Reduction of metal halates and recovery of metal halides

This invention relates to a process for reducing metal halates to metal halides, the process comprising reacting a metal halate with a reducing agent in an aqueous alkaline reaction medium having a pH above about 7, wherein the amount of reducing agent is sufficient to reduce substantially all of the metal halate to metal halide.

BACKGROUND OF THE INVENTION 
This invention relates to a novel process for the reduction of metal 
halates in aqueous alkaline solutions. More specifically, the invention 
relates to a process for decreasing the amount of metal bromate in aqueous 
alkaline solutions and recovering valuable metal bromide from such aqueous 
alkaline solutions. For example, in a scrubber system for the removal of 
halogens from process vent streams, an aqueous alkaline solution is 
generally used as a scrubbing medium. Minimizing the bromate content 
allows the use of low cost materials of construction, eliminates the 
potential for release of hazardous materials and achieves high bromide 
purity in the recovered product. 
The potential to form halates is known. The literature reports the 
formation of halate when a halogen and a basic metal compound react. Metal 
halides result when a basic metal compound and a reducing agent react with 
a halogen. Ammonium halide results when ammonia reacts with a halogen. 
The preparation of metal halide from a basic metal compound and a halogen 
in the presence of a reducing agent is well known in the art. Acidic 
conditions are reported, and the reaction conditions are such as to 
inhibit the formation of halates. In the absence of a reducing agent, a 
halogen compound reacts with a metal base in two steps, forming the 
hypohalide, which then tends to decompose to the halate. For example, U. 
S. Pat. No. 2,828,184 reports: 
EQU 3 I.sub.2 +6 KOH.fwdarw.3 KI+3 KIO+3 H.sub.2 O (1) 
EQU 3 KIO.fwdarw.KIO.sub.3 +2 KI (2) 
However, when a reducing agent is included in the reaction with the 
halogen, formation of the halate is prevented. 
U.S. Pat. No. 4,514,374 discloses preparation of aqueous metal bromide 
solution without the concurrent production of bromate by reaction of 
calcium hydroxide, bromine and methanol in aqueous solution. The reaction 
is depicted as follows: 
EQU 3 Ca(OH).sub.2 +3 Br.sub.2 +CH.sub.3 OH.fwdarw.3 CaBr.sub.2 +CO.sub.2 +5 
H.sub.2 O (3) 
In this process, only a stoichiometric amount of base is used, not excess 
base nor strongly alkaline reaction conditions. 
Other patent literature cites the use of ammonia, ammonium ion, or amines 
to form the halide directly from halogen. For example, the reaction with 
NH.sub.3 can be written as follows: 
EQU 3 Br.sub.2 +8 NH.sub.3 .fwdarw.6 NH.sub.4 Br+N.sub.2 ( 4) 
This reaction sequence is not without significant drawbacks including: 1) 
formation of explosive compounds such as halo amines as a by-product; and 
2) significant loss of ammonia to vent streams. 
Numerous examples of reducing agents are known in the prior art. For 
example, U.S. Pat. No. 2,269,733 discloses the use of ammonia, ammonium 
hydroxide, ammonium bromide, urea, formic acid, oxalic acid, ammonium 
carbonate, ammonium bicarbonate, and formamide as well as other metal 
oxides, hydroxides and carbonates. However, a small amount of free bromine 
is maintained during substantially the entire course of the reaction to 
form sodium bromide while the pH of the reaction is maintained below pH 7. 
In U.S. Pat. No. 3,431,068 there is disclosed a method of preparing alkali 
metal halides by reacting an alkali metal hydroxide with an elemental 
halogen in a liquid, saturated aliphatic or alicyclic alcohol or ketone, 
or a saturated aliphatic aldehyde. According to this process, the 
formation of unwanted halate salt by-product associated with alkali metal 
halide production is diminished or eliminated. It is also disclosed that 
the use of nitrogen containing compounds such as NH.sub.4 Br can result in 
explosive mixtures. The alcohol, aldehyde and ketone mentioned in this 
reference are not employed as reducing agents but as solvents and are 
present in large excess. Use of alcohols or ketones as reaction medium for 
scrubbing vent streams is prohibited by the emission losses that would 
occur. 
U.S. Pat. No. 4,083,942 discloses the use of formic acid as a reactant. The 
process is illustrated by the following equation: 
EQU Ca(OH).sub.2 +HCOOH+Br.sub.2 .fwdarw.CaBr.sub.2 +CO.sub.2 +2 H.sub.2 O(5) 
In the foregoing process, bromine and an alkaline compound are 
alternatively and incrementally added to an aqueous mixture of formic acid 
and a less than equivalent amount of metal compound, while maintaining the 
pH below 7.0. 
