snip
This reaction is favored at temperatures above 30C and at a pH below 7.
Under alkaline conditions and temperatures below 30C the hypochlorate will
remain unreacted. A typical chlorate cell is shown in Fig. 1.
Chlorate cells are operated at such a low temperature that not all of the
heat produced can be carried off by evaporation of water from the
electrolyte. Therefore, cooling coils are customarily employed. These coils
are usually of steel, located within the cell, and are bonded to the cathode
to provide protection against corrosion. External cooling of circulated
electrolyte may also be used. Owing to the decrease of overvoltage at
higher temperatures, and its effect on heat generation, the thermal
conditions are inherently stable and cooling water may be controlled by a
setting on a hand operated valve. The difference between the cell voltage
and the theoretical decomposition voltage (2.3 volts) represents the heat
which must be removed from the cell by cooling methods.
Electrical supply and arrangement of chlorate cell systems are similar to
those used in chlorine plants. Cells are usually connected in one or more
electrical series operating at up to 600 volts, but the electrolyte and
cooling water flows are in parallel.
The cell covers are provided with a gas duct system to vent the byproduct
gas. Cell gas, principally hydrogen contaminated with oxygen and chlorine to
the degree that it is usually not recovered, is vented to the atmosphere. In
some systems air is drawn in over the vapor space and mixed with the
hydrogen to drop its concentration below the explosive limit. Cells may
also be sealed and operated with a high hydrogen content in the vent gas.
Some facilities employ cells having bipolar electrodes which are arranged
to divide a long narrow cell container into a large number of parallel
chambers, each being an individual cell. One side of each electrode is
anodic with the opposite side being the cathode in the next chamber. Current
is introduced into the first electrode of the battery and leaves from the
last. This arrangement allows a very simple electrical bus system and a
compact arrangement of the cells.
No outstanding advantages can be attributed to any particular cell
design. The operating success of a plant is usually a function of the
chlorate plant in its entirety. Whereas past practices have involved cells
containing a large volume of dead space to allow retention time for the
conversion of hypochlorite to chlorate and various cascade systems to allow
for liquor flow through groups of cells in series, present practice tends
toward large cells having no dead space and simple parallel liquor flow
through individual cells. Advantages of this trend arise from the greater
electrolyzing capacity per unit area of floor space and from simplified
operation of the cells. These changes in design have not resulted in any
apparent loss in current efficiency.
Chlorate cells are not particularly sensitive to variations in operating
conditions and will produce satisfactorily within a wide range for each
variable. Thus, it is customary for each facility to operate it's cells in
such a manner as to obtain maximum fiscal economy, taking all plant expenses
into consideration. Therefore, it is not possible to set forth exact
operating data for chlorate cells and the following table merely contains a
representative range of characteristics based on the use of graphite anodes.
As in the case of chlorine cells, chlorate cells must be dismantled at
the end of the useful life of the anodes for cleaning and anode replacement.
The absence, of a diaphragm in the chlorate cell makes this repair
relatively simple.
It is customary to add several grams per liter of sodium chromate to the
electrolyte to assist in maintaining pH on the acid side and to reduce
corrosion effects on the metallic portions of the cell (not usable with
lead dioxide anodes). In the event that either the cell container or the
cooling coils are of metal, they are bonded to the cathode to provide
cathodic protection against corrosion. Fig. 2 is a basic flowsheet for
sodium chlorate production.
snip
loss of chlorine from the electrolyte. Therefore, the stream being returned
to the cells from the rundown tank is acidified with chlorine or
hydrochloric acid using automatic control based on pH.
The feed to the crystallization system contains hypochlorite ion which
must be removed by heating, air blowing, and/or acid or thiosulfate
treatment. The feed is also made slightly alkaline with caustic soda
following hypochlorite removal. Failure to remove hypochlorite and acid will
cause serious corrosion problems in the evaporator. The liquor is
subsequently filtered to remove anode mud and other solids. If required by
the concentration of chlorate, the purified liquor may then be evaporated in
continuous, multi-effect evaporators until chlorate just starts to
precipitate. The liquor is then transferred to either an evaporative or a
surface cooled crystallizer where crystallisation takes place. In some
systems "salting out" of chlorate with sodium chloride is employed.
Crystallizers may operate at temperatures as low as -1OC. The slurry
from the crystallizer is continuously centrifuged to remove chlorate
crystals, with the mother liquor being returned to the rundown tank.
SNIP
Chlorates are reasonably stable but, since they are very powerful
oxidizing agents, have a tendency to react strongly with reducing agents.
Accordingly, great care must be taken that all equipment and plant
facilities are kept clear of wood, oil, combustible organic materials,
sulfur, ammonium salts, dust, and easily oxidizable metals such as aluminum
and magnesium. Moving equipment must either be run dry or lubricated with
water or fluorinated lubricants. Pump packings must be of the noncombustible
type. Chlorate solutions are especially dangerous with respect to materials
that are both absorbent and oxidizable. Rubber and plastics, although
organic in nature, may be relatively safe for certain applications provided
that they do not become impregnated with solution, that they do not exude
organic materials such as oils and plasticizers, and that their temperature
is kept well below the ignition point. Good safety practice requires that
employees in chlorate facilities wear a complete change of clean clothing
every day and that it be washed after each shift. Clothing should also be
changed immediately in the event that it is splashed with chlorate solution
since, when dry, chlorate-saturated fabric becomes violently combustible.
Rubber shoes should also be worn since leather easily impregnates with
chlorate solutions and becomes hazardous. Chlorate solution will easily
creep into small cracks and other interstices in equipment and then form
crystals. Upon crystallization, expansion occurs which may cause serious
leaks. Chlorate solutions will also creep and deposit crystals in cell
vents. Therefore, daily washdowns of plant facilities are advisable.
Sodium chlorate is employed principally for pulp bleaching (reactant for
chlorine dioxide generation), as a herbicide, and as an intermediate for
the production of ammonium perchlorate, the oxidizer for most solid rocket
propellants. Potassium chlorate is used in matches, flares, and pyrotechnic
devices.
SNIP