REPORT

ON

SILICA REMOVAL

BY

PRECIPITATION

REPORT

ON

SILICA REMOVAL

BY

PRECIPITATION

BY

VIJAY RAVISANKAR

May 2013 – July 2013

ACKNOWLEDGEMENTS

Firstly, I like to thank my manager Mr. Arjun Bhattacharyya and my HR Manager

Mr.Vivek for giving me this opportunity to do my internship at John.F.Welch Technology

Center.

I also like to thank Mr. Soumik Chakraborty for guiding and helping me throughout my

project and in process help me learn a whole lot of new opportunities in chemical engineering. I

also thank Mr. Romit who helped me settle quickly at this new place. I thank each and every

member of this team who played an equally important role in helping and guiding me during my

tenure. Finally, I like to thank all the lab assistants who helped me during my lab work here.

SILICA

What is Silica??

Silica or Silicon Di-Oxide is a chemical compound that is an oxide of silica with the chemical

formula SiO2. Silica is most commonly found in nature in two forms, quartz and sand.

What are the uses of silica??

Silica is primarily used in the production of glass for windows, drinking glasses, bottles etc. The

majority of optical fibers for telecommunication are also made using silica. It is also a primary

raw material for many ceramics such as earthenware, stoneware and porcelain.

Crystal Structure of Silica

In the majority of silicates, the Si atom shows tetrahedral coordination, with 4 oxygen atoms

surrounding a central Si atom. The most common example is seen in the quartz crystalline form

of silica SiO2. In each of the most thermodynamically stable crystalline forms of silica, on

average, all 4 of the vertices (or oxygen atoms) of the SiO4 tetrahedral are shared with others,

yielding the net chemical formula: SiO2.

This is the tetrahedral shape of SiO4.

In this, the 4 oxygen atoms are each shared

between two silicon atoms and thus giving the

net formula as SiO2.

SILICA CHEMISTRY

Silicon dioxide is formed when silicon is exposed to oxygen (or air). A very shallow layer

(approximately 1 nm or 10 Å) of so-called native oxide is formed on the surface when silicon is

exposed to air under ambient conditions. Amorphous silica, silica gel, is produced by the

acidification of solutions of sodium silicate to produce a gelatinous precipitate that is then

washed and then dehydrated to produce colorless micro-porous silica.

The solubility of silicon dioxide in water strongly depends on its crystalline form and is 3–4

times higher for silica than quartz. The solubility also depends on the temperature.

Solubility curves comparing the solubility of

amorphous silica vs quartz in water at different

temperatures.

Solubility curves comparing the solubility of

amorphous silica vs quartz in water at

different pH.

DIFFERENT FORMS OF SILICA

Silica can be found in various different forms at different temperatures and pressures.

It can be found as α-quartz, β-quartz, Tridymite, Cristobalite, Coesite & Stishovite.

These can be found at the respective temperature and pressure ranges as shown in the phase

diagram below:

Phase Diagrams for different forms of Silica at various

temperatures and Pressures

SILICA IN WATER

Silica can be found in three different forms:

1. Soluble

2. Insoluble

3. Colloidal

The soluble silica and the colloidal silica are those which can be found in water. The colloidal

silica, though not visible to naked eye are large enough and are in suspended form and can often

be removed via filtration techniques. The soluble silica cannot be removed by filtration.

The soluble silica is generally removed by the method of precipitation with other salts.

The salts are generally salts of magnesium/calcium (as in lime softening process) or those of

aluminium/ferrous/ferric etc.

Silica is generally present in water in the form of silicilic acid polymer. It is generally silicilic

acid (H4SiO4) surrounded by water molecules.

This is the general method by which silicilic acid is formed:

Naturally occurring silicic acid is produced by a non-biological process called hydration

involving water, and quartz, which is known to be common on Earth. The reaction producing

silicic acid from quartz can be written as: Quartz + Water → Silicic acid, or (in balanced form):

SiO2 + 2 H2O → SiH4O4.

Silicilic acid can also be prepared in lab by acidification of sodium silicate in aqueous solution.

The problem of using silicilic acid in chemical synthesis is that they readily lose water to form

randomly polymeric silica gel, a form of silicon dioxide.

DISSOCIATION OF SILICILIC ACID IN WATER

The silicic acid monomer (H4SiO4) is a dominant species in solution over a pH range of 0-9 and

has a solubility of 100-140 ppm at 298 K. Above pH 9, the solubility-increases as the silicic acid

dissociates according to the following equilibrium reactions at 298 K.

SOLUBLE SILICA MEASUREMENTS

Silica determination can be done by various different methods. Two methods use Silica

determination via colorimetry (spectrophotometer) and the other method uses gravimetric

technique which involves the filtering, drying and precipitation of solids and then measuring

their exact weight.

