GB2231338A - Electrolytic conversion of bromide to bromate - Google Patents

Abstract

Bromide is converted electrolytically to bromate in the presence of one or more salts chosen from any of vanadate salts, tungstate salts, molybdate salts, manganate salts, niobate salts and tantalate salts. Alkali metal vanadate salts are preferred. The salts are used as substitutes from chromate depolarising agents which are carcinogens.

Description

TITLE Electrolytic conversion of Bromide to Bromate The present invention relates to the electrolytic conversion of bromide to bromate in aqueous solution with improved electrochemical efficiency.

The bromates of commerce are usually sodium bromate and potassium bromate, the former produced by electrolysis of sodium bromide solution, the latter either by double decomposition of sodium bromate solution with e.g.

potassium hydroxide solution or by electrolysis of potassium bromide solution. For many years chromate has been used as a "depolariser" in the electrolysis of bromide to bromate in aqueous solution. Without chromate, the electrochemical efficiency is only about 50%, the amount of chromate required for high electrochemical efficiency depending on the mass transfer characteristics of the electrolysis system. However, it is now known that chromate (CrO4--) and dichromate (Cr2o7--) are potential carcinogens and their use should be avoided where possible (CrO4 and Cr2o7 are interchangeable in aqueous solution by the addition of acid and alkali).

As is generally known to those involved in the process, electrolysis of bromide to bromate may be carried out in aqueous solution using either a static tank cell or a narrow spaced pumped cell with a platinum or platinised anode(s) and stainless steel or copper cathode(s) at a current density of a few thousand amps per sq. metre. The solution should normally be slightly alkaline and maintained at a temperature which minimises the electrical resistance of the solution without allowing excessive evaporation or electrode corrosion to occur.

Experiments have been carried out to examine the depolarising action of salts other than chromate in the electrolysis of bromide solutions, involving the use of a laboratory scale pumped cell with platinum anode and stainless steel cathode.

Initial solution com,oosition : 15.0%w/w NaBrO3 + 13.5%w/w Naer plus depolariser if any, at pH 9.5.

Charge weight : lOOOg 2 Current : 20 Amps.(2000A/m) Duration of electrolysis : 5 hours Temperature of electrolyte : 60 - 650C Theoretical linear flow of electrolyte across anode : 4cm/sec (but see below) At the end of five hours electrolysis, the electrolyte solution was recovered for weighing and analysis for calculation of overall electrochemical efficiency:- Depolariser Overall e.c.e.

None 500ppm w/w NTa2Cr207.2H20 100% 500ppm w/w Na2MoO4. 2H20 48% 500ppm w/w NaF 50% 500ppm w/w Na2B407.10H20 41% 500ppm w/w KMnO4 87% (But precipitation of MnO2) 500ppm w/w NaO4.2H2O 74% 500ppm w/w Na3VO4.14H2O 92% 50ppm w/w Na3VO4. 14H20 59% lOOOppm w/w Na3VO4.14H20 93% 500ppm w/w NaVO3 95% *500ppm w/w NaVO3 98% *lOOppm w/w NaVO3 89% *Theoretical linear flow of electrolyte across anode 25cm/sec.

It is known that vanadate and chromate have a similar effect on the ratio of hydrogen to sodium amalgam produced in the mercury chlor-alkali process but examination of the electrolytic conversion of bromide to bromate using substitutes for chromate does not appear to have been suggested.

According to the present invention there is provided an electrolytic process for converting bromide to bromate from aqueous bromide/bromate solutions in the presence of one or more salts chosen from any of vanadate salts, tungstate salts, molybdate salts, manganate salts, niobate salts and tantalate salts. Generally the salts are alkali metal salts, e.g. sodium or potassium salts.

Preferably vanadate salts are used, chosen from metavanadatesv

orthovanadates

pyrovanadates

and polyvanadates and hydrates thereof. The total amount of the vanadate salt(s) may range from lOppm w/w to 5% w/w of the vanadate ion, preferably in the range 50ppm w/w to 5000ppm w/w, based on the bromate/bromide solution to be electrolysed.

Preferably the electrolysis is carried out in a pumped cell wherein the theoretical linear flow rate across the anode is in a range lcm/sec to 100cm/sec.

