Since the technology
has been improving better and stronger these years, people are gradually getting
used to ignore the impact of little things that contribute a lot to the social
economic growth and the human development at the same time. In term of “little
things”, a species of little things called desiccants is significantly
noticeable but also easily despised due to their size and price. A variety of
desiccants are broadly used today and are playing significant roles in the daily
life, thus the discovery and advance of desiccants are highly 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 then was used in World War I for
the adsorption of
vapours and gases in gas mask canisters. Also
in World War II, silica gel
was used in keeping military equipment from moisture damage, as a fluid cracking catalyst for the
production of gasoline, and as a catalyst
support for the manufacture of butadiene from ethanol, feedstock for
the synthetic rubber program.
Realizing its value, a chemistry professor called Walter A. Patrick at Johns
Hopkins University has
invented certain new and useful improvements
and synthetic route for producing silica gel  in 1918. Other than carrying out simple ways to yield a
uniform product, the silica gel invented also possesses many pores inside its
hard shell, which when the water content is greatly reduced will be transparent
and showing a glassy appearance. To excel the property, the new silica gel is
also designed to be stable at high temperatures. 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 supposed to
be 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 due
to the instability of the mixture, an efficient stirring is needed to the
solutions. 3 The special part of the
invention is the physical adsorption of water vapor into its internal pores. There
is no chemical reaction involved and no by-products forming. Even when
saturated with water vapor, silica gel still maintains 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 important factors.4

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  Since
the silica gel has been improved greatly in the laboratory and already brought
much convenience into daily life, demands of this material by manufactures around
the world have been increasing promptly. There are several ways of
manufacturing silica gel in different plants. For instance, in Fuji Silysia Chemical LTD, Silica gel is commercially derived
from the manufacturing method, sol-gel, to
produce 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. These 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.5 (Figure 1)
And as in the 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.6

 

Figure 1. Flow chart of
manufacturing silica gel

  However, there are still potential risks that require
great attention while manufacturing the silica gel. 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 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.7 Therefore,
to keep more workers away from the 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)8.

 Not only widely manufactured in a variety
of commercial, industrial, and household applications, the silica gel is also
greatly used in water filtration, or as a food additive, which has been
bringing 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 is becoming 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.9

  Silica gel is commonly classified as one of the
most frequently-used desiccant today but still being
improved in characteristics and manufacturing costs.

1 Delta Adsorbents, “What is a desiccant”. 2013

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 Silica-gel Manufacturer, Fuji Silysia Chemical

Technology

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

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

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

9 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

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