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The textile manufacturing industry encompasses
many and diverse processes that rely heavily on the use of water, energy,
chemicals, and other resources. Wet spinning, sizing, desizing, scouring,
bleaching, mercerization, dyeing and printing are just a few. Monitoring and
controlling the pH, TDS/Conductivity/Salt Concentration, ORP (REDOX), and
Temperature of the aqueous solutions used in these processes conserves costly
resources, controls quality, and reduces the amount of pollution that must be
treated before discharge of effluent wastes. This can be done manually with
handheld instruments or automatically with in-line monitor/controllers.
pH
In all textile processes in which aqueous
solutions are used, balancing the pH of the solution is primary. pH control is
critical for a number of reasons. The effectiveness of oxidizing and reducing
agents is pH dependent. The amount of chemicals required for a given process is
directly related to the pH. The solubility of substances, such as dyes and
impurities, vary with pH. The corrosive and scaling potential of processing
solutions is also heavily influenced by pH. All of these issues affect quality
and costs.
Along with surface tension, pH plays an important
role in the wetting and saturating processes. For example, caustic solutions
cause interfibrillar swelling in cotton cellulose and cannot be squeezed out as
easily as water, which can reduce quality in subsequent processing.
The scouring of wool is a good example of a
process where maintaining the pH value permits a better solubilization of
certain impurities. For example, a pH of 10 is considered optimum for the
removal of wool wax.
In the instance of vat dyeing, pH controls the
solubilization of the dyes. Initially, the quantity of caustic soda present must
be adequate to ensure the solubility of the leuco form. Once the dye has been
exhausted, the pH is adjusted such that the dye returns to its insoluble form
and is mechanically trapped in the fiber.
Between the color kitchen and processing,
controlling the pH improves the lab-to-bulk reproducibility of color. Monitoring
and controlling pH ensures consistency of color from batch to batch, as well.
Maintaining the correct pH is also critical in
processes where a specific pH permits a reaction mechanism necessary for the
purification of fibers: bleaching with sodium hypochlorite or hydrogen peroxide,
desizing with oxidizing agents or removing soluble products and others.
To effectively bleach cellulose (e.g. cotton) with
a minimum amount of damage, the bleaching solution must be alkaline. This keeps
the hypochlorite stable and also prevents the presence of reducing groups that
cause an apparently well-bleached cloth to yellow with age. Additionally, an
acidic solution will form toxic and corrosive chlorine gas. Bleaching liquor is
therefore usually maintained at a pH of 9. The permanence of the white obtained
is thereby increased, and the bleaching is safe. Due to environmental concerns
in recent times, hydrogen peroxide bleaching has become more prevalent. Its
reaction products, oxygen and water, are relatively harmless. However, hydrogen
peroxide is a weak acid. Thus its conjugate base, HO2
-, is used to perform the actual bleaching. To ensure an adequate concentration
of HO2
-, the solution pH must be tightly controlled. Sodium hydroxide is used to
maintain the pH at a very alkaline level of 12-12.5.
pH must also be maintained in desizing, where
hydrogen peroxide, sodium bromite, or other oxidizing agents are employed to
remove the size of the yarn. A pH of >9 is required in the use of sodium bromite,
for example, as it decomposes at pH 8.
The pH of the boiler water in textile processing
must also be carefully monitored and controlled. To minimize corrosion, a pH as
high as 9 is desirable. However, care must be taken to avoid excess alkalinity.
Alkaline substances such as sodium hydroxide, carbonate, and phosphate have a
solvent action on the non-ferrous metal fittings. In pressure boilers caustic
embrittlement may also occur at riveted areas and places of high stress.
Additionally, it must be considered that acidic
waters cannot be softened by agents such as EDTA to improve processes.
MyronL
Company manufactures a variety of handheld instruments and in-line
monitor/controllers that measure and manage pH. Use the handheld electronic
Ultrameter II™
6P or TechPro
II™ TH1 to
measure the pH of solution. Use MyronL
in-line pH monitor/controllers such as the 720 Series
II
to establish and maintain an acceptable process solution pH setpoint
automatically.
Total
Dissolved Solids (TDS) and
Conductivity
The concentration of dissolved solids in solution, such as metal salts, is
determined by a measure of the conductivity of a solution and may be converted
to parts per million total dissolved solids.
TDS/Conductivity affects quality and energy
efficiency in many processes from the boiler water to the dye bath. A TDS
reading of 65-150 ppm is generally considered suitable for the textile industry.
If the source water has too high a TDS, it can significantly increase energy
costs and result in a poor quality product.