HETEROPOLY BLUE METHOD

Molybdenum Blue

Molybdenum blue is a term applied to:

Reduced heteropolymolybdate complexes, polyoxometalates containing Mo(V), Mo(VI),

and a hetero atom such as phosphorus or silicon

Reduced isopolymolybdate complexes, polyoxometalates containing Mo(V), Mo(VI)

formed when solutions of Mo(VI) are reduced

A blue pigment containing molybdenum(VI) oxide

The "heteropoly-molybdenum blues", are used extensively in analytical chemistry and as

catalysts. The formation of "isopoly-molybdenum blues" which are intense blue has been used as

a sensitive test for reducing reagents.

A sample of one kind of molybdenum blue with the formula Na15

[MoVI

126MoV

28O462H14(H2O)70]0.5 [MoVI

124MoV

28O457H14(H2O)68]0.5

USES IN QUANTITATIVE ANALYSIS

Colorimetric determination of P, As, Si and Ge

The determination of phosphorus, arsenic, silicon and germanium are examples of the use of

heteropoly-molybdenum blue in analytical chemistry. The following example describes the

determination of phosphorus. A sample containing the phosphate is mixed with an acid solution

of MoVI

, for example ammonium molybdate, to produce PMo12O403−

, which has a α-Keggin

structure. This anion is then reduced by, for example, ascorbic acid or SnCl2, to form the blue

coloured β-keggin ion, PMo12O407−

. The amount of the blue coloured ion produced is

proportional to the amount of phosphate present and the absorption can be measured using a

colorimeter to determine the amount of phosphorus.

DETERMINATION OF SILICA

GRAVIMETRIC METHOD

Principle: Silicon is converted to silicilic acid with hydrochloric acid and made insoluble. The

solution is then filtered, silica is ignited, weighed and then volatilized with hydrofluoric acid.

The residue is then ignited and weighed. The loss in weight represents silica (SiO2). Soluble

silica is found by spectrophotometry and combined with that found by gravimetry.

Procedure:

(a) Transfer approximately 1g of sample to weighing bottle and dry for 2 hrs. at 1400.

Stopper the bottle and cool for 1 hr. in a desiccator. Lift the stopper for instant, replace

and weight the bottle and sample. Transfer the sample, without brushing, to a 25ml

platinum crucible containing about 1.25g sodium carbonate. Reweight the bottle. The

difference is the weight of the sample.

(b) Thoroughly mix the dried sample and sodium carbonate by stirring with platinum rod.

Add another 1.25g portion of sodium carbonate and mix. Repeat with a third portion of

sodium carbonate and mix. Use 1.25g of sodium carbonate to clean the stirring rod and

then cover the mixture. Heat the covered crucible over a low flame for several min and

gradually increase the heat until the mass is molten. Using tongs slightly tilt and rotate

the crucible to fuse any particles that might cling with the crucible. Heat over open

flames for 20 min. Make sure nothing is sticking to the crucible, heat it gently for 20 sec,

cool it down.

(c) Cover the fused sample with water, transfer the contents to a platinum dish/ Teflon

beaker and wash the inside of the crucible and the lid. Cover the fusion with water and

allow to stand overnight. Reserve the crucible.

(d) Pulverize the cake and stir the solution. Place the tip of the funnel under the cover of

dish/beaker. Add 5ml of HCl to the reserved crucible. Slowly and cautiously, pour 40ml

of HCl (1:1) through the funnel. Transfer the contents of the reserved crucible through

the funnel. Rinse the crucible and cover with water and HCl repetitively. Wash the funnel

and funnel tip, and remove.

(e) Allow the solution to stand until reaction ceases. Rinse the cover, replace it on the

dish/beaker and hear on steam bath until carbon dioxide is expelled. IF any insoluble

residue is present, filter, wash the residue with water, transfer the paper to the crucible

used for fusion, ignite at low heat until carbon is removed. Fuse the residue with 2g

Sodium carbonate and add the cooled melt to the original solution. Wash the cover and

replace it on the dish/beaker and evaporate the solution to dryness with occasional

stirring.

(f) Add 50ml of HCl (1:4) to the dish/beaker, cover and heat for 10 min. Add 150mL of hot

HCl (1:19), stir to dissolve salts, and filter immediately by decantation into a 600mL

beaker.

(g) Transfer the filtrate and washing to the original platinum dish/beaker. Evaporate to

dryness on a steam bath. Add 50mL of HCl (1:4), digest and filter. Scrub the container 10

times with hot HCl and 3 times with hot water. Reserve the filtrate (A) and washing after

evaporation to about 200ml for spectrophotometric determination of soluble silica.

(h) Transfer the paper to the reserved platinum crucible, dry and hear slowly to 6000C until

carbon is removed. Finally ignite for 1 hr. at 12000C to constant weight. Cool in desiccator,

weigh after 30 min.

(i) Add about 1 mil of water and 6 drops of sulphuric acid (1:1) to the Silica precipitate. Add

15mL HF and evaporate to dryness and then heat at a gradually increasing rate until sulphuric

acid is removed. Ignite for 10 min at 1100 – 1150 0C, cool in desiccator, and weight. The

difference is weight of silica.

