Chemistry of inorganic compounds – With additive – Including anticaking or antihygroscopic function
Reexamination Certificate
2001-12-11
2004-10-05
Bos, Steven (Department: 1754)
Chemistry of inorganic compounds
With additive
Including anticaking or antihygroscopic function
C252S383000
Reexamination Certificate
active
06800263
ABSTRACT:
The present invention relates to non-caking sodium chloride (salt) compositions comprising iron, titanium and/or chromium complexes of hydroxypolycarboxylic compounds as an anti-caking additive, to a process to make such non-caking sodium chloride compositions, as well as to the use of such non-caking sodium chloride compositions to make brine, a solution of essentially NaCl in water, for electrolysis in, preferably, membrane cells.
GB 908,017 discloses the use of ammonium ferric salts of hydroxy-polycarboxylic compounds as anti-caking agents for salt. It is stated that, “Where the number of acidic, for example carboxylic, functions is in excess of the valence of the iron, the excess, in neutral compounds, is neutralised by a basic atom or molecule such as an alkali or alkaline earth metal atom or, preferably, an ammonium radical.” It is not stated that the compounds must be neutral. Also, only ammonium ferric compounds are disclosed, while it is not shown that neutralising species other than ammonia can be used. Furthermore, it is disclosed neither that salt is not caking when it is treated with ferric compounds neutralised with a product other than ammonia, nor that it is advantageous to use other neutralizing agents than ammonia. In this respect, reference is made to
British Chemical Engineering
Vol. 11, No 1 (January 1966), pages 34 and 35, where numerous compounds were evaluated for their efficiency in rendering salt crystals non-caking. The majority of the evaluated compounds were shown not to be effective. The effective compounds all contained one or more nitrogen atoms or undesired heavy metals. Therefore, heavy metal-free anti-caking agents were considered to inevitably contain nitrogen. Typically, the nitrogen is present in the form of cyanide or substituted ammonia groups. Till the present day, sodium or potassium ferrocyanide has been the product of choice. However, the use of anti-caking agents containing nitrogen, especially in the form of cyanide groups, is undesired. More particularly, there is an ongoing debate in respect of the desirability of a sodium or potassium ferrocyanide in table salt. Furthermore, the use of sodium or potassium ferrocyanide, or other nitrogen-containing anti-caking agents, gives difficulties in electrolysis operations because of the formation of NCl
3
. Especially when the NCl
3
accumulates, which is the case if chlorine gas is liquified as commercial in electrolysis operations, its formation is highly undesired because the resulting product is explosive. FR 69.36254 proposes to use ferric acetate, which is said not to suffer from these disadvantages, as an anti-caking agent for salt. However, ferric acetate was found not to be a sufficiently effective anti-caking additive for salt.
A further disadvantage of commercially used potassium ferrocyanide is the fact that the iron introduced by this agent can only be removed from brine produced from salt containing said anti-caking agent if special decomposition unit is used. Especially when the brine is used in membrane electrolysis cells, the iron that is not removed will precipitate, typically in the form of the hydroxide, in and on the membrane. This leads to less efficient membrane electrolysis operations.
For these reasons, the search for improved anti-caking salt additives has been ongoing and the need for improved non-caking salt compositions still exists.
Surprisingly, we have now found that non-caking salt compositions can be produced which do not suffer from the above-mentioned disadvantages. These non-caking salt compositions, where the salt is predominantly sodium chloride, are characterized in that they
are essentially nitrogen-free,
comprise a metal complex of hydroxypolycarboxylic acids, with the molar ratio between metal and hydroxypolycarboxylic acid being from 0.2 to 10, wherein the metal is selected from iron, titanium and/or chromium,
and have a pH of 1-10, preferably 3 to 9.
The pH requirement may be met without further additives being used, depending on the pH of the salt that is used. If the requirement is not met, then optional pH control agents can be used to obtain the desired pH.
The term “predominantly sodium chloride” is meant to denominate all salt of which more than 50% by weight consists of NaCl. Preferably, such salt contains more than 90% by weight of NaCl. More preferably, the salt contains more than 92% of NaCl, while a salt of more than 95% by weight NaCl is most preferred. Typically, the salt will contain about 2.5-3% water. The salt may be rock salt, solar salt, salt obtained by steam evaporation of water from brine, and the like.
The term “essentially free of nitrogen” is used for compositions that during electrolysis operation do not form NC
3
. Generally, this means that only traces of nitrogen-containing species (other than inert N
2
gas) are allowed in the composition. The amount of nitrogen atoms of said species in the composition is preferably less than 1 mg/kg, more preferably less than 0.1 mg/kg, while an amount of less than 0.01 mg/kg is most preferred. Higher amounts of such nitrogen are highly undesired, since they make the salt less suitable for use in membrane electrolysis operations.
The preferred pH range of the salt composition, measured as described below, depends on the type of hydroxypolycarboxylic acid used. For instance, it was observed that for iron-citric acid complexes, the preferred pH range is 6-10, since at this pH the best anti-caking performance was observed. For iron-mesotartaric acid complexes, on the other hand, the preferred pH range is 2-9, more preferably 3-7, while a pH ranging from 4 to 5 is most preferred. Which exact pH range works best for the other hydroxypolycarboxylic acids can be established simply by evaluating the caking behaviour of salt that has been treated with metal complexes of these products at the various pHs. The pH can be adjusted, if so desired, by means of any conventional acid or base. The acid or base can be added separately or together with the anti-caking agent. For the final composition to be nitrogen-free, the acid and base cannot be selected from nitrogen-containing products. Preferably, the pH of untreated salt is first adjusted to the desired level, after which a solution comprising one or more of the metal complexes of hydroxypolycarboxylic acids with the same pH is added to the salt. The way the anti-caking agent and the acid or base are introduced depends on the desired water content of the resulting salt and the water content of the salt to be treated. Typically, a concentrated solution of the agents is sprayed onto the salt.
If so desired, an additional pH buffer may be added to the salt and/or the treatment solution. The buffers to be used are of the conventional type. Preferably, they are organic acids. More preferably, they are carboxylic acids. Most preferably they are carboxylic acids that do not contain —CH
3
and/or —CH
2
—groups for the reason given below, such as formic acid and oxalic acid. The acid in the buffer of choice preferably has a pK value in aqueous solution around the desired pH, as is known in the art. The mesotartrate anti-caking agent was found to be best combined with formic acid as the pH buffer. The pH buffer can be used with or without the optional pH control agent being used. The pH buffer can be introduced into the salt composition by spraying the pure compound, a separate solution, and/or by introduction after mixing with the anti-caking treatment solution. Preferably, a treatment solution is sprayed onto the salt which comprises a metal source, hydroxypolycarboxylic acid, optionally a pH control agent, and optionally a pH buffer.
The metal source to be used to make the metal complexes of hydroxy-polycarboxylic acids according to the invention can be any conventional, water-soluble metal salt. Preferably, the salt is essentially nitrogen-free as in chlorides, sulfates, and the like. The metals that can be used are iron, titanium and/or chromium.
It was observed that the presence of other metals does not remove the beneficial non-caking effect of
Akzo Nobel N.V.
Bos Steven
Fennelly Richard P.
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