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

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Presentation on theme: "Chemical Clarification Precipitation Methods Prepared By Michigan Department of Environmental Quality Operator Training and Certification Unit."— Presentation transcript:

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 soluble insoluble

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 Hydroxide Precipitation Most Soluble Heavy Metals Will Precipitate When pH is Raised to a Given Point Cu(NO 3 ) 2 (aq) + 2 NaOH(aq) Cu(OH) 2 (s) + 2 NaNO 3 (aq) pH is Raised by Addition of Hydroxide

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

8 Sodium Hydroxide Caustic Soda, Soda Lye Available in Liquid or Dry Form 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 Hydrated Lime, Slaked Lime Available in Dry or Liquid Form Dry Form Available in Bags or Bulk Commercial Strength % Safety Caustic, Irritant, Dusty Avoid Contact with Eyes, Nose, Respiratory System Ca(OH) 2

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

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

12 Cu(NO 3 ) 2 (aq) + 2 NaOH(aq) --> Cu(OH) 2 (s) + 2 NaNO 3 (aq) Ni(NO 3 ) 2 (aq) + 2 NaOH(aq) --> Ni(OH) 2 (s) + 2 NaNO 3 (aq) 2 AgNO 3 (aq) + 2 NaOH(aq) --> Ag 2 O(s) + 2 NaNO 3 (aq) + H 2 O(l)

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

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

15 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 Hydroxide Precipitation

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

17 Precip +Precip + SettlingSettling + Filtration Cr0.1 to to 0.5 Cu0.2 to to 0.5 Ni0.5 to to 1.5 Cd0.2 to to 1.0 Zn0.2 to to 1.0 Expected Effluent Quality (mg/L) for Total Metals NaOH Mix TankClarifierFilter Hydroxide Precipitation

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 Chelates Can’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 A Big Problem in Wastewater Treatment

22 Commonly Used Chelating Agents Ammonia, NH 3 Phosphate EDTA Citrate Cyanide

23 Methods of Addressing the Problem 4.Use a precipitating chemical that steals the metal from the chelate Sulfide Sodium Dimethyldithiocarbamate (DTC) 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 1. Precipitate at high pH pH > 10 (may also add CaCl 2 ) Chelates

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 H 2 S in Lethal Concentrations

25 Ni(NO 3 ) 2 (aq) + Na 2 S(aq) --> NiS(s) + 2 NaNO 3 (aq) CdCl 2 (aq) + Na 2 S(aq) --> CdS(s) + 2 NaCl(aq) Cu(NO3) 2 (aq) + Na 2 S(aq) --> CuS(s) + 2 NaNO 3 (aq) Sulfide Precipitation of Soluble Metals

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27 Precip +Settling +Filtration Cr0.05 to to 0.20 Cu0.05 to to 0.25 Ni0.25 to to 0.50 Cd0.05 to to 0.25 Zn0.05 to to 0.25 OHOH + S Ca(OH) 2 Mix Tank ClarifierFilterMix Tank FeS

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

29 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

30 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)

31 Batch Treatment 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 Influent Wastewater NaOH Effluent Sludge

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

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

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

35 Review – Metal Precipitation 1 Sodium Hydroxide is hazardous to handle in that it is both _____________ and ______________. toxiccorrosive 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

36 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.

37 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

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


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