Title of Invention

"A METHOD FOR TREATMENT OF BOILER CHEMICAL CLEANING WASTES"

Abstract

The invention relates to a method for treatment of Boiler Chemical Cleaning Wastes generated from chemical cleaning of Boilers. Generally, hydrochloric acid of 5% strength, with additives to remove silica and copper, is used for cleaning the boiler. After the chemical cleaning operation, the spent boiler chemical cleaning waste (BCCW) contains solubilised metals like iron and copper in addition to insoluble cruds. This is highly toxic and beyond the environmental regulation limit. The method involves treatment of the waste following the process steps of (i) Hydroxide treatment for adjustment of pH; (ii) oxidation treatment for removal of iron and copper (iii) precipitation and (iv) filtration producing a non-toxic and safe waste product.

Full Text

The invention. relates to a method for treatment of Boiler chemical cleaning wastes generated from chemical cleaning of Boilers used in electric power generation, chemical processing, refineries and other industrial plants. There are scale formations on heat-transfer surfaces of boilers reducing its efficiency. It is thus essential that the heat - transfer surfaces are maintained periodically by cleaning and removing such deposited scales.Waterside deposits on the heat exchanger surfaces can adversely affect the unit performance if they are not periodically removed. Deposition can reduce heat transfer, restrict flow, foster corrosion of the underlying metal and may ultimately result in component failure, if it becomes excessive. The predominant constituents in such deposits are iron oxides, copper and copper oxides. The common method of cleaning of boilers of such deposits is chemical cleaning so that the deposit is completely removed by the selected chemicals without damaging the base metal. Generally hydrochloric acid of 5.0% strength, with additives to remove silica and copper, is used for cleaning the boiler. After the chemical cleaning operation, the spent boiler chemical cleaning waste (BCCW) contains solubilised metals like iron and copper in addition to insoluble cruds. There are disadvantages associated with the chemical cleaning with hydrochloric acid with additives. While the process is very effective in maintaining the boiler clean the resultant spent boiler chemical cleaning waste becomes very toxic and beyond the permissible standard of environmental regulations. Patent Numbers JP5341098 and JP60093999 and disclose methods of chemical cleaning of wastes. The treatment of chemical cleaning wastes suggested by the above two methods involves electrolysis. In boiler applications, where the cleaning wastes to be treated are of the orders of 150 to 200 tons, electrolysis becomes a very very costly and practically difficult proposition. The boiler chemical cleaning waste contains low pH Value with high iron, copper and suspended solids. The tables 1 and 2 shows the limit of various elements in the Industrial metal cleaning waste and a typical composition of the boiler chemical cleaning waste, generated from solvent packages based in hydrochloric acid with additives like thio-urea, ammonium bifluoride, contain residual acid, iron chloride, copper complex salts in addition to insoluble cruds. Attempts to treat the BCCW with lime (calcium hydroxide) met with limited success. Treatment with lime can fully meet the pH requirement only. The metals, iron and copper, are precipitated only partially and the residual concentration of these metals in the treated wastes will be more than the stipulated environmental values. Therefore the main object of the present invention is to provide a suitable process for treating the boiler chemical cleaning wastes which meets the environmental guidelines for all the four parameters, i.e. pH, Copper and suspended solids. Another object of this invention is to propose a suitable process for treating boiler chemical cleaning wastes which is non-toxic and safe. A further object of this invention is to propose a suitable process for treating boiler chemical cleaning wastes wherein all iron and copper is precipitated as hydroxide and are subjected to filteration. Yet another object of this invention is to propose a suitable process for treating boiler chemical cleaning wastes wherein the filterate pH is brought to acceptable level by addition of citric acid. According to this invention, there is provided a method for treating boiler chemical cleaning wastes (BCCW) generated by chemical cleaning of boilers with hydrochloric acid having additive mixed therewith comprising draining the BCCW into treatment tank, adding calcium hydroxides, sodium hydroxide or the mixture to adjust the pH to convert said waste from acidic condition to the alkaline condition by adjusting the pH of the waste, adding hydrogen peroxide to the said alkaline waste for oxidation of said waste followed by adding diethy! dithiocarbamate for precipitating copper or iron present in the waste and then adding polyacryl amide to form FLOC therein, subjecting said flocculated waste to the step of filteration and then adding citric acid to the filterate so as to adjust the pH thereof. The nature of the invention, its objectives and further advantages residing in the same will be apparent from the following description made with reference to non-limiting exemplary embodiments of the invention and the accompanying drawings: Fig 1 is a schematic diagram of the treatment scheme of BCCW. Chemical wastes is generated while cleaning by dissolving water - side deposits from the water wan tubes in 5% (W/V) hydrochloric acid, 0.5% (W/V) copper complexor (thiourea) and 0.2% (W/V) ammonium bifluoride. Hydrochloric acid is used for the removal of iron oxides; thiourea is used to remove copper as soluble copper - thiourea complex and ammonium bifluoride is used to remove silica deposits. The tables 1 and 2 below gives the regulation effluent limits and composition of toxic cleaning waste from Boiler. Table : 1 Industrial Metal cleaning waste effluent limits Constituents Concentration pH 6-9 Suspended solids (max) 100 mg/1 Iron ( max) 5.0 mg/1 Copper ( max) 