الكيماويات المستخدمة لضبط الاس الهيدروجينى فى معالجة الصرف الصناعى والصحى(حمض الكبريتيك/حمض الهيدروليك/حمض الفوسفوريك/حمض النيتريك/هيدروكسيد الصوديوم/هيدروكسيد الكالسيوم/هيدروكسيد الماغنسيوم/هيدروكسيد الامونيوم)

Chemicals Used for pH Adjustment

by

technolab el-bahaa group

general.dr

bahaa badr

water treatment advisor

The most commonly used neutralization chemicals for acid or base neutralization are 98% Sulfuric acid and 50% Sodium hydroxide.

In many cases these are very good choices, however, there are many considerations when selecting chemicals and these may not always be the best selection.

The selection of the chemicals used for the neutralization of an acid or base is almost as important as the design of the neutralization system.

There are many considerations ranging from health and safety to cost and convenience of operation.

Some of the major points to consider in the selection of chemicals are listed below:

Health and Safety

Cost and Convenience

Storage and Location

To neutralize an acid or base a source of hydroxide ions (OH-) or hydrogen ions (H+) are required, respectively.

An acid must be neutralized with a base, which, by definition, is characterized by an excess of OH- ions. Likewise, a base must be neutralized with an acid, which, by definition, is characterized by an excess of H+ ions.

For example: In a simple neutralization process hydrochloric acid (HCl) can be neutralized by using sodium hydroxide (NaOH).

For additional information and a detailed discussion covering water and neutralization reactions we have available technical discussions covering this topic in detail.

An explanation of chemical selection criteria follows.

Health and Safety:

Whenever mixing chemicals extreme caution must be exercised. Hazardous or noxious reactions may occur.

For example: adding any acid to a cyanide bearing solution will result in the release of deadly HCN gas.

Due to the complexity of the myriad of processes that are run in industry, most of which are either proprietary or undefined, Digital Analysis usually cannot comment on suitability of an acid or base selection for the neutralization of a wastewater stream.

Although adverse reactions are very rare, the possibility exists and must be considered.

Cost and Convenience: Most acids and bases will work in most applications. Therefore the determining criteria are usually cost and convenience.

Sulfuric acid (H2SO4), for example, is less costly and more potent than nitric acid, however, if sulfuric is not currently inventoried then nitric may look more favorable because it may already be on hand.

Concentrations are also an important consideration in evaluating cost. Sulfuric acid, for example, can be purchased in concentrations ranging from near 0% up to 98%.

If a 55 gallon drum of 50% sulfuric is purchased, obviously half of that purchase cost was for water. Higher concentrations are generally less expensive.

Storage Location / Environment: The physical properties of the selected reagent must be considered carefully.

50% Sodium hydroxide (NaOH), for example, begins to freeze at temperatures below 600F. In most places throughout the country, if not all,

the possibility of caustic freezing in tanks or in pipelines is a genuine concern. Decreasing the concentration to 25% eliminates this concern altogether. Hydrochloric acid (HCl), for example, out gasses severely.

The gas is highly corrosive and will attack all metallic objects including building structures, sprinkler heads, copper wiring, stainless steel, etc.

Therefore, if HCl is used it must be properly vented or used outdoors where the gasses can easily dissipate.

The most commonly used neutralizing chemicals are listed below, Each provides a link to a brief discussion of the selected chemical:

ACIDS:

Sulfuric Acid - H2SO4

Hydrochloric Acid - HCl

Nitric Acid HNO3

Phosphoric Acid H3PO4

Carbon Dioxide CO2

BASES:

Sodium Hydroxide - NaOH

Ammonium Hydroxide - NH4OH

Magnesium Hydroxide - Mg(OH)2

Calcium Hydroxide (Lime) Ca(OH)2

موضوع: حمض الكبريتيك الأربعاء مارس 20, 2013 1:10 pm

Sulfuric Acid

There are many chemicals available on the market today that are suitable for use as neutralization chemicals.

The most commonly used chemicals are discussed in an article available here:

Neutralization Chemicals.

Sulfuric Acid H2SO4

Sulfuric Acid H2SO4: The most widely used and produced chemical in the world.

Available in concentrations ranging from 0% to 98% sulfuric is also the least expensive acid to use.

Sulfuric acid is used almost universally for neutralization reactions.

It is easier and safer to use than HCl or HNO3 and is more potent than all of the other acids except for phosphoric.

