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
hydroxypolycarboxylic 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 (Jan. 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 NCl3. Especially when the NCl3 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
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 at least one metal complex of mesotartaric acid or of a mixture of
hydroxypolycarboxylic acids wherein at least 5% by weight of the hydroxypolycarboxylic
acid is mesotartaric acid, wherein the metal is iron, titanium and/or chromium,
and with the molar ratio between metal and mesotartaric acid or hydroxypolycarboxylic
acid being from 0.2 to 10, and
- have a pH of 1 to 10 when 100 g thereof is dissolved in 25 g of H2O
at 21 °C.
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 NCl3. Generally, this
means that only traces of nitrogen-containing species (other than inert N2
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 -CH3 and/or
-CH2-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 hydroxypolycarboxylic acids according to the invention can be any conventional,
watersoluble 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
It was observed that the presence of other metals does
not remove the beneficial non-caking effect of the metal complexes according to
the invention. Therefore, it is not necessary to use 100% pure metal sources. They
may be combined with other metals that are less active or inactive, or may be contaminated
with metals that are less desired, such as aluminium. Preferably more than 1, more
preferably more than 5, most preferably more than 10% by weight of all metal in
the composition is selected from iron, titanium and/or chromium. If the total amount
of metal in the formulation has to be kept to a minimum, it is preferred that more
than 25, more preferably more than 50, even more preferably more than 75, and most
preferably more than 90% by weight of all metal in the salt composition is selected
from iron, titanium and/or chromium.
For various reason, including the fact that iron can be
removed easily from brine if it is not complexed too strongly, as in the present
case, the use of iron complexes is most preferred.
The hydroxypolycarboxylic acid that is used according to
the invention is mesotartaric acid. Complexes of iron, titanium, and chromium with
this hydroxypolycarboxylic acid were found to render salt non-caking at low concentrations.
This hydroxypolycarboxylic acid does not comprise -CH2- and/or -CH3
groups, since the presence of such groups was found to result in the formation of
undesired chloroform and/or other chlorinated organic compounds in electrolysis
operations. Said chlorinated organics, i.e. chloroform, contaminate the chlorine
that is produced from brine containing said acids. The use of mesotartaric acid
has several advantages over the use of other hydroxypolycarboxylic acids because
its use i) results in an excellent anti-caking effect at the indicated pH range,
ii) gives a favourable strong dependency of anti-caking performance on pH, iii)
allows the easy removal of iron from brine produced with salt comprising the iron
complexes of this acid, and iv) because residual (meso)tartaric acid ions in brine
solutions do not interrupt membrane electrolysis operations. The product when used
in electrolysis operations did not result in the formation of NCl3, chloroform
and/or other chlorinated organic compounds. For this reason, and because mesotartaric
acid was found to be the most effective anti-caking agent, mesotartaric acid is
the most hydroxypolycarboxylic acid used. Because it was observed that mesotartaric
can be used in combination with one or more other hydroxypolycarboxylic acids without
a dramatic decrease in performance being observed, also such mixtures can be used.
If a mixture of acids is used, at least 5, preferably more than 10, more preferably
more than 20, even more preferably more than 35, and most preferably more than 50%
by weight of all acid in the formulation is mesotartaric acid.
A preferred mixture of tartaric acid, which includes mesotartaric
acid, can be prepared in a conventional way by treating a natural or synthetic tartaric
acid (CAS registry numbers 87-69-4 and 147-71-7, respectively) solution with concentrated
NaOH at temperatures above 100°C. Part of the L-, D-, and/or DL-tartaric acid
is then converted to the desired mesotartaric acid (CAS registry number 147-73-9).
The use of the nitrogen-free metal complexes of hydroxypolycarboxylic acids as an
anti-caking agent was also found to bring the additional benefit that water that
adheres to the salt is less likely to segregate upon storage.
