Chemistry of inorganic compounds – Halogen or compound thereof – Hydrogen halide
Reexamination Certificate
1999-11-05
2002-04-02
Nguyen, Ngoc-Yen (Department: 1754)
Chemistry of inorganic compounds
Halogen or compound thereof
Hydrogen halide
C423S150300, C423SDIG001
Reexamination Certificate
active
06365121
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a process for the preparation of superazeotropic hydrochloric acid from metal chloride solutions as well as to a method for processing ores and/or metals utilizing superazeotropic hydrochloric acid prepared according to the invention.
The term “superazeotropic” in this context refers to a mixture of water and hydrogen chloride, the content of hydrogen chloride of which is higher than at the azeotropic point of the system water/hydrogen chloride under normal pressure. The azeotropic point of the system under normal pressure is at approximately 20% by weight HCl.
BACKGROUND
Regeneration of hydrochloric acid from spent solutions containing metal chloride resulting from ore and/or metal processing, e.g. solutions from leaching of ores or spent pickling acids, is known in many ways from the prior art.
In particular, the regeneration of hydrochloric acid by thermal decomposition of the solutions containing metal chloride is known, e.g. by pyrohydrolytic treatment wherein the hydrochloric acid is attained in a gaseous state in the exhaust vapors and wherein the corresponding metal oxide may be recovered as a further product.
The hydrochloric gases so recovered are mostly absorbed in water, thereby obtaining hydrochloric acid. A characteristic of this prior art process is the fact that the hydrochloric acid recovered is mostly sub-azeotropic, i.e. that the proportion of hydrogen chloride is below the azeotropic point of the system water/hydrogen chloride under normal pressure.
The reason for this is that during the adiabatic adsorption of the hydrochloric acid from hydrochloric gases the azeotropic point cannot be exceeded due to the chloride content of the starting solution.
Typical proportions of HCl in the hydrochloric acid obtained in processes according to the state of the art are e.g. in the region of 18% to 20% by weight, if the chloride content of the starting solution is about 200 g/l.
Only in exceptional cases where the chloride concentration in the solution to be regenerated is so high that a superazeotropic mixture is present already after the thermal decomposition in the hydrochloric gases, may a hydrochloric acid, the content of HCl of which is above the azeotropic point, be recovered by condensation alone.
In the event of a chloride content in the starting solution of 300 g/l a hydrochloric acid having approx. 26% to 27% by weight may be recovered by condensation alone from the roasting gas of a pyrohydrolytic treatment.
An increased need has, however, come about recently to prepare superazeotropic hydrochloric acid from solutions containing metal chloride which, as in the first case mentioned above are so composed that by a mere absorption of the hydrochloric gas only a sub-azeotropic mixture is obtained.
A need exists, however, even for the second case mentioned above where superazeotropic mixtures are already obtained per se, to obtain a more highly concentrated hydrochloric acid.
Concentrated hydrochloric acid having a HCl-content of, for example, 30% by weight or more may be employed for a greater variety of purposes than a more diluted hydrochloric acid. A concentration of hydrochloric acid having a HCl content of 26% to 27% by weight is, however, not possible by mere evaporation of excess water.
Ways and means are described in the literature as to how, from a diluted hydrochloric acid, the HCl-concentration can be moved above the azeotropic point of the system HCl/H
2
O by shifting the H
2
O partial pressure. In this context one may avail oneself e.g. of an entrainer which binds water and thereby shifts the ratio HCl:H
2
O. It is further known to obtain superazeotropic hydrochloric acid by employing different pressures.
A process to this effect is disclosed in Ullmann's
Enzyklopädie der Technischen Chemie
(Encyclopaedia of Technical Chemistry), 3rd edition, 1964, volume XV, page 79. In this case a highly concentrated hydrochloric gas is discharged in a first tower at a pressure above atmospheric. The bottom product (diluted hydrochloric acid) is treated in a second tower at reduced pressure in which process water is evaporated off and diluted hydrochloric acid is obtained as the bottom product with the azeotropic composition typical of the reduced pressure in the second tower. This diluted hydrochloric acid can now be reintroduced to the first tower.
In order to move from the sub-azeotropic hydrochloric acid obtained by means of the known methods mentioned above or from the relatively low concentrations of superazeotropic hydrochloric acid towards concentrated superazeotropic hydrochloric acid, the prior art has up to date proposed to upgrade the hydrochloric acid obtained by the condensation or absorption of the entire hydrochloric gases in a separate installation.
However, this separate upgrading proves to be cost-intensive so that, instead of upgrading the regenerated hydrochloric acid one opts frequently for discarding the acid and for purchasing fresh hydrochloric acid for the required purposes. In many instances an 18% regenerated hydrochloric acid is thus not used any further but is neutralized which, in turn, may cause environmental problems due to the metals possibly still contained in traces in the hydrochloric acid, such as e.g. chromium or nickel.
U.S. Pat. No. 4,216,196 discloses a process in which from the total volume of the hydrochloric acid condensed from the hydrochloric gases a partial stream is upgraded to highly-concentrated HCl-gas by means of the above described entrainer method and in which this gas is reunited with the main stream of the condensate. A solution of that metal chloride which is also fed to the thermal decomposition is employed as an entrainer.
SUMMARY OF THE INVENTION
It is the object of the present invention to obtain superazeotropic hydrochloric acid from the regeneration of solutions containing metal chloride by means of a cost effective process.
This object is attained according to the invention in that the metal chloride solutions are thermally decomposed to form a hydrochloric gas, in that the hydrochloric gas is processed into a superazeotropic gas mixture of hydrogen chloride and water in a manner known per se, the superazeotropic gas mixture is condensed to yield a superazeotropic hydrochloric acid.
For the purposes of the present invention the term “processing into a superazeotropic gas mixture” in this context either means the upgrading of a hydrogen chloride water mixture having a sub-azeotropic composition into a superazeotropic mixture or the further upgrading of an already superazeotropic mixture. By “superazeotropic gas mixture” it is understood that the hydrogen chloride concentration in the gas mixture is so high that hydrochloric acid having the desired superazeotropic composition comes about after a condensation of the mixture.
The method according to the invention differs from the prior art methods in that the hydrochloric gases obtained by the thermal decomposition of the metal chlorides are upgraded to a superazeotropic gas mixture prior to their condensation so that, contrary to the prior art, it is not necessary to condense or absorb in water the entire hydrochloric gas and the hydrochloric acid obtained does not subsequently have to be treated or upgraded to a superazeotropic hydrochloric acid.
This offers the advantage that expensive upgrading plants may be kept smaller than in the case of processes according to the prior art. It is also found that the energy expenditure can be kept lower with the process according to the invention than in the case of prior art processes.
By performing the process according to the invention, the problems of the prior art are avoided in an astonishingly simple manner, enabling the person skilled in the art to prepare in a favorable manner even higher concentrations of superazeotropic hydrochloric acid from solutions containing metal chloride.
The process according to the invention is preferably so designed that the metal chloride solution is thermally decomposed in a spray-roasting-reactor or in a fluid
Jacobson & Holman PLLC
Nguyen Ngoc-Yen
S.A.D. EDV-Systemanalyse und-Systemdesign Gesellschaft m.b.H.
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