It
is undoubted that the modern technology has been improving better and stronger in
recent years, however, people are gradually starting to ignore the impact of little
things though that contribute a lot to the social economic growth and the human
development. In term of “little things”, one of the species called desiccants is
significantly noticeable in everyday life but is easily despised though due to
their size and price. As a matter of fact, desiccants which are broadly used by
us today are playing significant roles in different fields, thus the discovery
and advance of them are worthwhile to explore and develop.

Desiccant is defined as “a hygroscopic
material that serves to maintain a state of dryness1” and most commonly, one called silica gel was
in existence as early as the 1640s as a scientific curiosity2 and later was used in World
War I for the adsorption of vapours and gases in gas
mask canisters. Realizing its value, a chemistry professor called Walter
A. Patrick at Johns Hopkins University has invented certain new and useful improvements
and a synthetic route for producing silica gel in 1918. Other than
carrying out a simple way to yield a kind of uniform product, the new silica
gel invented is in the form of small beads but possesses abundant pores inside
its hard shell at the same time, which when the water content is greatly
reduced will be transparent and showing a glassy appearance. To excel the
property, the silica gel is also designed to remain stable at high temperatures
thus able to work under several conditions efficiently. In general, silica gel
is prepared in order by mixing solutions of sodium silicate or water-glass with
acid solutions, which prevents any other mixtures forming during the process.
After that, the excess acid and the salt formed in the reaction are then removed
from the solution, which is always done by a slow dialysis process. However,
the concentrations of acid and water-glass solution are required to be
determined very cautiously as it will give rise to a clear gel within 4 or 5
hours after mixing. Also, to avoid a rapid coagulation that will take place because
of the instability of the mixture, an efficient stirring is needed to the
solutions. 3 The special part of the invention is that the
physical adsorption of water vapor into its internal pores and with no
by-products forming. Even when contacted with water vapor, silica gel can still
maintain its property as a dry product with the unchanged shape. It will adsorb
up to one third of its own weight in water vapor. This adsorption efficiency is
approximately 35% greater than typical desiccant clays, making silica gel the
preferred choice where weight or efficiency are dominant factors to both
manufacturers and customers.4

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  As the silica gel has been greatly improved in
the laboratory and has brought so much convenience into daily life, soon the demands
of this material by various manufactures across the globe start to go up promptly.
There are several ways of manufacturing silica gel in different plants. For
instance, commercially in Fuji Silysia Chemical LTD, silica
gel is derived from a manufacturing method, sol-gel, a process where
micro particles or molecules in a solution (sols) agglomerate and under
controlled conditions eventually link together to form a coherent network (gel)5, to generate this material. Two raw
materials, sodium silicate and mineral acid, are used in a wet process to
create a reaction in generating monomeric silicic acid. These monomers
polymerize to generate primary silica particles, referred to as silica sol. The
particles then aggregate to form a three-dimensional structure in a gel state
maintaining a low refractive index of 1.46. During this process, growth
conditions affecting the primary particles (sizes 3-30nm) are controlled to
modify physical properties such as surface area, pore diameter and pore volume.6 (Figure 1) And according to another process
offered by National Research Development Corporation, clear stable solution of
sodium silicate after proper dilution with deionized water is filtered. The
filtered solution is then percolated from top through an ion-exchange column.
The operation is so synchronized that when the percolation is going on in one
exchange column, regeneration of the other columns is simultaneously carried
out. After that, the percolated silica sol is treated with ammonium hydroxide
and dried under sun where it becomes gelified. The hardened gel in the trays is
then conveyed into a compartmental tray drier and finally dried to 5 to 10
percent moisture content in an electric oven at 120 degrees and packed in air
tight plastic containers for dispatch.7

 

Figure 1. Flow chart of manufacturing silica
gel

  However, there are still potential risks that
could occur while manufacturing the silica gel, which requires great attention
to be paid. For example, the occupational exposure to airborne crystalline
silica such as during sand blasting, tunneling, or work in a quarry, does have
hazardous health impacts and may cause several respiratory diseases or even
lung cancer. Inhalation exposure to respirable crystalline silica can also cause
silicosis, which in severe cases can be disabling, or even fatal. Silicosis may
occur when respirable-sized crystalline silica dust is inhaled into lower
reaches of the lung and causes the formation of scar tissue, thus reducing the
person’s ability to take in oxygen.8 Therefore, in order to keep more workers away from these related diseases,
in Britain, RCS (Respirable crystalline silica) exposure has set up a workplace
exposure limit (WEL), which contains exposure below a set limit, preventing excessive
exposure. The WEL for RCS is 0.1 mg/m3 expressed as an 8-hour time-weighted
average (TWA). Exposure to RCS is also subject to the Control of Substances
Hazardous to Health Regulations 2002 (COSHH)9.

 Not only being extensively manufactured in a variety of commercial,
industrial, and household applications, the silica gel is also widely used in
water filtration, or as a food additive, which has brought about massive profit
to the manufactures. Owing to its simple manufacturing process, high specific
surface area and outstanding adsorption characteristics, silica gel is
considered as one of the cheapest desiccants around the world, which enhances the
demand in the use of silica gel significantly from the APAC region (excluding
China) and becomes one of the major manufacturing trends in the market in
recent years. According to the report, the increased demand for silica gel
in China, resulting from the increase in
economic and industrial development in the country these years, is the major factor
for the growth of the market and is also highly expected to benefit the entire Global
Silica Gel market in the future.10

  Today,
Silica gel is commonly classified as one of the most frequently-used desiccant but
still being improved in its characteristics and manufacturing costs. That is only one of the species of the desiccants that can physically
absorb water molecules from the humid air, caused by thousands of microscopic
cavities that create a lower vapor pressure inside it. It is strongly believed
that the other desiccants are also feasible to be fully used by triggering various
chemical reactions. Ideally, excess water is able be reacted or transferred
into other forms or products such as steam and heat, and therefore, apart from
obtaining a dry situation, the new products generated are very helpful in other
efficient use at the same time. The process of desiccants, from being
researched in the laboratory to a bulk chemical manufacturing plant, not only
shows us the improving of discoveries, but also implies a persistent spirit
that encourages people to learn from.

1 Delta
Adsorbents, “What is a desiccant”, 2013.  https://www.deltaadsorbents.com › Blog

2 Maryann
Feldman and Pierre Desrochers (March 2003). “Research Universities and Local Economic
Development: Lessons from the History of the Johns Hopkins University”. Industry
and Innovation. 10 (1):
5–24. Archived from the original on 2005-11-12.

3 WA LTER
A. PATRICK, OF BALTIMORE, MARYLAND SILICA GEL AND PROCESS OF MAKING SAME. March
18, 1919

4 Silica gel – Kurnool, site www.kurnool.ap.gov.in/departmentView.apo?…SILICA%20GEL1.pdf.

5 The
chemistry of the sol-gel process – ScienceDirect, https://www.sciencedirect.com/science/article/pii/016727388990338X

6 Silica-gel
Manufacturer, Fuji Silysia Chemical http://www.fujisilysia.com/technology/

7 Silica gel – Kurnool, www.kurnool.ap.gov.in/departmentView.apo?…SILICA%20GEL1.pdf.

8 OSHA Fact
Sheet at www.osha.gov/OshDoc/data_General_Facts/
crystalline-factsheet.pdf.

9 Silicosis – Lung disease, http://www.hse.gov.uk/lung-disease/silicosis.htm

10 Global
Silica Gel Market 2015-2019 with Clariant, Evonik Industries, Grace (WR), Huber
(JM) & Solvay Dominating. Apr 08,
2015, 06:29 ET from Research and Markets