It is noted that because of the pH dependency of the anti-caking
agent based on mesotartaric acid, it is possible to form blocks of salt from non-caking
salt merely by changing its pH to a value at which the anti-caking effect does not
exist and subsequently applying pressure. Such blocks can be used, for instance,
in salt dissolvers, e.g. in water softening installations, where such salt blocks
show less bridging. However, they may also be used as salt licks for animals. Residual
iron complexes of hydroxypolycarboxylic acid in such salt licks are not considered
to be a problem.
In membrane electrolysis operations, the use of mesotartaric
acid based anti-caking agents has the benefit that if mesotartaric acid enters the
electrolysis cell, it does not harm the membrane (no deposit is formed) while it
is rapidly decomposed in the anode chamber, releasing only harmless gaseous products
(typically just CO2). This is in contrast with various other hydroxypolycarboxylic
acids, such as citric acid which was found to generate chloroform. Furthermore,
it was found that brine obtained by the dissolution of non-caking salt according
to the invention can be purified, i.e. freed of iron, with greater ease than brine
containing alkali iron cyanide complexes, probably because of the weaker complexing
power of the hydroxypolycarboxylic acids. The improved removal of metal from the
brine prolongs the life of the membranes in brine electrolysis cells, also in view
of the fact that the voltage drop over the membrane remains more constant over time,
because less metal, i.e. iron, is carried into the cell and, consequently, less
metal hydroxide or oxide is deposited in and on the membrane. The fact that the
metal, i.e. iron, is now removed more easily allows for substantial savings in the
brine purification step and the electrolysis operation. The metal, i.e. iron, removal
step can be performed in the conventional way by increasing the pH of the brine
to precipitate the hydroxide and subsequent removal of the hydroxide by filtration.
For these reasons, a preferred embodiment of the invention is a membrane electrolysis
operation using brine obtained by the dissolution of a salt composition according
to the invention. More preferably, such an operation includes a process step wherein
metal ions are removed from the brine.
Because the valency of the metal in the salt may vary and
because different types of hydroxypolycarboxylic acids, with various amounts of
carboxylic acid groups per molecule, can be used according to the invention, the
molar ratio of metal to hydroxypolycarboxylic acid may vary over a wide range. If
iron is used as the metal, both di- and tri-valent ions (ferro- and ferri-ions,
respectively) are used with success. Practically, the metal in the final salt formulation
will be present in all valencies. Therefore, the term metal complex of hydroxypolycarboxylic
acid is used throughout this specification to denote compositions comprising metal
ions in various valencies and a hydroxypolycarboxylic acid moiety in ionic form.
If iron is used as the metal, ferro-compounds are preferred, because they were found
to give a slightly better anti-caking performance.
The amount of hydroxypolycarboxylic acid in respect of
the amount of metal ions will depend on the overall valency of the metal ions and
on the nature of the hydroxypolycarboxylic acid, particularly the amount of carboxylic
acid substituents per mole of acid. For the non-caking salt of the present invention,
a suitable molar ratio between hydroxypolycarboxylic acid and iron is from 0.2 to
10. However, for the various hydroxypolycarboxylic acids different optimum ratios
were found by simply evaluating the caking behaviour of the salt to which the products
were added. For mesotartaric acid salts the preferred range was found to be 1.5
The metal complexes of hydroxypolycarboxylic acids are
preferably used in an amount such that less than 20 mg of metal per kg is introduced
into the final non-caking salt formulation. More preferably, the amount used introduces
less than 10 mg metal per kg of the formulation, while most preferably, the amount
of metal introduced is less than 5 mg/kg. A preferred non-caking composition according
to the invention includes about 3 mg/kg of Fe" and 16 mg/kg of mesotartaric acid
The metal complexes of the hydroxypolycarboxylic acids
can be introduced or formed in and on the sodium chloride in various conventional
ways. However, a preferred way that resulted in much better control of the anti-caking
performance was to dissolve the metal source, the hydroxypolycarboxylic acid, and
optional further components in brine. To this end, one or more metal sources and
one or more hydroxypolycarboxylic acids are introduced into a solution of salt (NaCl),
optionally after the pH of said solution has been adjusted and/or buffered, with
a salt concentration from 10% by weight (%w/w) to saturated. More preferably, the
salt concentration in this solution is from 15 to 25%w/w. Most preferably, the salt
concentration is about 20%w/w in said solution. Preferably, the metal and the hydroxypolycarboxylic
acid(s) are provided on the salt crystals in a conventional way by spraying a solution
(preferably in brine) onto the salt. In a preferred embodiment, the solution sprayed
onto the salt comprises 20%w/w of salt, an iron source, such as FeCl2,
in an amount that will result in about 5 g/kg of Fe" in said solution, and about
25 g/kg of (meso)tartaric acid ions. If so desired, the salt is dried further after
the addition of the iron complexes of hydroxypolycarboxylic acids or solutions thereof.
