Preparation of Iodized Salt FOR Goitre Prophylaxis: M. A.M.Inst - Chem.E
Preparation of Iodized Salt FOR Goitre Prophylaxis: M. A.M.Inst - Chem.E
Preparation of Iodized Salt FOR Goitre Prophylaxis: M. A.M.Inst - Chem.E
SYNOPSIS
The methods employed for iodizing free-running salts are
discussed. They are not suitable for the iodization of coarse crystal-
line salts and a new process has been devised by the Chilean Iodine
Educational Bureau of London for the iodization of open-pan and
solar evaporated salts. This process is described and illustrated
by photographs of suitable plants. Attention is drawn to the
advantages of potassium iodate as an iodizing agent.
Spraying process
When the salt leaves the vacuum evaporators it is discharged into a
rotating filter and then conveyed to a final drier for the complete removal
of moisture.' Satisfactory and uniform iodization of the salt is achieved
by spraying a solution of potassium iodide on to the salt as it passes along
a conveyor belt prior to its entry into the final hot-air drier. The simplicity
of the operation is demonstrated in fig. 1, which shows salt from the vacuum
evaporators passing on the conveyor belt under a series of spray nozzles
through which the iodide solution is delivered in the desired quantity. In
the final drier the salt is stirred by paddles attached to a horizontal shaft,
and by this operation uniform iodization of the salt is ensured. When
iodized salt is to be produced in large quantities, the spraying process is
decidedly more economic than the dry-mixing process. It also permits
of more easy changes in the level of iodization to accord with the varying
requirements of authorities in different parts of the world.2
PREPARATION OF IODIZED SALT FOR GOITRE PROPHYLAXIS 233
FIG. 1. IODIZATION OF FREE-RUNNING SALT BY SPRAYING WITH POTASSIUM
IODIDE SOLUTION
Dry-mixing process
The general method is to prepare a stock mixture of ground potassium
iodide and a drying agent such as is usually included in free-running salts,
and to add the required quantities of this mixture to batches of salt. Since
the amount of drying agent used is considerably greater than the quantity
of iodide, the above procedure results in a very satisfactory uniform iodized
product.
The quantity of iodide added to the salt will of course vary from country
to country according to the recommendations of the medical authorities, but
the following outline of a recognized method of manufacture in a goitrous
area of the USA will serve to indicate the necessary process operations: 3
5
234 J. C. M. HOLMAN
97.75 pounds (44.3 kg) of calcium phosphate and 2.2 pounds (1.0 kg)
of potassium iodide are mixed together and ground to pass a 100-mesh
sieve. One pound (0.45 kg) of this mixture is added to 99 pounds (44.9 kg)
of dried salt which is then mixed for sufficient time to give a uniform
product as determined by analysis. As an added assurance of obtaining
a uniform final product, some manufacturers prefer to incorporate the
iodide-phosphate mixture in two stages by first thoroughly mixing it with
about 25 % of the salt and then adding the remainder of the salt.
The dry-mixing process is very suitable for preparing relatively small
stocks of iodized salt.
BULK CONTROLLER
+ \i | ~~~COMPONDl
40 ~~~~~~~~~~~PECISION FEDE
SALT~~~~~~~~~~~~~~~~~~~SL
H1 1 ~~~~~~~~MIXINGCONVEYOII
IODIZED|
The salt and the iodide or iodate mixture are intimately combined
during their passage along the remaining length of the worm-screw mixing
conveyor, and analyses have shown that the final product is satisfactory in
every respect. The iodized salt is collected directly into bags or other
containers at the end of the conveyor and the whole operation of the plant
requires the minimum of skilled labour.
The worm-screw of the conveyor and the paddle at the base of the bulk
controller are driven by single belts from a shaft attached to a small electric
motor or internal combustion engine and the rate of throughput of salt
can be adjusted in a few minutes by manipulating a simple set screw.
The precision feeder is geared to a 0.25-H.P. electric motor or can be
operated manually. It can be set at any desired rate by varying the gear-
wheels, and five sets of wheels are supplied with the feeder for this purpose.
