In this article we will discuss about the methods and techniques used to control liquid waste from electronic industries. Learn about:- 1. Preliminary Planning 2. Treatment Systems 3. Effectiveness of Treatment.
Preliminary Planning:
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Many of the chemicals used for microelectronic processes eventually end up as a waterborne waste problem. There is a large demand for soft water and deionised water in microelectronics. At present time zeolite softening techniques are employed using NaCl to regenerate the zeolite material.
The waste from the regeneration process contains high concentrations of soluble salts requiring proper disposal. The deionisation process utilises both strong alkali and strong acid in the regeneration of the ion-exchange resins. This regeneration process is relatively inefficient and high concentrations of soluble salts result in the effluent.
Etching processes produce waste that presents unique waste treatment problems. Etching consists of the removal of silicon, gold, copper and other metals from the various substrates and the etchant employed must be capable of dissolving the metals and maintaining the metals in solution.
The removal of the metal from the waste stream becomes very difficult because the primary purpose of the etchant is to maintain the metal in solution. Conversely the treatment process must be capable of reversing or neutralising the basic action of the etchant.
There are two waste streams generated from these processes—concentrated spent etchant and dilute rinse water containing a low concentration of the etchant. In some cases the concentrated waste can be considered for recovery of both etchants and metals to avoid a waste disposal problem. The dilute waste is acidic and contains a variable quantity of metal and acid depending on the individual operations.
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The cleaning or metal preparation waste consists of both acid and alkali materials and cyanide compounds. The spent concentrated cleaning bath must be disposed of periodically as well as the continuous flow of dilute rinse water following each cleaning operation.
The plating or metal finishing techniques employed by a specific shop can be numerous and widely different in the toxic wastes produced. Cyanide is used in many plating operations (copper, zinc, cadmium, gold, etc.).
Nickel and chrome plating are also widely used in microelectronics shops. The waterborne waste from these operations is limited to the diluted rinse water following the plating operations and the volume is variable because of the different rinsing techniques employed by each shop.
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The various process wastes must be segregated to allow for treatment by the different disposal techniques. For instance, deadly poisonous cyanide gas would be liberated if cyanide waste and acid wastes were mixed in the same sewer system.
Thus, the treatment techniques and the effluents dictate the complexity of the sewer collection system as well as the type of construction materials required for corrosion resistance.
For a typical microelectronics factory separate sewer systems may be installed for:
(i) Concentrated acids,
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(ii) Concentrated alkali,
(iii) Diluted acid-alkali,
(iv) Concentrated cyanide,
(v) Diluted cyanide, and
(vi) Chrome-wastes.
The construction material may vary from steel pipe for certain types of cyanide effluents to various reinforced plastics for more corrosive wastes. Unplasticised polyvinyl chloride (PVC) is probably the most common construction material used for corrosive wastes at temperatures less than 150°F.
The first step in the design of the waste treatment plant is the determination of the requirements established for the receiving stream or municipal sewer. These requirements are established by the regulatory agency having jurisdiction at the point of discharge of the waste and are set by a municipal sewer ordinance or the stream standards of a state agency.
The Water Quality Control Act of 1965 has required each state to establish stream standards for all interstate waters. These standards must be established to protect the present use of the stream as well as anticipated future use.
In some streams a zoning type approach has been applied where the upstream water quality requirement are extremely high and where a certain degree of pollution is to be tolerated in the lower or tidal reaches of the stream.
This necessitates that a high degree of treatments be employed. Stream standards also establish the effluent requirement for the municipal sewage treatment systems; thus, municipalities must enact appropriate sewer use codes to comply with these standards.
Table 22.1 presents typical limits for waste discharge to a municipal sewer system. Biological treatment processes are not capable of complete removal of the metals and therefore, a municipality must limit the concentration in order to comply with stream standards.
The regulatory agency having jurisdiction may have some specific requirement in regard to installation of industrial waste treatment systems. In some areas the agency may require a batch treatment in lieu of a flow through type system, back-up chemical feed equipment or duplicate instrumentation.
In addition, there may be regulations regarding detention time or the furnishing of metering and sampling facilities that must be determined prior to preparation of plans and specifications.
Waste Characteristics:
The next step is the determination of the actual quantity and quality of the waste by a complete inventory of all operations. This inventory includes an estimate of work flow through the shop, concentration of cleaning and plating baths, estimated rinse water flow, estimated drag-out, dump schedules of concentrated plating and cleaning tanks and other data. This information will determine the sewer system required to adequately handle the waste.
At this point consideration should be given to possible recovery of the chemicals, metals and water to reduce the amount of wastes to be treated. Many recovery procedures are well established and have recovered the associated capital investment in a relatively short time period.
Ion-exchange techniques for the recovery of specific metals such as chrome or gold and the use of evaporation facilities for recovery of chrome or cyanide solutions have been employed for many years. Similarly, recovery techniques for waste etchants from printed circuit board operations can be instituted to recover the large quantity of metal and etchant that often are wasted.
A separate recovery system would be required for each specific process to avoid contamination by different process effluents being served by the same recovery unit. Nonetheless, a waste treatment system is usually required because recovery systems will service only a part of the factory effluents.