U.S. Pat. No. 3,592,600 relates a process for the recovery of bromine 
and/or iodine from the reaction products of oxydehydrogenations of 
hydrocarbons in which bromine and/or iodine serves as a catalyst. The 
reaction product leaving the reactor, primarily hydrogen halide with some 
elementary halogen, is treated with an aqueous ammonia solution to which 
has been added an amount of hydrazine sufficient for the reduction of the 
elementary halogen. The aqueous phase is separated and the by-products 
containing bromine and/or iodine separated in the working up of the 
organic phase are burned with oxygen containing gases and the gas from the 
combustion is scrubbed with an ammonia solution containing hydrazine. 
However, this process also suffers the drawback mentioned above wherein 
explosive halo amines are formed. 
Amine halide is prepared by reaction of an amine with a metal base and a 
halide in solvent. For example, lithium carbonate, hydrazine and iodine 
react in water, as is disclosed by U.S. Pat. No. 4,111,991. Reaction 
conditions are maintained at pH 4.5 to 7.0. 
Japanese Patent. No. 47-3399 recites reaction of Br.sub.2 with aqueous NaOH 
to generate sodium bromide and sodium bromate, as in reactions (1) and (2) 
above. Excess bromine is used in the reaction with caustic, leaving the pH 
of the solution at less than 7.0. The resulting bromate is then reduced 
using sulfurous anhydride, stannous chloride, metallic zinc, formic acid, 
hydroxylamine, hydroquinone, phenylhydrazine, hydrated hydrazine or the 
like. However, SO.sub.4, Cl, etc., or impurities generated in the 
reduction may be admixed and their removal is difficult resulting in 
products with extremely low purity. This reference also teaches the 
treating of a mixed solution of sodium bromide and sodium bromate with 
formic acid in an amount equal to or less than the theoretical amount 
needed to reduce the sodium bromate at less than 40.degree. C. Then, 
hydrogenated hydrazine is added to bring the solution to a pH of 1 or 
lower. The solution is subsequently concentrated, neutralized, filtered 
and heated to about 120.degree. C. While this procedure may help prevent 
the unwanted formation of sodium formate, the strongly acidic conditions 
which it employs prevents its use in alkaline scrubber systems such as 
those taught in this invention. 
In U.S. Pat. No. 4,248,850, it was disclosed that metal bromides could be 
prepared by contacting in an aqueous medium a basic metal compound and 
bromine in the presence of added formaldehyde as a reducing agent. 
However, this method suffers from the draw-back of leaving unreacted 
formaldehyde in the product mixture which is difficult to remove. Also, 
acidic reaction conditions are used. 
Thus the prior art for limiting halate accumulation falls into two classes: 
1) metal hydroxides and a reducing agent in neutral to acidic conditions 
reacting with halogen whereby the formation of halate is inhibited, and 2) 
ammonia, ammonium ion, or amines reacting with halogen or halate to form 
halide. There remains a need to reduce halates under strongly basic 
conditions, for example at pH&gt;10 and preferably at pH&gt;13, whereby valuable 
by-products are recovered and the formation of explosive compounds is 
inhibited. 
The strongly basic reduction conditions are important in certain 
operations. For example, in scrubber systems for scrubbing bromine from 
process vent streams, caustic solution containing sodium bromide are 
maintained for long periods of time. Sodium bromate tends to accumulate in 
the scrubber solution. In such systems, if bromate content is allowed to 
accumulate, the solution becomes very corrosive. Also, if spent scrubber 
solution is disposed of, sodium bromate creates a disposal hazard since 
acidification will release bromine and any uncontrolled reaction with 
certain compounds, such as amines, could lead to explosive mixtures. When 
using a reducing agent such as methanol in a scrubber system for removing 
halogens from process vent streams, it is desirable to use less than a 
stoichiometric amount of methanol to reduce the halates and to perform 
such reduction with the vent streams diverted from the scrubber. This 
reduces the potential to emit methanol in the vent gases, and to form 
methyl bromide. 
It is therefore an object of this invention to reduce metal halates to 
metal halides in solutions which are strongly basic. It is another object 
of this invention to reduce the amount of corrosive bromate produced from 
the reaction of bromine and metal hydroxide or metal carbonate in vent 
streams by adding a reducing agent to a scrubber solution. It is a further 
object of this invention to recover substantially pure metal halide or 
metal halide solution from such scrubber solutions in order to conserve 
valuable raw materials. 
