CHAPTER 9
REELING WATER
Approximately 850-1,000 tons of water is used to manufacture 1 ton of raw silk. This chapter covers water issues pertinent to silk production.
9.1 Type of Water
All natural water sources contain minerals. The mineral constituents found in greatest abundance are bicarbonates, sulphates, calcium chlorides, magnesium and sodium. Carbon dioxide contents may be greater than expected as a result of the decomposition of organic matter. Three water sources are described here: well/spring water, lake water and surface water/river water.
Impurities in water are discussed under the following headings"
- Turbidity and colour
- Alkalinity
- Hardness
- Iron and manganese
Hardness is described as permanent and temporary; where permanent is caused by nitrates, chlorides and sulphates, and bicarbonates, which may be boiled to precipitate into carbonates and removing the hardness, cause temporary.
Ca(NCO3)2 ® CaCO3 + H2O + CO2
Hardness can be measured when a given amount of potassium palmitate or oleate solution is titrated using a given amount of water, concluding when the shaken liquid produces a permanent lather.
More sophisticated methods are now available where the concentrations of calcium and magnesium ions are quantified by titration with ethylene diaminetetra-acetic acid. Hardness is expressed in degrees, but the actual units of measure vary from country to country.
1 unit (CaCO3 ppm) of United States is equivalent to 0.056° dH of Germany.
If a given source is alkaline, iron in the form of ferrous bicarbonate may be removed by aeration when ferric hydroxide is precipitated and carbon dioxide released. Aeration also helps to raise the pH by reducing the content of dissolved carbon dioxide.
4Fe(HCO3)2 + O2 + 2H2O ® 4Fe(OH)3 + 8CO2
Manganese can be released in the same way but to ensure rapid oxidation, a pH greater than 10 is required through the addition of lime or caustic soda.
9.3 Methods of Water Softening
This section looks at industrial water treatment using lime-soda processing, base exchange and demineralization to remove all dissolved salts.
Ca(HCO3)2 +
Ca(OH)2 ® 2CaCO3
+ 2H2O
Mg(HCO3)2 + 2Ca(OH)2 ® 2CaCO3 + Mg(OH)2 + 2H2O
Calcium carbonate has a much lower solubility than magnesium carbonate. When water containing bicarbonates of calcium and magnesium is treated, it is the calcium carbonate, which is first deposited. For permanent hardness, the reaction is:
CaSO4 + Na2CO3 ® CACO3 + Na2SO4
MgC12 + Ca(OH)2 ® CaC12 + Mg(OH)2
The CaC12 formed in the last reaction is removed as carbonate by the sodium carbonate. It may be noted that, when temporary hardness is eliminated by this method, no salts remain but permanent hardness, an equivalent quantity of sodium salts is left behind.
If Z represents the silicate, the reaction may be represented as follows:
Temporary hardness Ca(HCO3)2 + Na2Z ® CaZ + 2NaHCO3
Mg(HCO3)2 + Na2Z ® MgZ + 2NaHCO3
Permanent hardness CaSO4 + Na2Z ® CaZ + Na2SO4
MgSO4 + Na2Z ® MgZ + Na2SO4
In contrast to the lime-soda process, the exchange process replaces the calcium and magnesium ions by an equivalent of sodium even when the hardness is temporary. The anions present are unaffected. The bed is made from Zeolite in granular form, sieved to a particle size around 1-2 mm in diameter; its capacity is limited and will eventually be depleted.
However, the process is reversible and employing a concentrated salt solution can regenerate the bed:
CaZ + 2NaC1 ® CaC12 +Na2Z
When the calcium and magnesium ions are removed as soluble chlorides, they are replaced by sodium. A cyclic procedure is created of softening, regeneration, water wash (to remove excess salt), softening, etc.
Between the softening and regeneration, it is sometimes desirable to backwash by passing water upwards through the bed for a short time to remove any matter, which has filtered out of the bed. Water from this process may have hardness values as low as 0.5 ppm.