Treatment Systems:
Design concept compliance with affluent code mandates the need for automatic controls with each treatment system. These controls pace the addition of chemicals to ensure complete treatment in the most economical manner.
Due to the variability of the waste a manual control system would cause fluctuation between over-treatment and incomplete treatment, with over-treatment usually predominating. Automatic control systems are of proven quality to produce the desired effluent characteristics.
The selection of either a batch type or a flow-through system is dependent on the waste flow unless the regulatory agency so specifies a required system. If the waste flow is greater than 50 gpm a batch treatment system is usually not economical since the tankage becomes excessive.
A batch treatment system requires a minimum of three duplicate systems to provide maximum flexibility of operation; one tank being filled, one tank being treated and the third tank being emptied. The flow-through or continuous system is used for the large flows but must always be preceded by an equalisation and surge tank.
This process is basically concerned with acidic and alkaline solutions and elimination of most of their acidic or alkaline characteristics. Acids and alkalies are the converse of each other; the hydrogen ions (low pH) of an acid will react with the hydroxyl ions (high pH) of an alkaline solution to form water.
The acid anion (mostly the chloride or sulphate ions) and the alkali cation (mostly the sodium ion) form a salt which normally remains in solution. Table 22.2 shows the basic equations involved in the pH adjustment process.
It is desirable to collect the strong acid waste and the strong alkali waste separate from the dilute waste in order to make the pH adjustment process controllable. These strong wastes are then pumped at a constant rate to the treatment system. The best features of the neutralisation process with optimum pH adjustment control are achieved with a continuous flow-through system.
Three tanks are used with each tank furnished with agitation capability. The initial pH adjustment should be made in the first tank while the final adjustment would be made in the third tank. Strong acid and strong alkali wastes are mixed with the dilute waste just ahead of the first neutralisation tank. (For a batch type treatment system the neutralisation tanks should be designed to mix the waste and neutralising agent effectively to make the process controllable.)
The system should be as fully automatic as is practical and be provided with chemical feed equipment and the instrumentation needed to continually control additions of the neutralising agent.
The treatment of acid waste is recommended to normally maintain a pH of zero mineral acidity in the first stage of neutralisation and a pH in the second stage of neutralisation as required in the subsequent combined waste treatment system.
The choice of the alkali to be used for neutralisation is dependent on local conditions and the actual requirements of the alkali. Being low in cost, lime is preferred for waste which requires extensive treatment.
Caustic soda is employed for waste having a relatively low treatment requirement because of its relative ease in handling. Lime is the currently accepted alkali used for removal of fluoride bearing wastes.
The partially soluble CaF2 is formed and removed as sludge, its solubility in water being 7 mg/l. A better method of fluoride removal is an important need of the microelectronics industry, since hydrogen fluoride (HF) is a commonly used etchant.
A continuous flow-through type system consists of a reaction tank furnished with agitation and facilities to control the addition of acid and the reducing agent followed by a retention chamber to ensure the complete reduction of chrome.
Preferably the trivalent chromium should not be precipitated in this system since the effluent will be discharged directly to the combined waste treatment system for further treatment. The reduction of chrome can be accomplished using either sulphur dioxide gas, one of the sodium salts of sulphur dioxide or ferrous sulphate as the reducing agent.
The pH of the waste can be maintained at the proper level with acids usually sulphuric acid. The reduction with ferrous sulphate can be accomplished over a wide pH range, but this process produces an excessive quantity of an iron bearing sludge.
The use of sulphur dioxide or one of its sodium salts reacts at a pH range of approximately 2.0 to 3.0 and produces a waste containing a minimum quantity of sludge.
The system should be provided with chemical feed equipment and instrumentation to continually control the process. The addition of the reducing agent is controlled to maintain a sulphite-trivalent chromium oxidation-reduction potential level, i.e. a small sulphite residual that ensures the completion of the reduction reaction. The instrumentation also controls the rate of acid additions to maintain a pH suitable for the reaction.
There are several oxidation methods that have been employed for cyanide destruction:
(i) Biological,
(ii) Chemical,
(iii) Electrolytic,
(iv) Incineration, and
(v) Radiation.
The chemical method employing the alkaline chlorination procedure is the most widely employed because:
(i) The method lends itself to automatic control,
(ii) The process can be controlled to stop the reaction at the cyanate level, and
(iii) The process ensures 100 per cent destruction of the cyanide.
This process is basically concerned with the oxidation of cyanide to carbon dioxide and nitrogen in an alkaline environment using chlorine as the oxidising agent in two stages of treatment.
In the first stage, the chlorine oxidises the cyanide to cyanogen chloride and this is then converted to cyanate. In the second stage the chlorine oxidises the cyanate to nitrogen and carbon dioxide. Table 22.3 presents the basic equation of the oxidation process.
In the first stage of treatment chlorine reacts with the cyanide radical (CN) at any pH to form cyanogen chloride (CNCl). This compound is so volatile and toxic that it is essential that it be converted by being hydrolysed to the non-volatile and less toxic cyanate as quickly as possible.
