ELECTRODIALYTIC PURIFICATION OF CALCIUM GLUCONATE
227
Table 3. Effect of the current density on the electrodialysis of a calcium gluconate solution. Linear flow velocity of the calcium
gluconate solution, 3.0 cm s–1; calcium gluconate concentration 218 g l–1; sodium bromide concentration 15.2 g l–1; temperature
20–30°C
Specific
Current
density, A
dm–2
Current efficiency by
calcium gluconate,
%
Yield of calcium
gluconate, %
Degree of
desalination, %
Energy consumption,
W h kg–1
throughput,
kg m–2 h–1
0.45
0.15
0.25
0.50
1.00
1.50
55.0
51.5
50.3
46.2
35.0
60.0
53.6
42.4
39.8
30.0
91.5
91.9
91.7
86.2
85.2
75.0
126.0
340.0
716.0
800.0
0.50
0.76
1.40
1.95
to electrodialytic desalination to remove sodium
bromide, was a suspension or a solution. The results of
desalination of the calcium gluconate suspension are
presented in Table 2.
electrodialysis of calcium gluconate solutions, instead
of suspensions. The results obtained in purification of
calcium gluconate solutions to remove sodium bromide
are presented in Tables 3 and 4.
It can be seen in Table 2 that sodium bromide can
be removed from the calcium gluconate suspension by
electrodialysis, with a 32.6–41.5% yield of the target
product, depending on the process conditions. An
increase in the yield of calcium gluconate is favored
by lowering the current density and the degree of
desalination. It was found that at degrees of desalination
of 85.0–90.0%, the sodium bromide concentration in
the mother liquor is 1.1–1.4 g l–1 and calcium gluconate
isolated from solution meets the requirements of FS
no. 121 in the content of bromides [4].
It can be seen in Tables 3 and 4 that the yield of
calcium gluconate and the current efficiency by this
compound noticeably decrease with increasing current
density. In addition, this also makes markedly higher
the power consumption by the process. The yield of
the target product also falls with increasing degree of
desalination of the calcium gluconate solution.
The study demonstrated that, as in the case of
a suspension, desalination of the calcium gluconate
solution should be performed to a residual concentration
of sodium bromide not exceeding 1.4 g l–1. At higher
salt concentrations in solution, calcium gluconate fails
to satisfy the requirements of FS no. 121 [4] in the
content of bromides.
In electrodialytic desalination, not only sodium
bromide, but also calcium gluconate pass into the
concentrate, with the loss of calcium gluconate being
13.5–17.0%. However, the whole amount of the
concentrate can be used to prepare the electrolyte in the
stage of electrochemical oxidation of glucose, because
the concentration of sodium bromide is 11.0–14.0 g l–1
and, when necessary, the electrolyte can be adjusted.
Asaresultofthestudy,theconditionsofelectrodialytic
desalination of calcium gluconate solutions containing
218–220 g l–1 of calcium gluconate and 15.2 g l–1 of
sodium bromide were found: current density 0.25–
0.50 A dm–2; temperature 20–35°C; linear flow velocity
of the calcium gluconate solution, 3.0 cm s–1; and degree
of desalination, 91.9–92.0%. Under these conditions, the
yield of calcium gluconate is 55.0–57.5% against 42.0%
for the existing technique, the energy consumption by
the process was 126–340 W h kg–1, and the specific
throughput of the electrodialyzer is 0.5–0.7 kg m–2 h–1.
It was found in electrodialytic desalination of the
calciumgluconatesuspensionthattheprocessisunstable,
which is manifested in the unsatisfactory reproducibility
of the results. The reason is that using the suspension
leads to mechanical loss of the product (10.0–20.0%)
in the purification chambers of the electrodialyzer. In
addition, the solid phase of calcium gluconate hinders
transfer of ions across the membranes and circulation of
the suspension through the electrodialyzer.
As follows from the data presented in Tables 1 and 4,
the sodium bromide concentration in the mother liquor
with washing water, obtained upon electrodialytic
desalination of calcium gluconate solutions, is
These shortcomings were eliminated by using
RUSSIAN JOURNAL OF APPLIED CHEMISTRY Vol. 85 No. 2 2012