Although adverse reactions are always a possibility, they are rare.

If calcium is present then calcium sulfate (CaSO4), also known as gypsum, will precipitate.

Unless calcium is abundantly present this is not an issue.

Sulfuric acid is typically used in concentrations ranging from 25% to 96%.

Generally this is a safe and inexpensive acid to use for the neutralization of alkaline materials.

Hydrochloric Acid

There are many chemicals available on the market today that are suitable for use as neutralization chemicals.

The most commonly used chemicals are discussed in an article available here:

Neutralization Chemicals.

Hydrochloric Acid - HCl

Hydrochloric Acid (HCl): Also known as muriatic acid, HCl is the second most commonly used acid in industry (sulfuric is number one).

HCl is a very effective and a relatively inexpensive acid.

At a maximum available concentration of 37% HCl is about 1/3 as potent as sulfuric acid, further contributing to the fact that HCl is more expensive to use than sulfuric.

Depending on temperature and agitation, HCl at concentrations above 10% will evolve a hydrogen chloride vapor that forms very corrosive vapors when combined with the water vapor already in the air (humidity).

The gas is very highly corrosive and will attack all metallic objects including building structures, sprinkler heads, copper wiring, stainless steel, etc.

Therefore, if HCl is used it must be properly vented or used outdoors where the gasses can easily dissipate.

For this reason alone we generally recommend against the use of HCl.

Generally this is a safe and inexpensive acid to use for the neutralization of alkaline materials, as long as precautions are taken to handle the corrosive gasses that are evolved..

موضوع: حمض النيتريك الأربعاء مارس 20, 2013 1:14 pm

Nitric Acid

There are many chemicals available on the market today that are suitable for use as neutralization chemicals.

The most commonly used chemicals are discussed in an article available here:

Neutralization Chemicals.

Nitric Acid - HNO3

Nitric Acid (HNO3): A widely used chemical in many industries.

Nitric does not enjoy the popularity that hydrochloric or sulfuric does, as such nitric is more expensive to use than either sulfuric or hydrochloric acid.

As with hydrochloric acid nitric will evolve a noxious gas that combines with water vapor already in the air (humidity) to form a very corrosive gas.

The gas is very highly corrosive and will attack all metallic objects including building structures, sprinkler heads, copper wiring, stainless steel, etc.

Therefore, if HNO3 is used it must be properly vented or used outdoors where the gasses can easily dissipate.

For this reason alone we generally recommend against the use of HNO3.

One precaution regarding nitric acid is that it can be a strong oxidizing agent and should not be used when organic material or organic bases are present.

Phosphoric Acid

There are many chemicals available on the market today that are suitable for use as neutralization chemicals.

The most commonly used chemicals are discussed in an article available here:

Neutralization Chemicals.

Phosphoric Acid - H3PO4

Phosphoric Acid (H3PO4): Very widely used in the production of agricultural fertilizers and detergent products.

Relatively inexpensive, however it still does not compete well with sulfuric and hydrochloric acid.

Due to its disassociation constant, phosphoric is a weak acid.

Unlike sulfuric or hydrochloric it will not fully disassociate in water at normal concentrations.

This renders phosphoric safer to use than sulfuric or hydrochloric and evolution of gasses is rarely, if ever, a problem.

Due to its weak disassociation constant, phosphoric acid does not react with the normal logarithmic response as a strong acid and tends to buffer neutralization reactions.

This makes for a slower reaction that is easier to control.

Due to its cost (as compared to sulfuric) and availability, phosphoric acid is not commonly used in neutralization systems.

Generally this is a safe and inexpensive acid to use for the neutralization of alkaline materials.

Carbon Dioxide

There are many chemicals available on the market today that are suitable for use as neutralization chemicals.

The most commonly used chemicals are discussed in an article available here:

Neutralization Chemicals.

Carbon Dioxide - CO2

Carbon Dioxide (CO2): The third most concentrated gas found in earth’s atmosphere (preceded by nitrogen and oxygen) CO2 is it self not an acid.

CO2 forms carbonic acid (H2CO3) when dissolved in water; and it is carbonic acid that leads to the neutralization of alkalinity in solution.

Carbon dioxide is not easy to use and its use is limited. However, for some applications CO2 can be a very effective choice.

The most appealing feature of CO2 is that it will not lower the pH of water below 7.0 (for practical purposes).

