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If contaminations exist in hydrochloric acid made by absorption, these may be related to the following material groups in the majority number of cases: Hydrogen halogens, and other materials that are capable of absorption by HCl, also less soluble materials such as gases (SO2 or NOx), or organic components and non-volatile materials such as heavy metal compounds or metal halogens (e.g. NaBr). The facility expense to remove these collateral materials should be as low as possible. This is especially achievable if one can combine cleaning steps with other process steps in one unit.
Separation of Non-Volatile Collateral Material
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Non-volatile collateral material, such as heavy metal salts, are held back simply by evaporating the feed acid. An according layout scheme is presented in illustration 1. Upon existence of metal halogens, one may take notice that such retains equilibrium in strongly HCl acid enviroment. The consequence is that the greater part of the metal halogen remains in the residue. The evaporated hydrochloric acid will display parts of the corresponding hydrogen halogens inside.
<<<< Illustr. 1:
Cleaning of Hydrochloric Acid by Evaporation
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An extra opportunity for separation unfolds due to the combination of an evaporator with a rectification column (Illustr. 2); even non-volatile materials such as HNO3 as a result of absorbed NOx can be separated. This is especially interesting if a concentrationing step is necessary for the overall process anyway. In the described case, the concentration is combined with the cleaning process.
<<<< Illustr. 2:
Cleaning of Hydrochloric Acid by Rectification
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Separation of Absorbed Gases (e.g. SO2) and Organic Materials
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In accordance with the specific solubility, it may be feasible that also other materials are absorbed into water, next to HCl. These are removed from the acid by implementing a stripping process by using steam. It is common to perform this cleaning step together with absorption in a facility, as displayed in illustration 3.
Absorption is performed in the upper part, whereas stripping of the also absorbed materials is performed in the lower part. If these materials are organic compounds and display themselves as volatile to steam, it may be recommended to implement falling condensation technology with final separation of organic components, as displayed in illustration 3. If the parts to be stripped are gases, such as SO2, condensation processes may also be implemented in a rising fashion.
<<<< Illustr. 3
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Separation of Bromide Ions
Some hydrochloric acid may also contain bromide ions, in form of HBr or as metal bromide. One must assume the existence of metal bromides in the acid directly upon performing absorption. Hydrogen bromide primarily exists if the acid already has processed through a condensation process in a first cleaning step or a combined evaporation / rectification process. Chlorine is preferred in order to perform separation of bromide ions. Chlorine gas production via the help of sodium (or similar materials) lends itself next to the use of chlorine gas. Bromide ions oxidize to basic bromide via chlorine. This bromide is stripped out as a light volatile component together with excess chlorine as displayed in illustration 4; then condensated at the column head, and separated.
This is also possible if one can combine cleaning steps with one or more other process steps in one unit. Thus, on the one side, it is possible to remove non-volatile materials via the residue of the evaporator, as displayed in illustration 5. On the other side, concentration of under-azeotrope feed acid to azeotrope concentration level and the stripping out of elementary bromides from oxidized bromide are performed.
Separation of Fluoride Ions
One notices the fact that non-volatile metal fluorides exist for the separation of fluoride ions. CaCl2 is an especially well-suited precipitant. This precipitation is not performed in a quantitative fashion so that only certain residue fluoride concentrations may be attained. A technology is available by employing the patented QVF process (European Patent EP 0506050, German Patent No. 4110177), which makes the removal of fluoride ions from hydrochloric acid possible, so that cleaned hydrochloric acid is generated that can be forwarded for use in industrial processing.
Illustration 6 displays the flow chart of such a process. In the course, diluted, contaminated hydrochloric acid is pumped from a storage tank into a reaction vessel and is mixed with calcium chloride solution. After precipitation the resulting calcium fluoride is removed in a decanter as a hydrochloric acid suspension. The remaining CaCl2 solution, which contains the main bulk of hydrochloric acid, is partly recycled to the reaction vessel for further precipitation and partly rectified in the column, whereby HF free hydrochloric acid is removed at a medium level and water is removed at the column head. The remaining salt solution in the bottom contains also HCl, and possibly other salts. A part of this CaCl2 solution is neutralized by adding milk of lime (CaO), and concentrated to pudding-like mass using a thickener. The remaining CaC2l solution is also mixed with CaO, and again recycled to the reaction vessel as precipitant in order to compensate for CaCl2 losses.
As CaCl2 is also used as extraction agent in extractive rectification processing to manufacture over-azeotrope acid, significant advantages are offered if one combines both process steps together. In accordance with the patented QVF technology (German Patent No 4110177), a part of the cooled and concentrated CaCl2 solution is extracted from the CaCl2 circle of the extractive rectification process, and mixed with the evaporated and pre-condensated acid (see also Concentration of Hydrochloric Acid) in an over-stoichiometric ratio.
The precipitating CaF2 is subsequently separated in a filtration step, and disposed of as waste product. The cleaned acid is pre-heated as a mixture displaying surplus amounts of CaCl2, and forwarded to the extractive rectification process. If the demand exists for further reduction of fluoride concentration in the product , this may be achieved by addition of small amounts of AlCl3.
Illustr. 6 >>>> QVF Technology to Separate Fluoride Ions from Hydrochloric Acid
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