The casings of the bulk controller and the mixing conveyor are made
of hard wood but the worm-screw paddle and blades are fabricated from
ordinary steel.
Fig. 3 and 4 show the plant now in use in Guatemala. The maximum
limiting capacity of this plant is about 5 tons of iodized salt per day. Where
greater quantities of iodized salt are required, a plant of similar design but
of larger size and capable of producing up to 25 tons per day can be supplied.
A second plant which operates on the same principles but differs in
layout has been constructed for use in the Union of South Africa. At
the request of the authorities this has been assembled on a light metal
frame in order that it can be lifted by porters and also carried on a lorry
without being dismantled. A view of this plant is provided in fig. 5.
236 J. C. M. HOLMAN
The bulk controller (left) and the precision feeder (centre) control respectively the hroughput
of salt and the level of iodization.
Reproduced by courtesy of Novadel Limited, London
PREPARATION OF IODIZED SALT FOR GOITRE PROPHYLAXIS 237
RItSUM1t
L'efficacite de l'iodisation d6pend en grande partie des methodes de production du
sel A partir des saumures ou de tout autre materiel brut.
Le sel produit dans les pays les plus developpes du monde, par evaporation
continue, dans le vide, est generalement iodise par pulverisation d'un compose iode, en
solution aqueuse, avant dessiccation finale par l'air chaud. Lorsqu'il s'agit de petites
quantites, on peut ajouter l'iodure de potassium, accompagne d'un deshydratant, sous
forme de poudre seche; ce procede est toutefois moins economique. L'iodisation du
sel cristallis6 brut produit par chauffage des saumures A l'air libre peut se faire par
addition de petites quantites d'iodure de potassium, tout au long du processus.
Aucune de ces m6thodes ne convient a l'iodisation des sels grossi6rement cristallis6s,
utilises dans les regions d'end6mie goitreuse de divers pays insuffisamment developpes.
A la demande de l'OMS, le Chilean Iodine Educational Bureau, A Londres, a elabore un
proc6de d'iodisation des sels grossiers. On a cherche A adapter A l'iodisation des sels
prepares par evaporation au soleil la methode par melange A sec. Le sel cristallise brut
passe A travers une tr6mie, A raison de 254 kg par heure, puis dans une vis sans fin,
m6langeuse et transporteuse. A sa sortie de la tr6mie, il reroit un m6lange exactement
dose d'une partie d'iodure ou d'iodate de potassium pulv6rulent et de dix parties de
carbonate de calcium, de fa9on que le sel contienne finalement la quantite d'iod?
prescrite (fig. 2). L'iodate est A conseiller lorsque les conditions de conservation ris-
quent de provoquer des pertes d'iode. Apres passage dans la vis, le sel est immediate-
ment mis en sacs ou en recipients appropries. Ce proc6de n'exige qu'un minimum de
personnel qualifie. Le dispositif est mu par un moteur electrique ou A combustion
intemne. Le distributeur de precision de l'iodure est relie A un moteur electrique d&
0,25 CV; il peut aussi etre actionne A la main; le debit est aisement r6glable. Le chassis
de l'appareil est en bois et le filet de la vis en acier ordinaire.
b See paper by Margaret M. Murray on page 211.
PREPARATION OF IODIZED SALT FOR GOITRE PROPHYLAXIS 239
REFERENCES
1. Badger, W. L. & Baker, E. M. (1941) Inorganic chemical technology, New York
2. Chilean Iodine Educational Bureau (1946) World goitre survey, London
3. Geagley, W. C. (1929) Amer. J. publ. Hlth, 19, 991
4. Hart, E. B. & Griem, W. B. (1939) Stabilization of iodine in salt and other feed
materials, US Pat. No. 2,144,150
5. Johnson, F. F. & Frederick, E. R. (1940) Science, 92, 315
6. Murray, M. M. & Pochin, E. E. (1951) J. Physiol., Lond. 114, 6P
7. O'Dell, A. E. (1952) Manufacture of salt, Brit. Pat. No. 682,709