The pH of the salt is measured in a conventional way using
a mixture of 100 g salt and 25 g H2O at 21°C.
Adhering water in the salt is determined by weight loss
measurement upon drying for 4 days at 35°C and 40% relative humidity. If the
ingredients are thermally stable, drying can take place at 120°C for 2 hours.
Caking is measured in triplicate by filling a cubic copper
mould of 5x5x5 cm with (treated) salt and pressing the lid at a pressure of 0.2
kg/cm2. Thereafter the resulting salt cubes are stored for 4 days at
a temperature of 35°C and 40% relative humidity. The force needed to break
up a fully supported cube by pressing at the top with a circular pad of 15 mm diameter,
is recorded. The higher the required force, the more the salt has caked.
A mesotartaric acid-containing treatment solution was prepared
by heating a solution of 95 g L(+)-tartaric acid, also known as d-tartaric acid,
in 1 kg of aqueous 30%w/w NaOH at 118°C for two hours. After cooling to room
temperature, HCl-solution was added to adjust the pH to 6. Depending on the amount
and the type of HCl solution used, NaCl or water was added in order to obtain a
solution containing 20%w/w of NaCl. Hereafter FeCl2 was added in such
an amount that the solution contained 4.8 g Fe" per 25.5 g of (meso)tartaric acid.
In the resulting treatment solution 29.8%w/w of the original L(+)-tartaric acid
was transformed into the meso form.
The treatment solution was sprayed onto salt (essentially
NaCl) with a water content of 2.5%w/w of which the pH was 6,' in an amount of 625
mg/kg. The resulting product showed no caking. The product can be used as road salt,
table salt, and the like. Brine solutions produced with the treated salt are pre-eminently
suited for use in membrane electrolysis cells.
Comparative Examples 2 and 3 and Comparative Examples A-C
The treatment solution of Example 4 was sprayed onto salt
with a carbonate content of about 0.2 mmol/kg and a moisture content of about 2%w/w
in an amount resulting in about 0.8 and 1.4 mg iron/kg being added. In comparative
tests, the same salt either remained untreated or was treated by the conventional
addition of about 8 mg/kg or 5 mg/kg of conventional K4Fe(CN)6•3H2O.
After having been stored for six months in 1,000 kg bags,
salt was taken from near the bottom of the bags, and the moisture content and the
caking behaviour were analyzed with the following results.
Anti caking agent
Caking Break force
0.8 mg Fe/kg
1.4 mg Fe/kg
This shows that the iron citrate is an effective anti-caking
agent that needs to be used in amount of only about 2 mg Fe/kg in order to be as
effective as 5 mg/kg conventional K4Fe(CN)6•3H2O.
Furthermore, it shows that the salt treated with iron citrate exhibited the least
segregation of water. This salt is pre-eminently suited for use in road salt and
Comparative Example 4
A citrate-containing treatment solution was prepared by
mixing 734.8 kg water, 250.0 kg NaCl, 6.5 kg ferro sulfate heptahydrate, 4.9 kg
citric acid monohydrate, and 3.8 kg sulfuric acid (96%). The molar ratio of iron
to citrate was 1:1. This treatment solution was sprayed (45l/h) onto salt on a conveyor
belt (35,000 kg/h) to result in a non-caking salt composition comprising the complexed
iron anti-caking agent in a concentration of about 1.7 mg/kg, expressed as the amount
of iron in the final product.