CALCULATION

% Silica = (A-B)/C * 100

Where:

A= initial weight, in grams, of crucible and impure silica.

B= final weight, in grams, of crucible and residue.

C = grams of sample used.

COLORIMETRIC ANALYSIS

(a) Molybdosilicate test

(b) Heteropoly blue method

Molybdosilicate test

The first method of silica colorimetric analysis is called the Molybdosilicate method. The sample

is treated with one milliliter of 1:1 acid reagent and two milliliters of ammonium molybdate

reagent. The sample is then inverted and allowed to set for five minutes. Two milliliters of

oxalic/citric acid is added to the sample and allowed to set for two minutes. The oxalic/citric acid

is used to remove any interferences from phosphate and decrease interferences from tannin. The

sample is then run photometrically. The down side to this method is the instability of the color

produced.

Heteropoly Blue Method

The second method of silica colorimetric analysis is called the Heteropoly Blue method. This

method is similar to the Molybdosilicate method except that an additional reagent is added to the

sample. A reducing agent made up of 1-amino-2-naphthol-4-sulfonic acid is used. When this

reagent is added to the sample, the reaction takes on a blue color. The blue color produced in this

reaction is more stable than the yellow color from the molybdosilicate method.

How Heteropoly blue method works??

Spectrophotometry

Spectrophotometry is the quantitative measurement of the reflection or transmission properties of

a material as a function of wavelength. Spectrophotometry involves the use of a

spectrophotometer. A spectrophotometer is a photometer that can measure intensity as a function

of the light source wavelength.

In order to determine the respective concentrations of sample, the light transmittance of the

solution can be tested using spectrophotometry. The amount of light that passes through the

solution is indicative of the concentration of certain chemicals that do not allow light to pass

through.

The concentration of substances are basically found by beer lambert’s law

DR 2800 Spectrophotometer

Beer Lambert Law

The Beer–Lambert law, also known as Beer's law or the Lambert–Beer law relates the absorption

of light to the properties of the material through which the light is travelling.

The law states that there is a logarithmic dependence between the transmission (or

transmissivity), T, of light through a substance and the product of the absorption coefficient of

the substance, α, and the distance the light travels through the material (i.e., the path length), ℓ.

The absorption coefficient can, in turn, be written as a product of either a molar absorptivity

(extinction coefficient) of the absorber, ε, and the molar concentration c of absorbing species in

the material

The transmission (or transmissivity) is expressed in terms of an absorbance which, for liquids, is

defined as A = -log (I/I0)

This implies that the absorbance becomes linear with the concentration (or number density of

absorbers) according to A = εlc

Thus, if the path length and the molar absorptivity (or the absorption cross section) are known

and the absorbance is measured, the concentration of the substance (or the number density of

absorbers) can be deduced

Deviations from Beer–Lambert law

1. Chemical – deviations observed due to specific chemical species of the sample which is

being analyzed.

2. Instrument – deviations which occur due to how the absorbance measurements are made.

The second one is basically related to the errors done by human/error in the instrument itself

(improper working of some component within the instrument leading to incorrect readings

produced by the instrument)

SILICA REMOVAL BY PRECIPITATION

One very important method involved with the removal of silica is by precipitation of soluble

silica with the help of inorganic salts. The exact method by which they are removed has never

been properly explained. It is generally believed that soluble silica tends to get adsorbed onto the

inorganic salts or they tend to form some kind of secondary bonds with the salts and then they

tend to be expelled out along with the inorganic salts when they are precipitated. All inorganic

salts have a certain pH beyond which they cannot remain soluble within the substance and are

precipitated out. Different pH’s are selected depending upon the salt, depending upon the ability

of salt to remove silica and then they are precipitated out along with some of silica.

Other methods by which silica removal can be accomplished are ion exchange, distillation,

Reverse Osmosis. Silica removal generally happens during the hot/soft lime softening process

using lime & soda ash. Generally Ferric chloride/Aluminium salts/Magnesium Hydroxide is

added in this lime process is enough amount of silica removal doesn’t take place.

The following examples will be explained in detail in this report giving a general idea on silica

removal.

1. Silica removal during lime softening process

2. Silica removal using sodium aluminate

3. Silica removal using magnesium salts and zinc compounds

SILICA REMOVAL DURING LIME SOFTENING PROCESS

The presence of silica in water used for boiler feed purposes is undesirable for many reasons.

When added to water, magnesium oxide reduces crystalloid or soluble silica to practically zero.

When used with softening agents, magnesium oxide produces better flocculation with a hardness

lower than normal. The most serious problem remaining in boiler feed water conditioning

today involves the prevention of siliceous deposits in boilers and turbines. Silica is

conventionally expressed in water analysis as SiO2. Actually silica exists in both the crystalloidal

and the colloidal forms. The latter form can normally be removed by proper coagulation and

filtration. It is silica in the soluble form that presents the major problem.