It is important to complete such conversion before the second stage reaction occurs because the release of the end-product gases, carbon dioxide and nitrogen, in the second stage reaction tends to increase the liberation of cyanogen chloride.
The rate of conversion of the cyanogen chloride to cyanate is dependent upon the pH of the waste, its conversion or hydrolysis being practically nil at pH 7.5 or less, fairly rapid at pH 8.0-8.5 quite rapid at pH 9.0-9.5 and exceedingly rapid (a matter of minutes) at a pH of 10 or above.
In the second stage of treatment chlorine reacts with the cyanate radical (CNO) to produce nitrogen gas and carbon dioxide. Part of the carbon dioxide reacts with carbonate or hydroxyl alkalinity to produce bicarbonates. The oxidation reaction is infinitely slow at a pH above 10, becomes increasingly rapid as the pH drops below 10 and reaches an optimum at pH 7.5.
Figures 22.3 and 22.4 illustrate flow-through and batch type treatment systems for cyanide destruction. The concentrated and the dilute wastes should be segregated so that the former can be pumped at a controlled rate together with the later to the treatment system.
The oxidation of the cyanide can be accomplished using either liquid chlorine or a hypochlorite solution as the source of chlorine. The pH of the waste must be maintained at the proper level in each stage of the process by the use of either acid or alkali.
Figure 22.3 illustrates a re-circulated flow-through system designed to perform the oxidation in two separate stages and consists of two sets of reaction tanks placed in series. The first tank will be employed for the first stage of oxidation, cyanide to cyanate, while the second series of tanks provides for the second stage of oxidation, cyanate to nitrogen and carbon dioxide.
Positive displacement or retention tanks should be installed following each stage of treatment to ensure the completion of the reactions prior to the discharge of the waste to the next stage of treatment.
The system should be provided with chemical feed equipment and instrumentation to control the system automatically.
The rate of chlorine application is controlled by an oxidation reduction potential (ORP) instrument to maintain a chloramine-cyanate level, i.e. a small chloramine residual in the first stage of treatment. Another ORP instrument controls the rate of chlorine application to the second stage to maintain a free chlorine (ORP) level, i.e. a small free chlorine residual in the second stage of treatment. Instruments should also be provided to automatically control the pH in each stage of treatment.
Figure 22.4 illustrates a batch type treatment system designed to perform the complete oxidation of cyanide. The system is a stepwise titration of the cyanide with chlorine to produce the desired results.
The titration is controlled automatically by ORP and pH instrumentation to attain the desired results. The first and second stages should be separated by the proper control of the pH. The flow diagram also shows the instrumentation required to automatically fill and empty the treatment tank.
Combined Waste Treatment:
The removal of toxic heavy metals is common to all three waste treatment processes. The metals are present as insoluble hydrous compounds, oxides and hydroxides in a finely divided particulate suspension.
Table 22.4 shows the general equations using lime to convert any remaining metal ions into hydroxides preparatory to coagulation. It is preferable to remove metals in a common treatment system consisting of coagulation and flocculation equipment followed by a high rate solids removal unit.
The three treated waste streams should be combined in a rapid mix tank for addition of coagulants and final pH adjustment as shown in Fig. 22.5. The metals are then completely precipitated from the solution to the point of minimum solubility or maximum insolubility.
The pH range at this point in the processing is limited by the narrow range over which the hydroxides of trivalent chromium and zinc are more or less totally insoluble. This pH is of the order of 8.2-9.2 with an optimum of about 8.7. In deference to the most favourable pH for the use of ferric iron salts as a coagulant the pH should never be less than 9.0.
Referring to Fig. 22.5, the properly flocculated waste will pass downward through the sludge blanket in the bottom of the reaction well (the center section of the solids removal unit) and then flow upward through the sludge blanket in the solids removal compartment (the outer periphery).
The flow-through the sludge blanket affords the final clarification. As the waste flows upward to the overflow flume it will be continually entering an ever widening area, causing the flow velocity to be progressively reduced.
The decreasing velocity will cause the remaining floe particles to separate from the waste stream and fall back into the sludge blanket. The clarified effluent will flow over V-notch weirs into the collecting flume and then by gravity to the effluent meter pit for flow measurement and final disposal.
The sludge drying beds, centrifuges and rotary drum vacuum filters have been utilised to concentrate the sludge. Each of those can produce a relatively dry sludge having the consistency of wet clay suitable for landfill.
Effectiveness of Treatment:
The installation of the properly designed treatment processes and the proper operation will provide an effluent having the following characteristics:
1. One that is free of hexavalent and trivalent chromium.
2. One that contains no cyanides except the iron cyanide complexes (the alkaline chlorination process does not destroy the less toxic iron cyanide complexes; it converts the ferrocyanide to ferricyanide).
3. One that has a pH of about 9.0 required for the efficient removal of the metal hydroxides (a final pH adjustment tank can be installed to maintain any required pH in the effluent).
4. One that will contain on the average not more than 5 mg/l, of suspended solids, mostly insoluble metallic compounds whose total metallic content is about 2.5 mg/l and consisting of a mixture of all the various metals initially present in the raw wastes.