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Chemical Clarification Precipitation Methods

Chemical Clarification Precipitation Methods. Prepared By Michigan Department of Environmental Quality Operator Training and Certification Unit. Some things (chemists use term compounds) dissolve in water – these are said to be soluble.

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Chemical Clarification Precipitation Methods

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  1. Chemical Clarification Precipitation Methods Prepared By Michigan Department of Environmental Quality Operator Training and Certification Unit

  2. Some things (chemists use term compounds) dissolve in water – these are said to be soluble. Some things (compounds) don’t dissolve in water – these are said to be insoluble.

  3. A precipitation reaction is a reaction in which soluble ions in separate solutions are mixed together to form an insoluble compound that settles out of solution as a solid. That insoluble compound is called a precipitate.

  4. Precipitate Formation solubleinsoluble

  5. Precipitation of Metals from Wastewater Metals Precipitation Transforms dissolved contaminants into an insoluble solid, facilitating the contaminant's subsequent removal from the liquid phase by sedimentation and / or filtration. Usually accomplished by: • pH Adjustment • Chemical Coagulation • Flocculation • Separation

  6. pH is Raised by Addition of Hydroxide Cu(NO3)2(aq) + 2 NaOH(aq) Cu(OH)2(s) + 2 NaNO3(aq) Hydroxide Precipitation Most Soluble Heavy Metals Will Precipitate When pH is Raised to a Given Point

  7. Hydroxide Precipitation Sodium Hydroxide, NaOH Caustic Soda Calcium Hydroxide, Ca(OH)2 Hydrated Lime

  8. Caustic Soda, Soda Lye Available in Liquid or Dry Form Sodium Hydroxide Liquid: Commercial Strength 50 % , 73 % NaOH Often Stored at 20 % 50 % Crystallizes at 53 Deg. F Dry: 99.9 % NaOH Bag, Drum, or Bulk Safety Caustic Poison Dangerous to Handle, Corrosive Prevent All Body Contact Protect Eyes

  9. Calcium Hydroxide Ca(OH)2 Hydrated Lime, Slaked Lime Available in Dry or Liquid Form Dry Form Available in Bags or Bulk Commercial Strength 82 - 95 % Safety Caustic, Irritant, Dusty Avoid Contact with Eyes, Nose, Respiratory System

  10. Hydroxide Precipitation Sodium Hydroxide VS Hydrated Lime Less Expensive Solids Settle Faster Higher Sludge Solids Conc Sludge Easier to Dewater Takes Longer to Neutralize More Complicated Feed System A Lot More Sludge

  11. Chemical Treatment Process Water .. Equalization Tank Treatment Tank Treated Water Clarifier Filter Press Back To Equalization Tank Sludge Tank Sludge Bin

  12. Cu(NO3)2(aq) + 2 NaOH(aq) --> Cu(OH)2(s) + 2 NaNO3(aq) Ni(NO3)2(aq) + 2 NaOH(aq) --> Ni(OH)2(s) + 2 NaNO3(aq) 2 AgNO3(aq) + 2 NaOH(aq) --> Ag2O(s) + 2 NaNO3(aq) + H2O(l) Hydroxide Precipitation of Soluble Metals

  13. Precipitation of Metals from Wastewater The Solubility of Metals is Dependent on pH

  14. Hydroxide Precipitation From Chart (“Book Value”) Minimum Solubility (Best Removal) Value for Some Metals Cadmium pH 11.0 Copper pH 8.1 Chromium pH 7.5 Nickel pH 10.8 Zinc pH 10.1

  15. Hydroxide Precipitation Need to Determine Optimum pH for Waste May or May Not be Close to Book Value Metals Precipitate at Various pH Levels Depending On: The Metal Being Precipitated The Chemistry of the Wastewater Other Metals Present Presence of Chelates

  16. Metal Precipitation Precipitation of more than one metal from the same solution: Adjust for Optimum pH for One Metal (With Lowest Discharge Permit Limit) Adjust for Compromise pH Operate in Stages Adjust pH for Each Metal in Stages Draw Off Sludge, or Transfer Supernatant Between Stages

  17. NaOH Mix Tank Clarifier Filter Hydroxide Precipitation Expected Effluent Quality (mg/L) for Total Metals Precip + Precip + Settling Settling + Filtration Cr 0.1 to 1.0 0.05 to 0.5 Cu 0.2 to 1.5 0.05 to 0.5 Ni 0.5 to 2.5 0.25 to 1.5 Cd 0.2 to 1.5 0.05 to 1.0 Zn 0.2 to 1.5 0.05 to 1.0

  18. Hydroxide Precipitation of Soluble Metals Things that Make Life More Difficult Many Metals are Amphoteric Metal hydroxides that re-solubilize if the pH increases or decreases from the optimum precipitation value

  19. Hydroxide Precipitation of Soluble Metals Things that Make Life More Difficult Many Metals are Amphoteric Double Solubility Curve pH Must be Carefully Controlled