In the case of boiler water, impurities reduce
efficiency of the boilers and can lead to corrosion of equipment. Scale also
forms, which is a poor conductor of heat. In addition to increased fuel
consumption then required, the presence of scale is dangerous as it causes
overheating, which softens the metal and ultimately causes it to fail.
TDS is also a good way to measure precise
concentrations of salts used in the exhaustion of direct dyes on cellulose
fibers, such as cotton. It is important to determine the optimum level of salt
as there is a maximum beyond which further additions may not yield much effect.
Using this optimum level conserves salt use and ensures the most rapid
exhaustion of vat dyes.
Measuring TDS of rinse water in the vat dye
process is useful in determining the effectiveness of the rinsing. By taking a
reading of rinse water from an ideal product and measuring it against readings
from batch to batch, you can determine when the product has been properly
rinsed. This ensures product quality while conserving water.
Monitoring and controlling TDS in the color
kitchen and in the dyeing process itself greatly improves color reproducibility
from the kitchen to the batch. Measuring and controlling TDS in the dye bath
from batch to batch also ensures quality consistency.
Salt concentration is also an important issue to
be managed in dyeing process wastewater, especially where reactive dyes are
used. Cotton batch dyeing operations typically use quantities of salt ranging
from 20 to 80 percent of the weight of the fabrics. High TDS in effluent can
also indicate the presence of inorganic pollutants. Dyeing effluents, for
example, can contain concentrations of some heavy metals such as chromium and
copper, but also zinc, lead, and nickel.
Use the handheld Ultrameter
II
4P or 6P, TechPro
II
TP1 or TPH1, or DS/pDS meter to manually test TDS/Conductivity and Salt
Concentration, cooling the water to the prescribed parameters for the
instrument, if necessary. To automatically monitor and control TDS/Conductivity
or Salt Concentration in any solution, use a 750 Series
II
Monitor/controller.
Oxidation Reduction Potential (ORP/REDOX)
ORP is the best way to measure the true oxidizing or reducing power of all
chemicals in solution. It is, therefore, an excellent way to measure and control
quantities of sodium hydrosulphite, for example, and other sulfur reducing
agents and bleaches. This drastically reduces the amount of error in determining
chemical quantities required in traditional dyeing processes, for example.
Precisely controlling the quantity of the reducing agent keeps the concentration
to a minimum and provides that there is enough to avoid oxidation of the dye. It
also prevents overreduction of dyes, which results in poor color yields. During
continuous reduction washing of disperse dyed or printed materials, maintaining
the proper ORP ensures the residual dyes are cleaned from solution and have no
chance of migrating and reattaching in undesirable areas of the fabric. ORP
measurement should be taken periodically during printing to ensure an adequate
quantity of reducing agent is present throughout the entire process.
In all of these processes, monitoring and
controlling ORP conserves chemicals and costs while reducing pollutants that
must be treated in wastewater before discharge. Another important benefit is a
substantial decrease in the noxious fumes present in the factory.
Use the Ultrameter
II
6P to manually monitor and control chemical levels using ORP and as a quick
check against automatic systems of control. Measurements can be taken of
automated system samples or in open-width washers, which is necessary for
printed fabrics. Use the 720 Series
II
pH/ORP Monitor/controller to establish a setpoint and automatically open and
close the valve to the oxidizing/ reducing agent to maintain the correct
chemical levels.
Temperature
Solution Temperature directly impacts the rate of processes and the solubility
and availability of chemicals and thus the quality of the product.
For example, it is important to monitor and
maintain the dyebath such that maximum exhaustion is obtained and dye uptake
optimized. For normal dyeing times during which the system does not necessarily
reach equilibrium, as the temperature increases, the amount of dye on the fiber
increases up to a limit, after which it decreases. This temperature of maximum
affinity varies from dye to dye and must be controlled.
The Ultrameter
II
4P, 6P and the TechPro
II
TPH1, TP1 and TH1 all conveniently measure temperature with each sample taken.
Both 720 and 750 Series
II
have temperature measurement/control as an option.
Wastewater Management
You can also use the Ultrameter
II
as a completely portable method of monitoring up to 6 basic parameters in your
wastewater: Conductivity; TDS; Resistivity; ORP (REDOX); pH; and Temperature.
The Ultrameter
II can store up
to 100 date-time-stamped readings in memory. You can instantly download stored
data to any computer using the optional uDock™ accessory package for easy data
entry, analysis, and reporting. To find out more about how MyronL
water quality instrumentation can improve your manufacturing processes, call
1-760-438-2021.
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