Additionally CO2 is not corrosive as a gas, however, since CO2 is heavier than air asphyxiation is always a hazard.

Carbon dioxide can be difficult to use because the gas must be dissolved into solution to be used.

This requires the use of a carbonator, or some method to dissolve the gas into solution.

Generally a tall tank must be used to ensure that there is sufficient fluid pressure to promote the dissolution of CO2 in water.

Significant out-gassing will occur, which is not a problem unless the process also requires the settling of solids.

The architecture of a system that uses a gas neutralizing chemical is much different than that which uses a liquid chemical.

Therefore the use of CO2 should be limited to unidirectional systems only (i.e. neutralizing alkaline materials only).

Bi-directional systems using CO2 require the use of a liquid chemical for the alkaline chemical and a gas for the acidic chemical.

Because of our unique design our pH adjustment systems can all be configured to use either liquid mineral acids (such as sulfuric acid) or carbon dioxide.

The efficacy of CO2 as a neutralizing agent should be confirmed in a laboratory, or through pilot testing, before use.

family of pH adjustment systems are specifically designed to use CO2 to neutralize alkaline influent wastewater particularly wastewater from concrete runoff, hydro-demolition, and mining applications.

Sodium Hydroxide

There are many chemicals available on the market today that are suitable for use as neutralization chemicals.

The most commonly used chemicals are discussed in an article available here:

Neutralization Chemicals.

Sodium Hydroxide NaOH

Sodium Hydroxide (NaOH): Also known simply as caustic, is the most widely used alkaline neutralizing chemical in use in industry today.

Sodium hydroxide is easy to handle, inexpensive, and very effective for the neutralization of strong or weak acids.

NaOH is available in concentrations of up to 50%, which is the most commonly used concentration.

One must be careful when using 50% NaOH because of the freezing point.

NaOH at a 50% concentration will begin to freeze at temperatures below 600F.

This happens to be a very common problem and can render a system useless.

The best way to circumvent this problem is to use lower concentrations.

We generally recommend using 25% NaOH since the freezing point at this concentration is below that of water.

Caustic has a high affinity for CO2 in the atmosphere.

The absorption of CO2 results in the formation of insoluble carbonate species.

This results in the formation of solids that can be problematic for small pumps.

A lower concentration of caustic not only alleviates the problem of freezing it decreases solids formation as a result of CO2 absorption.

Caustic is more expensive than lime or magnesium hydroxide, but, due to its solubility it is much easier to handle.

Neutralization reactions form salts as the pH is brought near the end point.

Sodium salts are normally quite soluble in water.

Therefore reactions using NaOH will not normally generate high solids, unlike calcium products (lime) or magnesium products (magnesium hydroxide).

Generally this is a safe and inexpensive base to use for the neutralization of acidic materials.

Ammonium Hydroxide

There are many chemicals available on the market today that are suitable for use as neutralization chemicals.

The most commonly used chemicals are discussed in an article available here:

Neutralization Chemicals.

Ammonium Hydroxide NH4OH

Ammonium Hydroxide (NH4OH): One of the more commonly produced chemicals in the United States, ammonium hydroxide is a poor choice for a neutralizing chemical.

It is relatively expensive, hard to handle, can evolve noxious and corrosive gasses and is a relatively weak base.

Nonetheless, we still receive inquiries, from time-to-time about using NH4OH for the neutralization of acids.

Although we can and will use NH4OH as a neutralizing chemical, we generally recommend against its use.

The storage tank and the treatment tank must be well ventilated and leaks can pose many hazards.

Ammonium hydroxide is not normally a good choice for a neutralization agent for acids, due to cost and safety reasons.

Magnesium Hydroxide

There are many chemicals available on the market today that are suitable for use as neutralization chemicals.

The most commonly used chemicals are discussed in an article available here:

Neutralization Chemicals.

Magnesium Hydroxide Mg(OH)2

Magnesium Hydroxide [Mg(OH)2].

Also commonly referred to as mag, this chemical is effective in neutralizing acids and has been pushed by some chemical companies.

Despite some attempts to advertise it as such, mag is no treatment panacea.

As with lime, magnesium hydroxide is more economical to use than caustic (NaOH), however, the difference is not significant unless substantial volumes are in use.

As with lime, magnesium hydroxide is also much more difficult to handle than caustic (NaOH).

Magnesium hydroxide is relatively insoluble in water at neutral pH values and higher.

Given this, mag has little or no effect on water alkalinity above a pH of 7.0.

