that upon filling a pressing vacancy with the finely divided solids before initiating the pressure, the pressing vacancy contains up to 77% by volume air or gas. Upon initiating the pressure, the density of the finely divided solids is gradually increased, and the air or the incorporated gas has to escape. The smaller the sizes of the particles of the finely divided solids, the longer it takes until incorporated air or gas has escaped. If at least one deaeration stroke is carried out in the process according to the invention, compacts are obtained which have greater strength than without at least one deaeration stroke.

(15) Particularly solid compacts are obtainable if in the process according to the invention two deaeration strokes a and b are carried out, and a at an initial pressure p.sub.1a, with the periods .sub.1a and .sub.1a, b at an initial pressure p.sub.1b, with the periods .sub.1b and .sub.1b, and p.sub.1a and p.sub.1b are identical or different, preferably different, and the periods .sub.1a and .sub.1b are identical or different, preferably different, and the periods .sub.1a and .sub.1b are identical or different, preferably different.

(16) Each deaeration stroke can be carried out with or without vacuum, preferably without vacuum, and be initiated with 0.2 to 1 kN/cm.sup.2, preferably 0.5 kN/cm.sup.2 pressing pressure, and for 2 to 10 s, preferably 5 s duration.

(17) Furthermore, it can be advantageous to eject the compact from the pressing vacancy in the process according to the invention with an ejection force of 7 to 8 kN/cm.sup.2. The ejection force has to be applied in order to drive the compact from the vacancy. It is known to the person skilled in the art from the field of hydraulic pressing. Unusually for the outcome of hydraulic pressing, however, no noises arise upon ejection.

(18) There is thus no jerky force pattern upon ejection of the compact from the vacancy which one would expect in the prior art on account of the significantly greater static friction compared with the sliding friction. The known Slip-Stick effect is surprisingly not present when carrying out the claimed process.

(19) After carrying out the process according to the invention, a surprisingly high filling factor is found. In the context of the invention, a filling factor is understood as meaning the ratio of the height with which the pressing vacancy is filled with the finely divided solids to the height of the compact produced according to the invention. If, for example, the vacancy has been filled to a height of 96 mm, and, after the process according to the invention has been carried out, a compact with a height of about 22 mm is obtained, this compact is present with a filling factor of
96/22=4.36.

(20) This filling factor is unexpectedly high compared with filling factors of customary pressing masses, for example of standard commercial sands, of ceramic granules for the sintering for producing tiles. For example, in the case of pure sand and the same pressing pressure, it is lower by about a factor of 2.

(21) Consequently, the invention likewise provides a compact which is obtained according to the claimed process and which is characterized by a filling factor of the finely divided solids to be hydraulically pressed of 3.9 to 4.5.

(22) The process according to the invention will be illustrated in more detail below by reference to examples.

(23) Pressing Experiments on Finely Divided Solids from the Chlorosilane Production.

(24) In the examples described hereinbelow, the finely divided solids were filter dust from the hot filter of a fluidized-bed reactor for the production of chlorosilanes, so-called hot gas filter ash.

(25) A hydraulic press of the Alpha 1500_120 type from Alpha Ceramics was implemented. Here, a cylindrical DM 41 mm 4-hole cylinder mold without conical opening was used. Its upper and lower punches consisted of hardened steel, and the pressing vacancies used were sheaths made of high-strength silicon nitride ceramic (Si.sub.3N.sub.4). Here, it was ensured that the mold surface was drawn off cleanly. If this cannot be effected, it is also possible to press from above using a mirror plate. However, cakings of the compact on the contact surfaces can then be expected.

(26) In the experiments, upper punches were driven to enter into the mold. For this, these upper punches were aligned.

(27) On account of the very fine ash, a circulating punch play of at most 0.05 mm was traveled, and also a targeted removal by suction between the lower punches was used since, during the initiation of the pressure, a blowing-out of material along the lower punches was observed.

(28) Each pressing vacancy was filled up to a height of 96 mm with the hot gas filter ash. The experiments differ as a result of the pressing force, which was 6, 8, 10, 12 or 14 kN/cm.sup.2 and had been initiated in each case at a pressing rate of 0.5 kN/cm.sup.2s.

(29) The ejection force was in each case 7.5 kN/cm.sup.2.

(30) The extent and the densities of the compacts obtained according to the invention are summarized in table 1.

(31) TABLE-US-00001 TABLE 1 Overview of the ascertained parameters of the compacts as a function of the pressing pressure. Pressing Compact pressure Height Weight Density Filling kN/cm.sup.2 Diameter [mm] [mm] [g] [g/cm.sup.3] factor 6 41.25 24.18 71.17 2.20 3.97 8 41.26 22.95 74.63 2.43 4.18 10 41.17 22.39 72.70 2.44 4.29 12 41.18 21.72 70.65 2.44 4.42 14 41.16 21.52 71.37 2.49 4.46

(32) For each experiment which has been carried out with the pressing pressure noted in table 1, two deaeration strokes were used.

(33) The first deaeration stroke at 2 kN/cm.sup.2 pressure was initiated with 0.5 kN/cm.sup.2s for 4 s, not held, and decreased within 1 s.

(34) The second deaeration stroke was initiated at 4 kN/cm.sup.2 pressure with 0.5 kN/cm.sup.2s for 8 s, held for 1 s and decreased within 1 s.

(35) It was found that the hot gas filter ash cannot be compacted much more than was achieved with a pressing pressure of 14 kN/cm.sup.2. Instead, it was observed that density and filling factor as a function of the pressing pressure exhibited a saturation behavior, shown in FIG. 1. Use of higher pressing pressures, not shown here, resulted in layer formation in the compacts.

(36) In the case of a pressing pressure 10 kN/cm.sup.2 to 12 kN/cm.sup.2, the compacts obtained according to the invention withstood a drop from a height of about 2 m without breaking open. Consequently, these have a sufficiently good stability to be able to be returned to the reactor.