EXAMPLES The comparative example and Examples 1 and 2 which illustrate the invention all used a single pumped cell of electrode area 10cm x 10 cm, electrode spacing 4mm with stainless steel cathode and Pt/Ir oxide anode. Electrolyte was circulated from and to a 700ml glass reservoir, electrically heated, using a PTFE diaphragm pump.

COMPARATIVE EXAMPLE No Depolariser Initial charge 1000g : 14.86% NaBrO3, 13.77% NaBr, pH 9.1 Flow Rate 1 litre/min (equivalent to a linear flow across the electrode faces of 4.2cm/sec) Electrolyte circulation was started and heat applied to the reservoir until solution temperature reached 600C.20A current was then applied, initial voltage 2.85V. Heat input was reduced to maintain temperature at 60-650C. Current was turned off after exactly 5 hours, final voltage 3.n3V. The solution was recovered for weighing and analysis, the pump, tubing and reservoir being rinsed out with distilled water, the rinsings being combined with the bulk of the solution.

Total solution recovered : 1025g : 18.96% NaBrO3, 10.31% NaBr.

Therefore : Sodium bromate in 148.6g II " out 194.35g gain 45.75g.

Theoretical NaBrO3 production at 20A = 18.76g/hour.

Therefore theoretical gain over 5 hours = 93.8g.

Therefore apparent current efficiency = 45.75 x 100% = 48.8%.

93.8 EXAMPLE 1 500ppm Na,V04.14H,O Initial charge 1000g : 15.12% NaBrO3, 14.3% NaBr plus 0.5gNa3VO4.14H20 pH 9.6 Flow rate 1 litre/min.

Procedure as in Comparative Example. Initial voltage was ca. 2.5V rising to 2.91V after 5 hours. A 2.1g sample of solution was taken for analysis after 2h30m and contained 19.17% NaBrO3.

Total solution recovered: 1047g : 22.65% NaBrO3, 7.95% NaBr plus sample.

Therefore : Sodium bromate in 151.2g out 237.55g gain 86.35g.

Therefore apparent current efficiency = 86.35 x 100% = 92.1%.

93.8 EXAMPLE 2 500ppm NaVO : Higher flow rate Initial charge 1000g : 14.20% NaBrO3, 16.5% NaBr plus 0.5g NaVO3 pH 9.5 Flow rate 6 litres/min (equivalent to a linear flow across the electrode faces of ca.25cm/sec) Procedure as previously. Initial voltage ca.2.5V, Final voltage 2.82V.

A 4.5g sample was taken for analysis after 2h 30m and contained 18.74% NaBrO3.

Total solution recovered : 1059.5g : 22.02% NaBrO3 plus sample.

Therefore : Sodium bromate in 141.95g " " out 234.15g gain 92.2g.

Therefore apparent current efficiency = 92.2 x 100% = 98.3%.

93.8

Claims (9)

1. CLAIMS 1. An electrolytic process for converting bromide to bromate from aqueous bromide/,bromate solutions in the presence of one or more salts chosen from any of vanadate salts, tungstate salts, molybdate salts, manganate salts niobate salts and tantalate salts.
2. 2. A process according to Claim 1, wherein the salts are alkali metal salts.
3. 3. A process according to Claim 2, wherein the alkali metal salts are sodium or potassium salts.
4. 4. A process according to any preceding claim, wherein the salts are vanadate salts.
5. 5. A process according to Claim 4, wherein the vanadate salts are chosen from metavanadates

   orthovanadates

   pyrovanadates

   and polyvanadates and hydrates thereof.
6. 6. A process according to Claim 4 or Claim 5, wherein the total amount of the vanadate salt(s) ranges from lOppm w/w to 5% w/w of the vanadate ion based on the bromate/bromide solution to be electrolysed.
7. 7. A process according to Claim 6, wherein.the total amount of the vanadate salt(s) ranges from 50ppm w/w to 5000ppm w/w based on the bromate/bromide solution to be electrolysed.
8. 8. A process according to any preceding claim, wherein the electrolysis is carried out in a pumped cell in which the theoretical linear flow rate across the anode is in a range 1cm/sec. to 100cm/sec.
9. 9. A process according to Claim 1 substantially as herein described and exemplified.