30,000 kg of the treated salt were stored indoors in a
heap. After 5 weeks the heap could easily be picked up with a shovel, demonstrating
that the treated salt is non-caking.
Comparative Example 5, Example 6 and Comparative Example D
An aqueous solution of 500 mg/l citric acid or mesotartaric
acid was treated under electrolysis conditions, viz. at 85°C and pH 3, in the
presence of chlorine. Decomposition was virtually complete within 6 minutes, since
at that moment the amount of acid was below the detection limit. The citric acid
was found to have generated some chloroform. The carbon of the mesotartaric acid
was converted into carbon dioxide. A comparative test wherein a cyanide-containing
anti-caking agent was used resulted in the formation of undesired NH2Cl,
NHCl2, which under membrane electrolysis operation conditions (in the
presence of high chlorine concentrations) is converted to undesired NCl3.
Examples 8 and 10 and Comparative Examples 7, 9, E and H
Solutions were prepared containing 250 g/l NaCl and anti-caking
agent in an amount such that 400 µg Fe/I was present. In the Examples, the
molar ratio of hydroxycarboxylic acid to iron was 1:1.
These solutions were stirred at 55°C for 15 minutes
at either pH 6.7 or pH 10.9 (measured at 20°C). Any iron hydroxide precipitate
formed was removed by filtration using a filter with 0.05 µm pores. The amount
of residual iron in the filtered solution was analyzed in a conventional way.
The following results were obtained:
residual iron (µg/l)
This shows that the use of iron mesotartrate greatly facilitates
the removal of iron from brine. Therefore, non-caking salt containing iron mesotartrate
as its anti-caking agent is pre-eminently suited for use in membrane electrolysis
operations where iron is detrimental.
Examples 11-15 and Comparative Example I.
To an aqueous brine solution (20%w/w NaCl) containing iron
and mesotartrate ions in a molar ratio of 1:2 (the amount of mesotartrate being
about 3%w/w based on the total composition) an aqueous solution of 15%w/w NaOH was
added to obtain a pH as indicated in the table below. This solution was sprayed
onto salt such that a total of 3 mg iron was present per kg of the NaCl. In a blank
experiment, the salt was not treated. The breaking force of the salt after caking
experiments was measured. The results were as follows:
Caking Break force
Clearly, the iron mesotartrate complexes can be used over
a wide pH range while remaining active as an anti-caking agent. The treatment solution
of Example 15 became turbid over time.
Example 14 was repeated, except that a mixture of iron
and aluminium ions (in a 1:1 weight ratio) was substituted for the iron in the treatment
solution and said solution was sprayed onto the NaCl such that, per kg of NaCl,
1 mg of Al and 1 mg of Fe ions were present. The caking break force of the so obtained
salt was 6 kg/cm2, showing that complexes of mixtures of metals and mesotartrate
may be used. However, since aluminium is less desired in both electrolysis operations
and food products, the use of just iron may be preferred.
Comparative Example J
Example 14 was repeated, except that the treatment solution
was brought to pH 5 using NH4OH instead of NaOH. The treatment solution
became turbid over time and the nitrogen-containing salt composition so obtained
led to the formation of undesired NCl3 upon electrolysis of brine produced
therefrom, making the use of NH4OH neutralized complexes less desirable.
Examples 17 and 18
Example 14 was repeated, except that chromium and titanium,
respectively, were substituted for the iron. The breaking force of the so obtained
salt was 6 and 30, respectively, showing that mesotartrate complexes of these two
metals are effective anti-caking additives for NaCl. Based on the unoptimized results,
iron and chromium complexes of mesotartrate are preferred.
Comparative Examples K-N
Example 14 was repeated, except that Ca, Mg, Sr, and Ba,
respectively, were substituted for the iron. The breaking force of the so obtained
salt was equal to or greater than the breaking force of untreated salt.