THE INVENTION 
Thus, this invention relates to a process for reducing metal halates to 
metal halides which comprises reacting a metal halate with a reducing 
agent in an aqueous alkaline reaction medium having a pH above about 7 
wherein the amount of reducing agent is sufficient to reduce substantially 
all of the metal halate to metal halide. 
This invention also relates to a process for reducing the amount of 
residual halate in an scrubber system and recovering metal halide, the 
process comprising reacting halogen with an aqueous alkaline scrubbing 
solution and adding, either periodically or continuously, a reducing agent 
wherein the amount of reducing agent is sufficient to convert 
substantially all of the halate to halide and subsequently recovering the 
metal halide product. For the purposes of this invention, the term 
"substantially all" means that from about 0 wt. % to about 0.5 wt. % 
halate based on the total weight of the aqueous alkaline reaction medium 
plus halate remains unreacted. Thus, in both embodiments, a minor amount 
of halate is left unreacted so as to insure the complete conversion of the 
reducing agent. In another embodiment, the minor amount of halate 
remaining in the aqueous alkaline reaction medium is reacted with 
sufficient additional reducing agent, preferably hydrazine, in order to 
convert the minor amount of halate to halide. 
Without being bound by theory, it is believed that the reaction of halogen 
(X) in nitrogen rich vent streams, utilizing a strong alkaline scrubber 
solution, and methanol as a reducing agent may be illustrated as follows. 
During scrubbing of a nitrogen vent stream: 
EQU 3X.sub.2 +6MOH(aq).fwdarw.+5MX(aq)+MXO.sub.3 (aq)+3H.sub.2 O(I). 
Reduction of halate in a scrubber solution: 
EQU MXO.sub.3 (aq)+CH.sub.3 OH+MOH.fwdarw.MX(aq)+MCHO.sub.3 (aq)+2H.sub.2 
O(II). 
In reaction II less than the stoichiometric amount of methanol is normally 
used. Sufficient methanol is added periodically or continuously to 
eliminate essentially all of the residual halate according to Reaction II, 
and then HX is added to release CO.sub.2 and to recover additional MX. 
Removal of residual halate: 
EQU MHCO.sub.3 +HX.fwdarw.MX+CO.sub.2 +H.sub.2 O (III). 
wherein M is an alkaline or alkaline earth metal. The limitation of 
residual halate in process streams aids in the inhibition of corrosion of 
steel process equipment. 
The metal halates referred to in this invention include NaBrO.sub.3, 
NaClO.sub.3, NaIO.sub.3, MgClO.sub.3, MgBrO.sub.3, MgIO.sub.3, KBrO.sub.3, 
KClO.sub.3, KIO.sub.3, and the like. Preferably the metal halate is an 
alkali metal halate and most preferably NaBrO.sub.3. One way that these 
halates may be formed is in situ in a scrubber system; however, halates 
may be formed in any alkaline process stream which contains a halogen and 
an alkaline reaction medium. In the context of a scrubber system, halate 
concentration is maintained within the range of from 0 wt. % to about 2 
wt. %, preferably from about 0.05 wt. % to about 0.5 wt. %, and most 
preferably from about 0.1 wt. % to about 0.3 wt. % by reducing a portion 
of the halate after it is formed. 
The halogens referred to in this invention include Br.sub.2, Cl.sub.2, and 
I.sub.2. Likewise, the halides referred to in this invention include 
chloride, bromide, and iodide and the halates include chlorate, bromate, 
and iodate. Bromine, bromide, and bromate are the preferred forms. 
The reducing agents which can be employed in this invention include 
alkanols of from 1-8 carbon atoms, H.sub.2 O.sub.2, hydrazine, hydrazine 
hydrate, urea, hydrogen, formaldehyde, carbon, sulfur dioxide, and 
(C.sub.1 -C.sub.5)polyhydroxyalcohols, such as ethylene glycol. The 
preferred reducing agent is methanol due to its low cost and ease of 
handling. When used in a scrubber system, the reducing agent is typically 
added periodically to the aqueous alkaline medium in an amount less than 
the stoichiometric amount required to reduce all of the halate formed 
during the scrubbing operations. During the addition of the reducing 
agent, it may be desirable to divert the vent streams from the scrubber 
system so as to ensure that the reducing agent is reacting with the halate 
in the scrubbing solution and not reacting with the incoming halogen in 
the vent stream and also to prevent loss of the reducing agent. 
