خطوات المعالجة الكيميائية الكهروميكلنيكية للاسطح المعدنية(النسخة الانجيليزية)

Chemical surface treatments can be grouped into four categories:
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• Pickling

- acids that remove impurities (including high temperature scale from welding or heat treatment) and etch the steel surface. 'Pickling' means some of the stainless steel surface is removed.

• Passivation

- oxidising acids or chemicals which remove impurities and enhance the chromium level on the surface.

• Chelating

agents are chemicals that can remove surface contaminants.

• Electropolishing

- electrochemical treatments that remove impurities and have the added beneficial effect of smoothing and brightening the surfaces.

• .
Pickling

Mixtures of hydrofluoric (HF) and nitric acid are the most common and are generally the most effective. Acids are available as a bath, a gel or a paste.
Commercially available mixtures contain up to about 25% nitric acid and 8% hydrofluoric acid. These chemicals etch the stainless steel which can roughen and dull the surface.

Care is required with all these chemicals because of both occupational health and safety and environmental considerations. HF is a Schedule 7 poison which has implications for sale or use in most states.

Passivation

Nitric acid is most commonly used for this purpose. Passivation treatments are available as a bath, a gel or a paste. Available formulations contain up to about 50% nitric acid and may also contain other oxidisers such as sodium dichromate. Used correctly, a nitric acid treatment should not affect the appearance of the steel although mirror polished surfaces should be tested first.

Passivation works by dissolving any carbon steel contamination from the surface of the stainless steel, and by dissolving out sulphide inclusions breaking the surface.

Nitric acid may also enrich the proportion of chromium at the surface - some chelants are also claimed to do this.

Pickling and passivation (L-R): before and after treatment of fuel tanks for storing helicopter fuel on ships. Photos courtesy of Alloy Engineers and MME Surface Finishing.

Chelants

Chelants have chemical 'claws' designed to selectively clean the surface.
The carboxylic acid group COOH is the basis for many chelants which are used in cleaners, water softening and lubricants. The pH and temperature must be correct for the chelant to do its job. Turbulent rinsing of pipes and vessels afterwards is important.

Cleaning by chelating agents tends to be based on proprietary knowledge and systems, and is less standardised than the other methods described.
The successful use of these systems needs to be established on a case by case basis.

Electropolishing

Most commonly phosphoric and sulphuric acids are used in conjunction with a high current density to clean and smooth (by metal removal) the surface of the steel.

The process preferentially attacks peaks and rounds valleys on the surface and raises the proportion of chromium at the surface.

The technique can have substantial effect on the appearance increasing lustre and brightness while only changing the measured roughness by about 30%.

Electropolishing of the example on the right effectively removed contamination including heat tint and smoothed the surface lifting lustre and reflectivity compared with the untreated example on the left.

Precautions

For chemical processes that etch the stainless steel, reaction times will increase with increasing grade.

More care is required with 'free machining' grades and these will usually require substantially less aggressive chemicals. The sulphur addition in these steels makes them readily attacked by chemical treatments. Care is also required when treating martensitic or low chromium ferritic stainless steels.

Detailed recommendations for each grade of stainless steel are given below.
The four categories of treatment are detailed in a number of Standards, but the most commonly used are:

• ASTM A380 Cleaning, Descaling and Passivation of Stainless Steel Parts, Equipment and Systems.

• ASTM A967 Chemical Passivation Treatments for Stainless Steel Parts.

• ASTM B912 Passivation of Stainless Steels using Electropolishing.

These very useful documents give detailed recommendations on many aspects of selection, application and evaluation of these treatments. Highly recommended reading.

Dirt and grease will mask the surface from treatments outlined above. Therefore, the steel surfaces must be free of these agents before applying chemical treatments.

Many of the chemical treatments described contain strong acids. Before disposal they will require neutralisation. Check with your local authority concerning the requirements for trade waste, neutralisation and disposal.

Many of the chemicals described above will be classified as hazardous substances under State OHS legislation, with implications for purchasing, transport, storage and handling.

Chemical treatments are useful tools in cost effectively achieving peak performance with stainless steels. With appropriate training, hazards associated with their use can be managed.

What is chemical treatment?

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Treatment by this method uses various chemicals to destabilize, de-emulsify, or absorb into the oil phase of a metal removal fluid, thereby allowing the water and oil phases to separate. Click for more details about the different methods of chemical treatment.

At a minimum, one storage tank, one processing tank, and one oil-sludge tank are necessary. A processing tank should be large enough to handle at least one average day of flow. If the system is being set up for continuous flow, a series of cascading tanks with at least a half- hour retention time per tank is necessary.

