Patent Number: 045004498
Section: description

DETAILED DESCRIPTION OF THE INVENTION In the method according to the invention a mole ratio of sodium to boron of either 0.25 or 0.7 is set by adding sodium hydroxide to the residues, corresponding to a pH-value ranges of 7.3 to 8.0 for a mole ratio of 0.25, and 9.8 to 10.2 for a mole ratio of 0.7. For, it has been found that, contrary to the embedment conditions up to now, for instance, with a mole ratio of about 1, the following disadvantages are avoided with the mole ratios according to the invention: The resistance of the bitumen to strong caustic solutions is relatively poor. Therefore, the leaching resistance of the end product (bitumen with embedded residues) is improved substantially by the invention because of the smaller amount of caustic solution. In addition, because of the reduced requirement for sodium hydroxide, the mole ratio according to the invention results in a reduction of up to 50% in waste volume. The composition of the embedded borates is substantially less aggressive, i.e. reactive with respect to the bitumen, so that the danger of liberation of flammable vapors, previously observed, is practically completely avoided. The maximum embedment quantity of the residues is obtained with a mole ratio of 0.7. There, in accordance with a further feature of the invention, the processing and storage are carried out at temperatures of at least 50.degree. C. and preferably 80.degree. C. in order to avoid crystallization, which occurs for this mole ratio at lower temperatures. Of course, unduly high temperatures such as would cause evaporation of the concentrate in storage should not be employed. For this purpose the containers used for storing the residues can be provided with heating means such as a steam jacket or coil, so that the heating to the above-mentioned temperatures can be carried out not later than when the sodium hydroxide is added in accordance with the invention. With the above-mentioned mole ratio of sodium to boron of 0.7, the pH value is between 9.8 and 10.2 at 20.degree. C. The solubility of the borate is 125 g boron/kg at 80.degree. C. and about 5 g boron/kg bitumen at 20.degree. C. The solids produced during the embedding in bitumen have approximately the following composition: EQU 2Na.sub.2 O.times.3B.sub.2 O.sub.3 .times.4H.sub.2 O. These solids are present in the form of a melt at the operating temperatures of the worm dryer used as the mixing apparatus, so that abrasion and salt incrustation in suction lines associated with the mixing device are minimized. Also the leaching resistance of the end product is very favorable because of the low solubility of these borates at low temperatures. With a mole ratio of sodium to boron of 0.25, on the other hand, the processing and storage temperature can be in the range of room temperatures. For this ratio, a pH-value of 7.3 to 8.0 is obtained at 20.degree. C. The solubility of the borates is about 100 g boron/kg at 80.degree. C. and 20 g boron/kg at 20.degree. C. The solids have the composition: EQU Na.sub.2 O.times.4B.sub.2 O.sub.3 .times.4H.sub.2 O. Operating with a sodium-boron mole ratio of 0.25 prevents any salt encrustation in exhaust domes and lines and reduces the wear of the dryer worms. The leaching resistance of the end products is good. The pH-value of the bound solids of about 7.5 would seem to ensure the leaching resistance even after extended periods of storage. For adjusting the sodium-boron mole ratio of 0.25, only small amounts of sodium hydroxide are required. An advantage is that the storage of the residues can be carried out at temperatures of 20.degree. C.