Alternatively, part of the soluble solution may be isolated for reaction 
with reducing agent. When periodically added to the scrubbing solution, 
the reducing agent is preferably added all at once, but it may be added 
incrementally over a period of time such that the heat of reaction is 
controlled. When methanol is used as a reducing agent, the amount of 
methanol is preferably within the range of from about 0.5 mole to about 
2.0 moles per mole of halate, most preferably 0.95 to 1.05 moles per mole 
of halate. 
The aqueous alkaline reaction medium can be any of the alkaline or 
alkaline-earth metal hydroxides and carbonates, such as NaOH, Na.sub.2 
CO.sub.3, Mg(OH).sub.2, MgCO.sub.3, Ca(OH).sub.2, CaCO.sub.3, KOH, K.sub.2 
CO.sub.3 and the like. The preferred reaction medium is aqueous NaOH. In 
the context of a scrubber system, from about 15 wt. % to about 30 wt. % 
NaOH solution is charged to the scrubber system to provide a sufficient 
scrubbing medium. 
The pH range of the aqueous alkaline reaction medium is a critical feature 
of this invention. The pH must be maintained above 7 in order to insure 
that free halogen is not released from the scrubbing medium. Preferably, 
the pH is maintained within the range of from about 9.0 to about 14.0; 
most preferably within the range of from about 10.0 to about 13.5. 
However, corrosive halate is formed as a by-product of the reaction of a 
base, such as NaOH, with residual halogen which may be present in the 
process streams. According to the present invention, a reducing agent such 
as methanol may be added under strongly basic reaction conditions in order 
to reduce substantially all of the halate to halide, thus essentially 
eliminating the formation of free halogen. 
The temperature and pressure of the reaction are not critical. Generally, 
the temperatures may range from about 0.degree. C. to about 90.degree. C., 
with temperatures within the range of from 15.degree. C. to 60.degree. C. 
being preferred. Likewise, the pressures employed during the reaction may 
vary widely. Suitably, these pressures may range from about 0.07 KPa to 
about 10 KPa or more, with pressures of from about 1 KPa to about 2 KPa 
being most preferred. 
For the purposes of this invention, the term scrubber system is used to 
identify any means of gas absorption whereby one or more of the 
constituents of a gas mixture, in this case nitrogen and minor amounts of 
free halogen, may be dissolved or absorbed in a circulating liquid 
scrubbing medium, in this case an aqueous alkaline reaction medium 
comprising either alkaline or alkaline-earth metal hydroxides or 
carbonates, for the purposes of gas purification and product recovery. The 
gas absorption is typically carried out in a vertical countercurrent 
column wherein the aqueous alkaline reaction medium is fed into the top of 
the column and the gas mixture is introduced into the bottom of the 
column. As the gas mixture travels upward through the column, it contacts 
and reacts with the downward travelling aqueous alkaline reaction medium. 
The reaction medium collects in the lower part of the column or in a 
separate circulation tank where it can be recirculated to the top of the 
column. 
The column may be packed, plate or a simple spray column. Packed columns 
generally comprise a shell filled with a packing material designed to 
disperse the liquid an bring it into close contact with the rising gas 
mixture. Solubility of the absorbed gas and rate of mass transfer as well 
as many practical details must be considered during design and 
construction of such scrubber columns. Alternately, a contactor (packed 
column, Venturi scrubber, or in-line scrubber, or combinations of the 
same) with co-current flow, can be used. Those skilled in the art are 
familiar with design and construction techniques of scrubbers and scrubber 
systems. 
For the purpose of this invention, the term process stream is used to 
identify any liquid or gaseous stream propelled through equipment lines 
used to transport reagents or products in industrial scale reactions. 
A preferred use of this invention is in the reduction of the amount of 
bromate in aqueous solutions to inhibit the corrosion of process lines and 
storage vessels, with the added benefit of recovering saleable NaBr or 
NaBr solution. In practice, a scrubber solution comprising metal hydroxide 
or metal carbonate is available for scrubbing bromine from vent streams 
and process streams. However, bromate concentration increases during the 
course of this reaction. Occasionally, process vent streams containing 
bromine may be diverted and a small amount of reducing agent may be added. 
Thus, bromate concentration is maintained at a substantially low level. 
Corrosion of process lines and storage vessels is dramatically decreased. 
There is essentially no loss of reducing agent in vent streams from the 
scrubber system, nor is there substantial formation of methyl bromide. 
The reaction of the preferred embodiment can be illustrated as follows: 
EQU NaBrO.sub.3 +CH.sub.3 OH+NaOH.fwdarw.NaBr+NaHCO.sub.3 +2 H.sub.2 O 
In the applications where it is advantageous to recover the spent scrubber 
solution, additional processing might be required, depending on the 
reducing agent and product quality. For example, HBr could be added to the 
scrubber solution, provided that the pH of the solution is not allowed to 
fall below about 7.0, in order to convert NaHCO.sub.3 to NaBr and 
CO.sub.2, when methanol is the reducing agent. The HBr also converts any 
residual NaOH to NaBr. 