This means that even in a poorly designed system mag will not raise the pH above a pH of 7.0, rendering this chemical safe to use from a control point of view.

Additionally, magnesium hydroxide, also known as milk of magnesia, is quite safe to handle and is not normally toxic.

The characteristic that limits mag’s ability to raise the pH above 7.0 (solubility) also contributes to very long reaction times.

Typical reaction times for complete neutralization are in the order of 90 minutes.

This means that a single reactor tank must have the capacity of 90 minutes of flow.

In a 100 gpm system, for example, the first stage reactor must be approximately 9,000 gallons in volume.

The alternative is to discharge with an incomplete reaction.

This means that the pH will continue to change as the effluent is discharged (never favorable) and an excessive amount of mag must be used.

Also as a result if solubility the use of magnesium hydroxide will significantly increase solids loading in the effluent.

Also as a result of solubility, magnesium hydroxide is difficult to handle.

Magnesium hydroxide is a slurry that will rapidly separate from solution.

The storage tank must be constantly agitated and chemical delivery lines must be kept in motion.

Typically recirculation loops are employed with a metering valve, inline, for chemical delivery.

Static lines are not acceptable because the slurry will separate, and lines will plug, over time.

There are a few applications where magnesium hydroxide will produce favorable results, however, due to its solubility and long reaction time, magnesium hydroxide is not normally a good choice

Calcium Hydroxide -Lime-

There are many chemicals available on the market today that are suitable for use as neutralization chemicals.

The most commonly used chemicals are discussed in an article available here:

Neutralization Chemicals.

Calcium Hydroxide Ca(OH)2

Calcium Hydroxide [Ca(OH)2].

Also commonly referred to as slaked lime or hydrated lime;

calcium hydroxide is formed as a result of hydrating lime (calcium oxide, CaO). Lime is by far the most economically favorable alkaline reagent to use for acid neutralization.

Lime is significantly cheaper than caustic (NaOH), but is much more difficult to handle.

As with magnesium hydroxide, Lime is not very soluble in water.

Although the reaction times of lime are substantially less than magnesium hydroxide, lime is difficult to handle because it is handled as a slurry.

Ca(OH)2 is divalent, yielding two moles of (OH)2 for every one mole of Ca(OH)2.

When compared to caustic (NaOH), which is monovalent, twice the neutralizing power is available for a given molar volume of lime, thus contributing to the economy of lime.

As with magnesium hydroxide, lime is normally delivered in dry crystalline form.

This must then be mixed with water to form a slurry to be delivered to the process.

The ease with which caustic (sodium hydroxide) can be handled makes it far more favorable than lime, at least for low volume applications.

Lime is a slurry that will rapidly separate from solution.

The storage tank must be constantly agitated and chemical delivery lines must be kept in motion.

Typically recirculation loops are employed with a metering valve, inline, for chemical delivery.

Static lines are not acceptable because the slurry will separate, and lines will plug, over time.

Lime offers very significant advantages when the precipitation of metals or fluorides is the goal.

Calcium salts are normally quite insoluble, and due to the fact that lime is divalent, sludge densities are normally much higher than those formed with caustic (NaOH).

Lime is an excellent choice for acid neutralization.

If volumes are relatively low, and precipitation of metal or fluoride ions is not paramount, then caustic (NaOH) may be the better choice because it is easier to handle.

Neutralization of Sulfuric Acid (H2SO4)

For one mole (98 lbs) of 100% Sulfuric Acid to be neutralized, the following chemical reactions occur:

AQUAMAG® Mg(OH)2 + H2SO4 → MgSO4 + 2H2O
(58.3 lbs) (98 lbs) (120.3 lbs) (36 lbs

Hydrated Lime Ca(OH)2 + H2SO4 → CaSO4• 2H2O ⇓ *

(74 lbs) (98 lbs) (172 lbs) (36 lbs)

Caustic Soda 2NaOH + H2SO4 → Na2SO4 + 2H2O
(80 lbs) (98 lbs) (142 lbs) (36 lbs)

Soda Ash Na2CO3 + H2SO4 → Na2SO4 + CO2 ⇑ + H2O
(106 lbs) (98 lbs) (142 lbs) (44 lbs) (18 lbs)

Caustic Potash 2KOH + H2SO4 → K2SO4 + 2H2O
(112 lbs) (98 lbs) (174 lbs) (36 lbs)

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