Use with lime and soda ash softening

Silica removal by magnesium oxide can be carried out in the same container or softener as the

softening by lime and soda ash. Removal of silica by magnesium oxide can proceed

simultaneously with the removal of hardness from water by lime and soda ash.

Effect of magnesium oxide quantity on silica removal

Tests using magnesium oxide for silica removal showed that the amount of magnesium oxide (in

ppm) required to almost completely remove soluble silica present in water is 5 times that of silica

content present (i.e.) Magnesium oxide : silica = 5:1

Effect of Temperature on silica removal

Tests performed on silica removal showed that silica removal was very low at temperatures close

to room temperatures and removal rate increased with the increase in temperature. It was seen

that the maximum removal of silica was at around 95 0 C and the minimum was at 15

0 C.

So, a higher temperature was preferred for better silica removal.

Effect of retention time on silica removal

The retention time had some effect on silica removal up to a certain point beyond which any

increase in retention time didn’t produce significant increase in the silica removal. The effect of

retention time on silica removal could be felt up to a time period of one hour beyond which any

further increase in retention time had insignificant effect on silica removal.

Effect of sodium hydroxide quantity on silica removal

The maximum removal of magnesium oxide is in general found to occur at pH = 10.2. On

addition of different quantities of sodium hydroxide, it was found that addition of NaOH beyond

a certain limit resulted in an increase in the alkalinity of the solution and thus reduced the

amount of silica removal.

SILICA REMOVAL USING SODIUM ALUMINATE/FERRIC SUPLHATE

The presence of silica in water to be used for boiler feed purposes is very undesirable since it

may react with any calcium present to form a hard, dense deposit of calcium silicate. If the silica

content in boiler feed water is kept below 5ppm, the possibility of its concentration and

subsequent precipitation as silicate scale is greatly reduced. Since it was known that hydrous

aluminum oxide, precipitated either from sodium aluminate or other aluminium salts, has definite

coagulating and adsorptive properties, it was used to check the effect on silica removal. Silica

mas determined using spectrophotometer by reading the molybdenum blue formed by the

reduction of the silicomolybdate by means of l-amino-2-naphthol-4-sulfonic acid reagent.

Effect of pH on silica removal

pH control was found to be a very important factor in silica removal. It was found that maximum

silica removal was possible only within a narrow pH range and at any other pH outside the range,

it was observed that there was not much silica removal. In the case of hydrous aluminum oxide ,

it was found that maximum silica removal happened at a pH range of around 8.3 – 8.7

Effect of concentration of sodium aluminate added on silica removal

It was found out that as the concentration of sodium aluminate increase , the efficiency silica

removal also started increasing faster. Therefore all treatments were generally made at higher

concentrations of sodium aluminate and the pH was adjusted accordingly to the pH of around 8.5

with the help of lime or hydrochloric acid.

Effect of concentration of ferric sulfate added on silica removal

It was found out that ferric sulfate, just like sodium aluminate could also be used for silica

removal. Similar to sodium aluminate, ferric sulfate also gave maximum silica removal within a

narrow range. It was found out that silica removal was maximum at a pH of around 9.0. It was

found that ferric sulfate had a better effect on silica removal than sodium aluminate at smaller

concentrations but as the concentration was increased, there seemed to be very small increase in

similar removal as compared to large amount of silica removal with increase in concentration of

sodium aluminate.

SILICA REMOVAL USING MAGNESIUM SALTS AND ZINC COMPOUNDS

Some chemical clarification methods for silica removal were compared. The combination of

magnesium compound, sodium hydroxide precipitation and zinc sulfate coagulation was

investigated to control the silicate scale. The results indicated that silica was removed mainly

through magnesium compound, pH regulator and zinc sulfate. Zinc sulfate (ZnSO4·.7H2O) was a

sort of coagulant with good effect. The concentration of silica (calculated with SiO2) was

reduced to less than 50 mg/L in the optimal Condition. In addition, temperature and settle time

showed effects on silica removal. High temperature (70–900C) and long settle time (> 1.0 h) in a

mixing jar were advantageous to the silica removal. All the processes involved were mainly for

the water being used in steam boiler in the production in oilfields. General methods such as acid

washing cannot remove silica scale, apart from that they may make boiler tube and steam tube

perforated. Chemical methods are the main techniques to remove silica. Recent studies on the

removal of have been based on the chemical processes such as precipitation and coagulation.

Coagulation is a kind of physicochemical technique which uses metal oxide or metal hydroxide

to adsorb or coagulate silica.

Experimental Setup

The samples of heavy oil wastewater from the outlet of a crude oil–water separator were

mixed in an adjusting tank. The chemicals such as coagulants, pH regulators and silica removal

agents were added into a mixing jar. After all chemical agents completed reaction with waste-

water, the upper cleaned water was transferred to a filter

.