1.0 mg/1 Table : 2 Typical composition of BCCW from Hydrochloric acid Constituents Concentration pH 1.0 - 1.8 Iron 4000 - 8000 mg/1 Copper 300 - 500 mg/1 Suspended solids 100 - 300 mg/1 The process steps of the present invention for the treatment of the "laboratory - made " toxic wastes consists of the followings: (i) Hydroxide treatment (ii) Oxidation treatment (iii) Precipitation (iv) Filtration The constituents present in the treated wastes are iron (II) chloride (Fe Cl2), iron ( III) Chloride (Fe Cl3), Copper chloride ( Cu cl), copper thiourea complex and residual acid. The Table 3 below gives a typical composition of treated waste which is non toxic and meets the environmental regulation standards. Characteristics of BCCW before and after treatment (Table Removed) Generally the wastes are neutralised with either lime (calcium hydroxide) or caustic soda ( sodium hydroxide) for increasing the pH and precipitating the solubilised metals as relatively insoluble hydroxide. Instead of using lime or sodium hydroxide independently, a mixture of these two chemicals is used in the present invention for neutralisa¬ tion to pH 11.0 to 12.0 from pH 1.0, which facilitates both faster formation of precipitates as well as quick settling hydroxide precipitates in addition to neutralisation of residual acid. The fluoride is precipitated as calcium fluoride. (Equation Removed) . ...(l) (Equation Removed) .(2) Though I con II chloride and copper react in the similar way, their hydroxides are partially soluble and hence complete precipitation o:C Iron and Copper and its removal is not possible to meet the regulations. The strong alkaline pH 11.5 to 12.0 also helps the hydroly- ses of Thiourea, the copper complexer. (Equation Removed) (3) After conversion of waste from highly acidic to strong alkaline condition, the waste is subjected to oxidation process with hydrogen peroxide. Air, which is used for thorough mixing of chemicals is also useful for enhancing oxidation. The oxidation process helps the conversion of lower state of metals to their higher oxidation state and thereby precipitating the metals as insoluble Hydroxides. (Equation Removed) (4) Then reaction No. (2) is followed. The oxidation with alkalne H2O2 converts thio urea, a carcinogen to urea. {.* (Equation Removed) ..... . (5) Although effactive iron removal is achieved by hydroxide treatment, the copper will not readily precipitate as hy droxide. To achieve complete removal of copper as that of iron, an in insoluble copper complex precipitate is formed by the addi tion diethyl dithip carbamate (DADTC). (Equation Removed) (6) After the precipitation process, the flocculant poly acrylamide is added, so that easily filterable floes are formed. Then the treated wastes are filtered through sand bed for complete removal of iron and copper precipitates. Incorporation of activated carbon bed in the sandbed facilitates the removal of left over organics. The filterate has been tested and the results have been satisfactory in meeting the guidelines with respect to iron, copper and suspended solids. Finally the pH of the filterate is adjusted for the range of 6.0 to 9.0 with dilute solution of citric acid. Thus BCCW treated by the invented process are found to have acceptable pH range ( 6.0 - 9.0), iron content ( The invented process can also be used for industries producing wastes of similar nature. The neutralisation part and quantity of lime and caustic required for the treatment process depends on strength of unused acid, Iron content in the wastes and the type of treatment i.e. batch type or continuous. The quantity of H2O2 required for the oxidation treatment depends on concentration of Ferrous ion, and Thio urea in the wastes. The quantity of diethyl dithio carbanate required depends on the concentration of copper in the waste. The quantity of flocculant, polyacrylamide, is to be decided based on the characteristics of the wastes and determined individually as warranted by the environmental regulation. The quantity of the chemicals for treatment not only depends on the concentration of the metals but also on the total quantity of wastes to be treated. The design and size the treatment tank, pump, filter depends on the total quantity of the wastes to be treated, type of process namely batch or continuous. The invention described hereinabove is in relation to a non-liaiting embodiment and as defined by the accompanying claims. WE CLAIM: 1. A method for treating boiler chemical cleaning wastes (BCCW) generated by chemical cleaning of boilers with hydrochloric acid having additive mixed therewith comprising draining the BCCW into treatment tank, adding calcium hydroxides, sodium hydroxide or the mixture to adjust the pH to convert said waste from acidic condition to the alkaline condition by adjusting the pH of the waste, adding hydrogen peroxide to the said alkaline waste for oxidation of said waste followed by adding diethyl dithio carbamate for precipitating copper or iron present in the waste and then adding polyacryl amide to form FLOC therein, subjecting said flocculated waste to the step of filteration and then adding citric acid to the filterate so as to adjust the pH thereof. 2. A method as claimed in claim 1 wherein the step of oxidation is carried in the presence of air. 3. A method as claimed in claim 1 wherein said floculent comprises polyacryl amide. 4. A method as claimed in claim 1 wherein the step of filteration is carried by using sand bed having activated carbon bed disposed therein as a filter medium. 5. A method as claimed in claim 1 wherein the pH of the alkaline waste is adjusted between 11.5 to 12. 6. A method as claimed in claim 1 wherein the pH of the filtered is adjusted between 6 to 9. 7. A method for treating boiler chemical cleaning wastes substantially as herein described and illustrated.

Documents:

2406-del-1996-abstract.pdf

2406-del-1996-claims.pdf

2406-del-1996-correspondence-others.pdf

2406-del-1996-correspondence-po.pdf

2406-del-1996-description (complete).pdf

2406-del-1996-drawings.pdf

2406-del-1996-form-1.pdf

2406-del-1996-form-19.pdf

2406-del-1996-form-2.pdf

2406-del-1996-gpa.pdf