A small laboratory bench with a dedicated pH meter and calibrating buffer solutions, a magnetic stirring mixer, chemicals used within the process and pipettes for measuring chemical additions are all good to have on site.

Truck spill containment may be required for oil hauling pump-outs by a contract service.

What are the advantages and disadvantages of chemical treatment?
Advantages:

Energy consumption is low.

Diluted solutions are easier to separate.

Separation times can be very rapid (30 minutes).

Disadvantages:

Corrosive chemicals are required for use (sulfuric acid, sodium hydroxide).

Chemical treatment is very sensitive to changes in emulsifier (surfactant) chemistry.

Specialized instruments are required (pH meter).

Instruments require frequent calibration (pH meter).

Chemical changes and/or meter malfunctions can result in poor water quality without notice.

Balancing chemical reactions, at times, can be more an art than a science.

Synthetic fluids cannot be effectively treated by this method.

The basic concepts of this method are abstract and are not easily understood by persons without some chemistry background.

Methods of Chemical Treatment of Waste MRF

There are three basic methods:

1. Chemical splitting with polyvalent metastable salts
2. Chemical de-emulsification with polymers
3. Various combinations of #1 and #2 above

Treatment by polyvalent metastable salts

The typical metastable salts are:

Compound Type
Sodium Chloride monovalent cation
Calcium Chloride divalent cation
Magnesium Chloride divalent cation
Magnesium Sulfate divalent cation
Ferrous Sulfate divalent cation
Ferric Chloride trivalent cation
Aluminum Sulfate trivalent cation

The most common methods are:

1. Acid – alum – caustic split
2. Acid – calcium chloride – caustic split

Both of these methods use a salt, either alum (aluminum sulfate) or calcium chloride, to provide the necessary positive charges (aluminum with three positive charges per ion, as Al+++, or calcium with two positive charges per ion, as Ca++).

The concurrent addition of acid to a pH of 2.5 to 3.0 and one of the above salts helps to destabilize the emulsion. The most common and least expensive acid (per pound) to lower the pH to this level is sulfuric acid. This is due to the fact that most surfactant chemistry works best in the alkaline pH ranges of 8.0 to 13.0. The net amount of aluminum or calcium varies between 300 mg/L to 3,000 mg/L depending on the emulsion stability of the solution being treated.

After about 15 minutes of contact time with the acid and aluminum or calcium, the pH is raised (typically with sodium or calcium hydroxide) to somewhere between 5.5 and 8.5 depending on the surfactant chemistry present. If there is a sufficient amount of oil present, the resulting destabilized oil phase will gradually float, and a water-like phase will separate to the bottom.

MRF Effluent Characteristics After Sulfuric Acid, Aluminum Sulfate, Sodium Hydroxide

Chemical Method
BOD5 COD O&G pH
Fluid A 500 850 80 5.0
Fluid B 1,100 2,500 200 .5.2
Fluid C 2,000 6,500 500 5.5
Fluid D 1,200 20,000 250 5.0
Fluid E 1,500 22,000 240 5.3
Fluid F 120 28,000 110 6.0
All readings are in mg/L except pH, which is in standard units.
Treatment by Polymers

Polymers can contain metastable salts and/or complex proprietary organic chemistries that have a unique affinity for oil. The polymers draw the oil phases into their organic molecular structure, thus causing a separation. Careful polymer selection can result in very good oil separation, with very high oil/water density ratios. These oil water concentrations after polymer treatment can be as high as 80% (volume/volume).

With very stable emulsions, the addition of an acid, metastable salt, positively charged (cationic) polymer, sodium hydroxide, and a negatively charged polymer (anionic) in a series reaction may be required to produce any effective separation.

Some solutions with strong amounts of chelating compounds, such as sodium EDTA can be virtually impossible to treat by any chemical method.

Overall, these treatment methods can be done in a batch process or in a series reaction with cascading tanks, with each treatment stage in separate tanks. Separation can be enhanced with the use of fine micro-bubble air injected near the bottom of the final process tank. Dissolved air flotation is one common method to provide enhanced micro-bubble separation.

The chemical cost to treat by this method varies by the concentration of oil in the spent metal removal solution and / or the strength of the emulsifiers that are present. The chemical costs, for a 5% volume/volume spent metalworking solution can be between 0.6 and 1.2 cents per gallon. These costs can further vary by the type and amount of chelating chemistries present since they interfere with the precipitation and flocculation process.