Schematic Diagram of the experimental setup

Effect of magnesium oxide on silica removal and hardness level

The concentration of SiO2 was reduced from 140.5 mg/L to 70.3 mg/L, and the efficiency of

silica removal reached 50%; the residual hardness changed from 29.8 mg/L to

18.2 mg/L when the dosage of MgO ranged from 0 mmol/L to 35 mmol/L (0–1400 mg/L)

Effect of magnesium chloride on silica removal and hardness level

The dosage of MgCl2·6H2O ranged from 0 mmol/L to 6.9 mmol/L (0–1400 mg/L). The

efficiency of the silica removal was high from 76.7% to 95.8% with the dosage of MgCl2·6H2O

between 2.9 and 6.9 mmol/L. The concentration of SiO2 declined to less than 50 mg/L, even

dropping by 5.9 mg/L. The mechanism of silica removal different from that of MgO [15] is that

magnesium chloride can remove silica through producing sediments of CaCO3 and MgSiO3 at

pH 10–11.

The theory of chemical reactions are shown as follows:

HCO3- + OH

- H2O + CO3

2-

Ca2+

+ CO32-

CaCO3 (ppt)

Mg2+

+ SiO32-

MgSiO3 (ppt)

Mg2+

+ OH- Mg(OH)2 (ppt)

Effect of zinc sulfate/other coagulants dosage on silica removal

The coagulants improve the efficiency of magnesium sediment and are advantageous to

the removal of silica because the silica is adsorbed selectively by the hydration complex

ion of the zinc, aluminum and ferric salts. By this means, silica is adsorbed on the surface of

deposits. The flocs produced by zinc sulfate are weighty and dense, the speed of adsorption and

reaction with silica sediments and settling rate are faster than by aluminum and ferric salts. The

faster settling rate and better coagulation power by zinc sulfate result in the higher efficiency of

silica removal.

Effect of temperature on silica removal

Higher temperature helps the removal of silica The speed of silica reacting with Ca2+

or Mg2+

and adsorbed by the sediment increase with the increasing of temperature. The concentration of

silica dropped by about 60% (to56.5 mg/L) at 500C and dropped by 83.2% at 90

0C. The

temperature did not have any effect on the hardness of water.

EXPERIMENTAL WORK

The following salts were used for testing the effect on silica removal at room temperature at a

constant stirring speed. Only the amount of substances added for testing were changed while

keeping all other parameters constant

1) Magnesium Oxide

2) Magnesium oxide + Calcium Hydroxide

3) Zinc Chloride

4) Ferric Chloride

5) Alum

6) Magnesium Chloride

7) Ferrous Sulfate

8) Dolomite

9) Magnesium Chloride + Lime

10) Magnesium Hydroxide

11) Alum + Lime

12) Zinc Chloride + Lime

The following were the data obtained for each of the salts. The tests were generally performed in

five to six 600mL beaker with 200mL of sample taken in each of the beakers and the amount of

salts/coagulants/flocculants were fixed and added at regular intervals allowing enough mixing to

happen in between each addition. The tables and graphs show the results for each of the salt

taken. The initial silica content of feed was found to be around 200 ± 20 ppm

RESULTS

1) MAGNESIUM OXIDE

0.00

5.00

10.00

15.00

20.00

25.00

30.00

35.00

40.00

45.00

0 500 1000 1500 2000 2500 3000

% S

ilica

re

mo

val

MgO (ppm)

%Silica removal vs Mgo

Sample Code

MgO (ppm) Coagulant(ppm)

Flocullant (ppm) Feed pH

pH after Mgo Final pH

Silica (ppm) % removal

C1 500 2 2 9.21 9.26 9.28 167.5 10.43

C2 1000 2 2 9.23 9.29 9.3 152.5 18.45

C3 1500 2 2 9.22 9.31 9.33 144 22.99

C4 2000 2 2 9.22 9.33 9.34 137.5 26.47

C5 2500 2 2 9.22 9.33 9.36 112 40.11

2) MAGNESIUM OXIDE+ LIME

0.00

5.00

10.00

15.00

20.00

25.00

30.00

35.00

40.00

45.00

50.00

0 500 1000 1500 2000 2500 3000

% S

ilica

re

mo

val

Lime (ppm)

% Silica removal vs Lime @ MgO=2500ppm

Sample Code

MgO (ppm)

Coagulant(ppm)

Flocculant

(ppm)

Lime (ppm)

Feed pH

pH after Mgo

pH after Lime

Final pH

Silica (ppm)

% removal

A1 2500 2 2

500 9.22 9.35 9.35 9.36 151 29.44

A2 2500 2 2

1000 9.23 9.34 9.37 9.39 144.5 32.48

A3 2500 2 2

1500 9.23 9.34 9.4 9.43 132 38.32

A4 2500 2 2

2000 9.22 9.34 9.41 9.44 127.5 40.42

A5 2500 2 2

2500 9.22 9.34 9.44 9.48 113.5 46.96

3) ZINC CHLORIDE

*Repeatability was tested and found to produce similar results

0.00

10.00

20.00

30.00

40.00

50.00

60.00

70.00

80.00

90.00

0 500 1000 1500 2000 2500 3000

% S

ilica

re

mo

val

ZnCl2 (ppm)