The following examples illustrate a process for converting bromine and 
sodium bromate in a scrubber system to sodium bromide at a pH within the 
range of from about 10.0 to about 13.5.

EXAMPLE I 
A 2000 gal. volume of 25 wt. % caustic solution is charged to a scrubber 
system and process streams containing bromine are vented through the 
scrubber and circulating scrubber solution at ambient temperature and 
pressure. Initially, the amount of bromate in the scrubber solution 
increases to about 0.6 wt. % at 23.5 wt. % caustic and about 2.5 wt. % 
sodium bromide is formed. At this point, the vent stream is diverted and 
about 2.8 pounds of methanol (0.1 mole methanol per mole bromate) is added 
to the scrubber solution in one minute. Thus, sufficient methanol is added 
to convert part of the bromate to bromide, while maintaining a residual 
bromate content at about 0.5 wt. %. With resumed scrubbing, the bromate 
content again increases to 0.6 wt. %. Then scrubbing is interrupted for 
another methanol addition. With extended use, the caustic content of the 
system reduces to about 1 wt. % and the sodium bromide content increases 
to &gt;32 wt. %. The small amount of bromate is maintained in the system to 
assure complete conversion of the methanol in the desired reaction. If 
sodium bromide is to be recovered when the scrubber solution is spent, 
additional methanol can be added to reduce the bromate content to about 
0.05 wt. % or less. Then, HBr is added to convert NaHCO and residual 
caustic to sodium bromide at a pH of about 7.0. The final solution 
contains about 43 wt. % NaBr. 
EXAMPLE II 
A 2000 gal. volume of 25 wt. % caustic solution is charged to a scrubber 
system and process streams containing bromine are vented through the 
scrubber and circulating scrubber solution at ambient temperature and 
pressure. Initially, the amount of bromate in the scrubber solution 
increases to about 1.6 wt. % at 21 wt. % caustic and about 5.4 wt. % 
sodium bromide is formed. At this point, the vent stream is diverted and 
about 50 pounds of methanol is added to the scrubber solution in ten 
minutes. Thus, sufficient methanol is added to convert part of the bromate 
to bromide, while maintaining a residual bromate content at about 0.5 wt. 
%. With resumed scrubbing, the bromate content again increases to 1.6 wt. 
%. Then scrubbing is interrupted for another methanol addition. With 
extended use, the caustic content of the system reduces to about 1 wt. %. 
Recovery of NaBr follows procedures as used in Example 1. 
The following examples illustrate another feature of this invention. 
EXAMPLE III 
To a strongly basic solution containing 0.49 wt. % sodium bromate, 2.29 wt. 
% sodium bromide and about 13 wt. % NaOH was added 6.23 g of hydrazine 
monohydrate solution (0.82 wt. % hydrazine, confirmed by HPLC). The amount 
of nitrogen formed was quantitatively measured as 28.45 g. Without being 
bound by theory, it is believed that the reduction of sodium bromate to 
bromide (with nitrogen gas evolution) may be illustrated as follows: 
EQU 2 NaBrO.sub.3 +3 N.sub.2 H.sub.4 .fwdarw.3 N.sub.2 +2 NaBr+6 H.sub.2 O 
The hydrazine content of the reagent solution was 115 wt. % of the 
theoretical requirement to reduce the sodium bromate to sodium bromide 
(with nitrogen gas evolution) according to the above reaction. The 
reaction proceeded to completion within 30 seconds. The volume of gas 
evolved was 43.82 mL and was determined to be nitrogen with traces of 
water vapor by GC/MS. The experimental volume of gas was 112% of the 
expected theoretical value of 39.28 mL. The bromate present after reaction 
was measured to be 63 ppm (a 98.4% conversion). The bromide content 
increased from 2.29 wt. % as sodium bromide before reaction to 2.78 wt. % 
after reaction. No hydrazine was found in the sample after the reaction. 
EXAMPLE IV 
To 120 g of a field sample containing 15.5% NaOH, 10.2% NaBr, and 1.5% 
NaBrO.sub.3 was added 4.75 g methanol at ambient temperature. The sample 
color changed from yellowish to water-white in about 5 minutes at ambient 
temperature. The reacted sample was analyzed, and the result was 34 ppm 
NaBrO.sub.3.