Liquid purification or separation – Processes – Chemical treatment
Reexamination Certificate
1999-06-08
2001-02-20
Simmons, David A. (Department: 1724)
Liquid purification or separation
Processes
Chemical treatment
C210S754000, C210S903000
Reexamination Certificate
active
06190567
ABSTRACT:
The present invention relates to a process for breaking down hydroxylamine in aqueous solutions, especially in wastewaters, by treatment with an oxidizing agent.
A range of chemical processes produce wastewaters which contain hydroxylamine, whose bacterial toxicity gives rise to problems in the disposal of these wastewaters. Hydroxylamine is only moderately stable and decomposes on heating to give NH
3
and N
2
in alkaline solution and NH
3
and N
2
O in acidic solution, for example. However, the rate of these reactions is too low for the effective breakdown of hydroxylamine in wastewaters.
Hydroxylamine is a strong reducing agent and is therefore amenable in principle to oxidative decomposition; however, it is known that the salts of hydroxylamine possess considerable oxidation stability. To ensure that the hydroxylamine is readily oxidizable, the reaction with oxidizing agents takes place in a moderately or highly basic medium in which the hydroxylamine is in the form of the free base, which is less stable. Suitable oxidizing agents include oxygen, ozone, nitrites, nitrates, halogens, such as chlorine, bromine and iodine, hypohalites, such as sodium hypochlorite, potassium hypochlorite and calcium hypochlorite, hydrogen peroxide, peroxo compounds, such as peroxomonosulfates and peroxodisulfates, manganese dioxide, permanganates, chromates and dichromates.
SU 617 366 (inventor's certificate), for instance, describes a process for decomposing concentrated calcium hypochlorite solutions using hydroxylamine, which must be used in excess. U.S. Pat. No. 5,062,966 describes a process for decomposing concentrated solutions of hydroxylammonium salts by adding a base to establish a pH of >8, preferably >12, and then carrying out reaction with hypochlorite.
The known processes have the disadvantage that when other nitrogen bases are present, such as ammonia, which may be present in the hydroxylamine as a result either of the preparation process or of decomposition events, these bases are oxidized as well. This raises the consumption of oxidizing agent and may result in the formation of unwanted byproducts. In addition, the strongly basic wastewater has to be neutralized, resulting in an undesirably high salt load.
It is an object of the present invention to provide a process which enables aqueous solutions comprising hydroxylamine, especially wastewaters comprising hydroxylamine, to be broken down over a wide concentration range and down to a low residual concentration at high reaction rates. The process should also feature sufficient selectivity for hydroxylamine in the presence of other organic substances, especially ammonia and amines.
We have found that this object is achieved and that hydroxylamine can be broken down selectively in aqueous solutions, especially wastewaters, if said wastewaters are treated with hypohalites, peroxomonosulfates or peroxodisulfates at a pH <8. This could not have been expected in the light of U.S. Pat. No. 5,062,966, according to which oxidization with hypochlorite is effective only in the strongly alkaline range. In particular it was surprising that the breakdown of hydroxylamine takes place selectively at a pH<8.
The present invention therefore provides a process for breaking down hydroxylamine in aqueous solutions, especially wastewaters, by treatment with an oxidizing agent, which comprises using as said oxidizing agent an alkali metal hypohalite or alkaline earth metal hypohalite or peroxomonosulfuric or peroxodisulfuric acid or a salt thereof and carrying out treatment with the oxidizing agent at a pH<8.
The treatment with the oxidizing agent is preferably carried out at a pH in the range from 3 to 7.8, more preferably in the range from 5 to 7.5 and, with very particular preference, in the range form 6 to 7.5. Where the aqueous solutions containing hydroxylamine have a pH which is outside these ranges, it is necessary to add acids (e.g. sulfuric or hydrochloric acid) or alkalis (e.g. NaOH) to establish a pH within the range of the invention. In general, it is sufficient to establish the pH at a suitable level within the range of the invention once, before beginning the addition of the oxidizing agent; however, depending on the concentration of the hydroxylamine and on the nature and concentration of any additional substances present in the wastewaters, it may be advantageous to monitor the pH during the addition of the oxidizing agent and to regulate it accordingly if necessary.
The process of the invention is particularly suitable for breaking down hydroxylamine in aqueous solutions, especially wastewaters, having a hydroxylamine concentration of up to 20 g/l, and generally results in a residual hydroxylamine concentration of less than 100 ppm, in particular less than 25 ppm and, with particular preference, less than 10 ppm. A further particular advantage is that this process permits selective breakdown of hydroxylamine even in the presence of other organic substances, especially ammonia and amines, it being possible for the ammonia concentration to be up 10 g/l.
The process can be operated continuously or batchwise, preferably batchwise by adding the oxidizing agent to the collected hydroxylamine solution. For the batchwise breakdown of large quantities of hydroxylamine-carrying wastewaters it is advantageous to operate in alternation using at least two vessels or reactors.
Suitable oxidizing agents for treating the hydroxylamine-carrying aqueous solutions are alkali metal hypohalites, such as alkali metal hypochlorites and hypobromites, especially hypochlorites, preferably sodium hypochlorite and potassium hypochlorite, alkaline earth metal hypohalites, such as alkaline earth metal hypochlorites and hypobromites, especially magnesium hypochlorite and calcium hypochlorite, peroxomonosulfuric and peroxodisulfuric acid and their salts, such as the alkali metal salts and ammonium salts, especially sodium peroxodisulfate and potassium peroxodisulfate. Particular preference is given to sodium hypochlorite and sodium peroxodisulfate. Mixtures of two or more of the oxidizing agents can also be used.
In general, the process of the invention employs the oxidizing agents in the form of aqueous solutions. Suitable concentrations of the solutions lie within the range from 5 to 70% by weight, in particular from 10 to 50% by weight. A particularly preferred concentration range for the sodium hypochlorite solutions is from 10 to 20% by weight (from 10 to 13% active chlorine content) and, for the sodium peroxodisulfate solutions, from 20 to 50% by weight. The oxidizing agent is generally employed in an amount of from 1.1 to 3, in particular from 1.1 to 1.5, molar equivalents, 35 based on the hydroxylamine content of the aqueous solution to be treated. The oxidizing agent is added continuously or a little at a time, preferably continuously, over a period of generally from 5 to 90 minutes, preferably from 5 to 45 minutes.
For a sufficiently high breakdown rate of the hydroxylamine in combination with a very low consumption of oxidizing agent it is advantageous to ensure thorough mixing between the hydroxylamine-carrying aqueous solution and the oxidizing agent. Suitable means which enable thorough mixing are known to the skilled worker (see also Ullmanns Enzyklopadie der technischen Chemie, Vol. 1, 3rd edition, 1951, p. 701 ff.). Mixing can, generally, be carried out by means of stirrers, such as propeller, blade or paddle stirrers, in-line mixers, jet introduction devices, etc.
The reaction time to complete breakdown is an hour or less. Especially when using hypochlorite, reaction times of 10 minutes or less are required.
Preferably, process parameters such as pH, redox potential, gas evolution, temperature development and the like are used to control the addition of oxidizing agent automatically. Particular preference is given to controlling the addition of oxidizing agent by on-line measurement of the redox potential during the treatment of the invention. Electrodes suitable for measuring the redox potential are known to the skilled wor
Hanus Frank
Leutner Bernd
Schneider Hans-Michael
Watzenberger Otto
Wegmann Uwe
BASF - Aktiengesellschaft
Hoey Betsey Morrison
Keil & Weinkauf
Simmons David A.
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