% Silica removal vs ZnCl2 (ppm)

Sample Code

ZnCl2 (ppm) Coagulant(ppm)

Flocculant (ppm) Feed pH

pH after ZnCl2 Final pH

Silica (ppm) % removal*

Z1 500 2 2

9.16 9.12 9.14 129.5 34.92

Z2 1000 2 2

9.15 9.12 9.13 108 45.73

Z3 1500 2 2

9.15 9.08 9.11 79 60.30

Z4 2000 2 2

9.14 9.07 9.09 63.5 68.09

Z5 2500 2 2

9.14 9.04 9.07 46.5 76.63

4) FERRIC CHLORIDE

0.00

10.00

20.00

30.00

40.00

50.00

60.00

70.00

0 500 1000 1500 2000 2500 3000

%si

lica

rem

ova

l

FeCl3 (ppm)

%Silica Removal vs Ferric chloride

Sample Code

FeCl3 (ppm) Coagulant(ppm)

Flocullant (ppm) Feed pH

pH after FeCl3 Final pH

Silica (ppm) % removal

F1 500 2 2

9.17 9.16 9.15 133.5 34.72

F2 1000 2 2

9.18 9.14 9.12 99 51.59

F3 1500 2 2

9.19 9.11 9.1 94.5 53.79

F4 2000 2 2

9.18 9.1 9.07 89.5 56.23

F5 2500 2 2

9.18 9.06 9.03 80.5 60.64

5) ALUM

0.00

10.00

20.00

30.00

40.00

50.00

60.00

0 500 1000 1500 2000 2500 3000

% s

ilica

re

mo

val

Alum (ppm)

% Silica Removal vs Alum

Sample Code

Alum (ppm) Coagulant(ppm)

Flocullant (ppm) Feed pH

pH after Alum Final pH

Silica (ppm) % removal

AL1 500 2 2

9.09 9.14 9.15 137.5 34.99

AL2 1000 2 2

9.1 9.14 9.14 121 42.79

AL3 1500 2 2

9.1 9.12 9.13 114.5 45.86

AL4 2000 2 2

9.11 9.12 9.12 101 52.25

AL5 2500 2 2

9.12 9.13 9.13 95 55.08

6) MAGNESIUM CHLORIDE

0.00

5.00

10.00

15.00

20.00

25.00

30.00

0 500 1000 1500 2000 2500 3000

%si

lica

rem

ova

l

MgCl2 (ppm)

% Silica Removal vs MgCl2

Sample Code

MgCl2 (ppm) Coagulant(ppm)

Flocculant (ppm) Feed pH

pH after MgCl2 Final pH

Silica (ppm) % removal

M1 500 2 2

9.15 9.12 9.1 193 8.96

M2 1000 2 2

9.14 9.11 9.1 180 15.09

M3 1500 2 2

9.14 9.11 9.1 174 17.92

M4 2000 2 2

9.15 9.1 9.09 169 20.28

M5 2500 2 2

9.14 9.1 9.08 157.5 25.71

7) FERROUS SULPHATE

** Value couldn’t be found due to sampling problems

0.00

5.00

10.00

15.00

20.00

25.00

30.00

35.00

40.00

45.00

50.00

0 500 1000 1500 2000 2500 3000

% s

ilica

re

mo

val

FeSO4 (ppm)

% Silica Removal vs FeSO4

Sample Code

FeSO4 (ppm) Coagulant(ppm)

Flocculant (ppm) Feed pH

pH after FeSO4 Final pH

Silica (ppm) % removal

FS1 500 2 2

9.23 9.21 9.19 150.5 28.33

FS2 1000 2 2

9.25 9.19 9.18 135 35.71

FS3 1500 2 2

9.25 9.16 9.17 117.5 44.05

FS4 2000 2 2

9.26 9.15 9.16 111.5 46.90

FS5 2500 2 2

9.26 9.14 9.16 ** **

8) DOLOMITE

0.00

5.00

10.00

15.00

20.00

25.00

30.00

35.00

0 500 1000 1500 2000 2500 3000

%si

lica

rem

ova

l

Dolomite (ppm)

% Silica removal vs Dolomite

Sample Code

Dolomite(ppm) Coagulant(ppm)

Flocullant (ppm) Feed pH

pH after Dolomite Final pH

Silica (ppm) % removal

DO1 500 2 2

9.23 9.23 9.26 207 6.33

DO2 1000 2 2

9.25 9.28 9.28 198 10.41

DO3 1500 2 2

9.25 9.3 9.3 178 19.46

DO4 2000 2 2

9.26 9.32 9.32 160.5 27.38

DO5 2500 2 2

9.26 9.32 9.32 155 29.86

9) MAGNESIUM CHLORIDE + LIME

0.00

5.00

10.00

15.00

20.00

25.00

30.00

35.00

0 500 1000 1500 2000 2500 3000

% S

ilica

Re

mo

val

MgCl2 (ppm)