  20. Hydroxide Precipitation of Soluble Metals Things that Make Life More Difficult Many Metals are Amphoteric Double Solubility Curve pH Must be Carefully Controlled Presence of Chelating Agents will Interfere with Ability to Precipitate Metals

  21. ChelatesCan’t Live With ‘em, Can’t Do Without ‘em An agent that will form a compound with a heavy metal ion, with the intended purpose of keeping the metal in solution. Highly Desired in Plating Solutions ABigProblem in Wastewater Treatment

  22. Commonly Used Chelating Agents Ammonia, NH3 Phosphate EDTA Citrate Cyanide

  23. Chelates Methods of Addressing the Problem 1. Precipitate at high pH pH > 10 (may also add CaCl2) 2. Add a material which the chelate prefers over the metal to be precipitated. Ferrous Sulfate 3. Use a reducing agent to convert the heavy metal from the ion back to the metal Sodium Borohydride • Use a precipitating chemical that steals the metal from the chelate • Sulfide • Sodium Dimethyldithiocarbamate (DTC)

  24. Sulfide Precipitation of Soluble Metals Extremely Low Metal Solubilities Often Preferred for Wastewaters Containing Chelating Agents More Negative Environmental Impacts More Costly (About Twice $ as for OH) May Produce H2S in Lethal Concentrations

  25. Cu(NO3)2(aq) + Na2S(aq) --> CuS(s) + 2 NaNO3(aq) CdCl2(aq) + Na2S(aq) --> CdS(s) + 2 NaCl(aq) Ni(NO3)2(aq) + Na2S(aq) --> NiS(s) + 2 NaNO3(aq) Sulfide Precipitation of Soluble Metals

  26. Ca(OH)2 FeS Mix Tank Mix Tank Clarifier Filter OH OH + S Precip + Precip + Settling + Settling + Filtration Filtration Cr 0.05 to 0.5 0.01 to 0.20 Cu 0.05 to 0.5 0.01 to 0.25 Ni 0.25 to 1.5 0.05 to 0.50 Cd 0.05 to 1.0 0.01 to 0.25 Zn 0.05 to 1.0 0.01 to 0.25

  27. Sodium Dimethyldithiocarbamate (DTC or SDDC) Organic sulfur compound Controls chelated waste streams Not pH sensitive – any pH above 3 Non amphoteric Very toxic Agricultural insecticide Industrial biocide

  28. Sodium polythiocarbonate (PTC) Thio-Red Much less toxic than DTC 50% less sludge than DTC, lime or iron chemistries; Requires lower dosing of flocculent chemistry for liquid/solid separation Treated waters pass toxicity tests for National Pollutant Discharge Elimination System (NPDES) discharges

  29. Hydroxide Precipitation of Soluble Metals Things that Make Life More Difficult Metals are Amphoteric Double Solubility Curve pH Must be Carefully Controlled Presence of Chelating Agents will Interfere with Ability to Precipitate Metals Hydroxide Sludges May be Difficult to Dewater Metal Finishing Sludge is Classified as a Hazardous Waste by Federal & State Law (RCRA)

  30. Usually Preferred for Small Flows Either a Holding Tank and Reaction Tank, or Two Reaction Tanks Reaction Tanks Serve Also as Clarifiers Allows for Maximum Control of Effluent Quality Batch Treatment Influent Wastewater NaOH NaOH Effluent Effluent Sludge Sludge

  31. Continuous (Flow-Through) Treatment Usually Used For Larger Flows Addition of OH Controlled by pH Meter / Controller NaOH pH React Storage Clarifier Flocculation

  32. Sludge Recirculation Precipitated Solids Act as Seed Structure for Forming Precipitate Batch Treatment Portion of Settled Sludge Left on Tank Bottom that Re-suspends When Mixing For New Batch Begins

  33. Sludge Recirculation Continuous Treatment Sludge is Recirculated from Sludge Hopper of Clarifier to the Influent or Reaction Tank

  34. Review – Metal Precipitation 1 Sodium Hydroxide is hazardous to handle in that it is both _____________ and ______________. toxic corrosive 2. List three options for precipitating two metals with different optimum pH. a. b. c. Optimize for One Metal Find a Compromise pH Operate as a Two-Stage System

  35. 3. Metal hydroxides that resolubilize if the pH increases or decreases from the optimum precipitation value are said to be __________________. amphoteric 4. Ammonia, phosphates, and EDTA are _______________agents used in plating solutions. 5.Describe the problem that may occur when precipitating metals from wastewater where the materials described in #4 are present. chelating Chelates hold metals in solution, not allowing them to precipitate.

  36. 6. List 3 ways a wastewater treatment plant operator may be able to overcome the problem in #5 that is caused by the materials listed in #4. a. b. c. 7. Recirculation of _________________ may be beneficial in that it provides “seed” material for the precipitation process. Precipitate at high pH Add a metal to tie up chelate Add a chemical that steals metal from the chelate (Sulfide) precipitated sludge

  37. Chemical Clarification Precipitation Methods Prepared By Michigan Department of Environmental Quality Operator Training and Certification Unit

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