CaC12 + H2R ® CaR + 2HCL
Ca(HCO3)2 + H2R ® CaR + 2CO2 + 2H2O
The resultant carbonic acid breaks down into water ad carbon dioxide, which can be removed by aeration, but the mineral acids remain. Regeneration of the exchange process may be carried out by the use of hydrochloric acid.
Table 29. Ion-exchange types and the quality of water treated
(K.E. Song and Y.W. Lee, 1973)
Type of treatment |
Original water |
Base type |
Hydrogen type |
pH |
6.4 |
6.8 |
3.3 |
M-alkalinity (CaCO3 mg/1) |
60 |
81 |
0 |
Acidity (CaCO3 mg/1) |
30 |
0 |
74 |
Whole hardness (CaCO3 mg/1) |
88 |
8 |
2 |
It is essential to carefully select the quality of reeling water, as
it impacts reeling efficiency, raw silk percentage of cocoons and the raw silk
quality. Table 31 gives a standard for quality of reeling water and Table 31
shows the pH values and water hardness desirable for cocoon cooking water.
Table 30. Standard quality of reeling water (B.H. Kim, 1983)
Items |
Standard concentration |
Range of concentration |
(1) Colour and cleanness |
Colourless and clear |
|
(2) smell |
No smell |
|
(3) Suspension and sediment |
No |
|
(4) pH of water |
6.9 |
6.6-7.2 |
(5) pH of water after being boiled |
8.3 |
7.9-8.6 |
(6) specific electro conductivity (micrombo/cm) |
100 |
40-300 |
(7) Hardness (° dH) ° dH x 17.85 CaCO3 ppm |
2.0 |
0.5-4.0 |
(8) M-alkalinity (CaCO3 ppm) |
30 |
20-40 |
(9) Total acidity (CaCO3 ppm) |
5 |
3-15 |
(10) Heavy metal iron (Fe2O3 ppm) |
Under 0.1 |
0-0.3 |
(11) Residue after evaporation (ppm) |
85 |
50-200 |
Table 31. pH values and water hardness suitable for cooking parts
Parts |
Items |
Good reelable cocoons |
Poor reelable cocoons |
Dipping part |
pH |
4.5-5.5 |
4.5-5.5 |
Water hardness (° dH) |
0 |
0 |
|
Low temperature permeating part |
pH |
5.5-6.5 |
6.0-7.0 |
Water hardness (° dH) |
0-2 |
0-2 |
|
Cooking adjusting part |
pH |
5.5-6.5 |
6.5-7.5 |
Water hardness (° dH) |
2-4 |
1-3 |
|
Finishing part |
pH |
6.5-7.0 |
7.0-8.0 |
Water hardness (° dH) |
1-3 |
1-2 |
Notice:
a, Total acidity (CaCO3)
b,
Amount of N/50 NaOH sol. Consumed for titration (ml)
c,
Amount of sample water (ml)
* Phenolphthalein indicator: 0.15g phenolphthalein and 0.05g thymol blue
dissolved in 100 ml of 50% ethanol.
Notice:
a, M-alkalinity (CaCO3)
b,
Amount of N/50 H2SO4 sol. Consumed
for titration (ml)
c,
Amount of sample water (ml)
*Methyl red indicator:
0.02g methyl red and 0.1g bromcresol green dissolved
in 100 ml water
° dH = a x 0.056
|Notice: a, Total hardness (CACO3)
b, Amount of N/50 H2SO4 sol. Consumed
for titration (ml)
c,
Amount of sample water (ml)
* Buffer solution: | the total volume to be 1 litre by adding the
distilled water after 67.5 g of NH4C1 is dissolved into 570 ml of NH4OH. |
*E.B.T. indicator: | the total volume to be 100 ml by adding the ethanol after mixing 9.5g of Eriochrom Black T. and 30 ml of triethanolamine. |
*E.D.T.A. Standard solution: | 3.7225g of the purified Ethylene diamine Tetraacetic Acid disodium salt is dissolved into distilled water and until a total volume of 1 litre is achieved. |