%Silica Removal vs MgCl2 @ 2500ppm Lime

Sample Code

MgCl2 (ppm)

Coagulant (ppm)

Flocullant

(ppm)

Lime (ppm)

Feed pH

pH after

MgCl2

pH after Lime

Final pH

Silica (ppm)

% removal

ML1 2500 2 2

500 9.24 9.32 9.4 9.39 209 6.07

ML 2 2500 2 2

1000 9.26 9.34 9.41 9.42 203.5 8.54

ML 3 2500 2 2

1500 9.27 9.34 9.4 9.4 194.5 12.58

ML 4 2500 2 2

2000 9.27 9.36 9.39 9.4 177.5 20.22

ML 5 2500 2 2

2500 9.26 9.35 9.38 9.38 152 31.69

10) MAGNESIUM HYDROXIDE

0.00

5.00

10.00

15.00

20.00

25.00

30.00

0 500 1000 1500 2000 2500 3000

% S

ilica

re

mo

val

Mg(OH)2 (ppm)

% Silica Removal vs Mg(OH)2

Sample Code

Mg(OH)2 (ppm) Coagulant(ppm)

Flocullant (ppm) Feed pH

pH after Mg(OH)2 Final pH

Silica (ppm) % removal

MH1 500 2 2

9.13 9.16 9.16 198 5.49

MH 2 1250 2 2

9.12 9.16 9.17 182 13.13

MH 3 2000 2 2

9.12 9.18 9.18 167 20.29

MH 4 2500 2 2

9.13 9.19 9.2 150 28.40

11) ALUM + LIME

0.00

10.00

20.00

30.00

40.00

50.00

60.00

70.00

80.00

90.00

0 500 1000 1500 2000 2500

% S

ilica

Re

mo

val

Lime (ppm)

% Silica removal vs Lime @ 2500ppm Alum

Sample Code

Alum (ppm)

Coagulant (ppm)

Flocullant

(ppm)

Lime (ppm)

Feed pH

pH after Alum

pH after Lime

Final pH

Silica (ppm)

% removal

AM1 2500 2 2

0 9.23 9.19 9.19 9.19 77 60.91

AM2 2500 2 2

500 9.22 9.18 9.21 9.21 60 69.54

AM3 2500 2 2

1000 9.24 9.19 9.22 9.23 51.5 73.86

AM4 2500 2 2

1500 9.22 9.17 9.21 9.22 43 78.17

AM5 2500 2 2

2000 9.23 9.16 9.22 9.23 32.5 83.50

12) ZINC CHLORIDE + LIME

0.00

10.00

20.00

30.00

40.00

50.00

60.00

0 500 1000 1500 2000 2500

% S

ilica

re

mo

val

Lime

% Silica Removal vs Lime @ 500ppm Zinc Chloride

Sample Code

ZnCl2(ppm)

Coagulant (ppm)

Flocullant

(ppm)

Lime (ppm)

Feed pH

pH after ZnCl2

pH after Lime

Final pH

Silica (ppm)

% removal

ZL1 500 2 2

0 9.24 9.21 9.19 9.18 138 33.01

Zl2 500 2 2

500 9.23 9.2 9.2 9.21 122.5 40.53

ZL3 500 2 2

1000 9.23 9.21 9.22 9.22 113.5 44.90

ZL4 500 2 2

1500 9.23 9.21 9.23 9.23 107.5 47.82

ZL5 500 2 2

2000 9.23 9.22 9.25 9.25 94.5 54.13

13) ZINC CHLORIDE + HYPO

0.00

5.00

10.00

15.00

20.00

25.00

30.00

35.00

40.00

45.00

50.00

0 20 40 60 80 100 120 140 160

% S

ilica

re

mo

val

Hypo

% Silica Removal Hypo@ 500ppm Zinc Chloride

Sample Code

ZnCl2(ppm)

Coagulant (ppm)

Flocullant

(ppm)

Hypo (ppm)

Feed pH

pH after ZnCl2

pH after Hypo

Final pH

Silica (ppm)

% removal

ZH1 500 2 2

0 9.21 9.2 9.19 9.19 133.5 37.18

ZH 2 500 2 2

50 9.21 9.19 9.19 9.2 138 35.06

ZH 3 500 2 2

100 9.23 9.21 9.21 9.22 128 39.76

ZH 4 500 2 2

150 9.22 9.2 9.22 9.23 112.5 47.06

EFFECT OF SILICA REMOVAL BY SALTS ALONE

COMPARISON OF THE BEST POSSIBLE RESULTS

0

50

100

0 1000 2000 3000

%si

lica

rem

ova

l

Salt (ppm)

% Silica removal vs Salt (ppm)

MgO

ZnCl2

FeCl3

Alum

MgCl2

FeSO4

DISCUSSIONS & CONCLUSION

Various salts were used for the analysis and some salts were used along with other substances to

check the effect on silica removal. Of all the salts used, it was found that zinc chloride produced

the maximum amount of silica removal of around 76% and alum and ferric salts showing around

55 to 60%. It was also found that magnesium salts did not have any considerable effects at this

pH. Various magnesium salts were considered (Magnesium oxide, Magnesium chloride, dolomite

, Magnesium hydroxide ) but the removal remained around 30-40%. Not much effect was also

seen when lime was also added along with this magnesium salts to support silica removal. Apart

from this, Zinc chloride and alum were tested in the presence of lime and in both cases it was

found to give 20% more than removal by the salt alone. These showed that addition of lime had

considerable effect on silica removal by providing enough hydroxide ions for the metal salts to

react with and thereby precipitate out as respective metal hydroxide and in the process remove

more silica than usual amounts. The reasons for much higher removal in case of zinc/alum/ferric

when compared to magnesium salts can be seen from the figure below. It shows that

zinc/ferric/alum have low solubility close to a pH of 9 which was found to the pH of the treated

water thereby implying that most of the salts added were precipitated out along with silica. The

total dissolved content of the wastewater was also very high and the alkalinity was also very high

and therefore the water almost acted like a buffer solution and pH adjustment was found to be

very difficult without changing much of the volume of sample.. The buffer like solution and the

solubility vs pH graph therefore show that zinc/alum/ferric salts tend to be a better choice for

silica removal while treating this sample of water.

Solubility vs pH graph for various salts

pH adjustment using Lime

9.24

9.25

9.26

9.27

9.28

9.29

9.3

9.31

9.32

9.33

0 500 1000 1500 2000 2500 3000

pH

aft

er

Lim

e a

dd

itio

n

Lime added (ppm)

pH adjustment using NaOH

pH adjustment using HCl

9.245

9.25

9.255

9.26

9.265

9.27

9.275

9.28

9.285

9.29

9.295

0 50 100 150 200 250

pH

aft

er

NaO

H a

dd

itio

n

NaOH added (ppm)

0

1

2

3

4

5

6

7

8

9

10

0 20 40 60 80 100 120

pH

HCl (mL)

pH vs HCl volume added

APPENDIX

CHEMICAL OXYGEN DEMAND (COD)

Chemical oxygen demand is an indirect method of determining the amount of organic

compounds present in water. It is expressed in milligrams/liter(mg/L) also known as parts per

million (ppm) which indicates mass of oxygen consumed per liter of solution.

The most important idea in the COD determination is that almost all the organic compounds can

be fully oxidized to carbon di oxide using a strong oxidizing agent in acidic conditions. In

general the strong oxidizing agent used is potassium dichromate depending upon the amount of

organics present. Potassium dichromate is generally used because of its strong oxidizing nature

and therefore able to oxidize almost all the organics present and give an accurate result of COD.

The sample to be tested is added to the test tube containing a fixed volume of excess volume of

the oxidizing agent. This is then heated for some time for the digestion process to be completed.

The final COD value is then determined by finding the difference between the excess volume of

sample and excess volume of the oxidizing agent in a spectrophotometer.

The blank for testing is prepared by adding the same amount of distilled water as the sample to

another similar test tube containing oxidizing agent and the difference between these are

calculated as COD.

Interferences in COD measurement

The most common interferences in COD measurement is the presence of inorganic oxidizable

material present in the distilled water to be added to the blank. Due to the presence of these

oxidizable substance , some of the oxidizing agent reacts with these and therefore the final COD

measurement of the sample deviates from the actual value. One of the most common interfering

substances are chlorides present in water. They react with potassium dichromate in the following

manner :

The table below shows the general inorganic interferences and some ways of eliminating :

One important factor related to COD is the wastewater treatment. Generally, in all countries the

maximum amount of COD present in wastewater is restricted and these conditions must be

satisfied before the wastewater can actually be released into the environment.

Vials containing oxidizing agent in which the sample is

taken and heated before analyzing the COD content

using a DR2800 spectrophotometer

OZONOLYSIS

The increasing use of ozone in the treatment of wastewater effluents has been stimulated by the

need to achieve higher effluent quality and greater compliance with physicochemical and

microbiological quality standards before discharge. Ozone (O3) is a highly reactive chemical

with a high oxidation–reduction potential. Its use in aqueous conditions usually leads to the

simultaneous production of secondary oxidants, such as radical species (OH•), whose oxidation

power is much greater than molecular ozone. This factor, taken with the absence of any halogen

constituent, has made ozone a valuable chemical in water and wastewater treatment. With

respect to wastewater treatment, the high reactivity of ozone makes it appropriate for the reaction

of chemical oxygen demand (COD).

The main process involved the consumption of air from the external environment and allowing

only oxygen to pass through a filter into an ozonator in which the oxygen was subsequently

converted into ozone which was then passed into the wastewater sample. The ozone being a very

strong oxidizing agent then reacted with the organics present within the water to reduce them to

carbon di oxide. Samples were taken at regular intervals so as to find out the time it takes to

reduce COD to desirable levels. The COD was tested using the method as explained previously

(i.e.) using a DR 2800 spectrophotometer.