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Chemical ClarificationChemical Clarification
Precipitation MethodsPrecipitation Methods
Prepared ByMichigan Department of Environmental Quality
Operator Training and Certification Unit
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.
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.
Precipitate Formation
soluble insoluble
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
Hydroxide Precipitation
Most Soluble Heavy Metals Will PrecipitateWhen pH is Raised to a Given Point
Cu(NO3)2(aq) + 2 NaOH(aq) Cu(OH)2(s) + 2 NaNO3(aq)
pH is Raised by Addition of Hydroxide
Sodium Hydroxide, NaOH Caustic Soda
Calcium Hydroxide, Ca(OH)2 Hydrated Lime
Hydroxide Precipitation
Sodium HydroxideCaustic Soda, Soda Lye
Available in Liquid or Dry Form
Liquid:Commercial Strength
50 % , 73 % NaOHOften Stored at 20 % 50 % Crystallizes at 53 Deg. F
Dry:99.9 % NaOHBag, Drum, or BulkSafety
Caustic PoisonDangerous to Handle, CorrosivePrevent All Body ContactProtect Eyes
Calcium Hydroxide
Hydrated Lime, Slaked Lime
Available in Dry or Liquid Form
Dry Form Available in Bags or BulkCommercial Strength 82 - 95 %
SafetyCaustic, Irritant, DustyAvoid Contact with Eyes, Nose, Respiratory System
Ca(OH)2
Sodium Hydroxide
Hydrated LimeLess ExpensiveSolids Settle FasterHigher Sludge Solids ConcSludge Easier to Dewater
VS
Takes Longer to NeutralizeMore Complicated Feed SystemA Lot More Sludge
Hydroxide Precipitation
Process WaterChemicalTreatment
..
Equalization Tank Treatment Tank
Treated Water Clarifier
Filter Press
Sludge Bin
Sludge Tank
Back ToEqualization Tank
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)
Precipitation of Metals from Wastewater
The Solubility of Metals is Dependent on pH
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
Need to Determine Optimum pH for WasteMay 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
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)
Precip + Precip + Settling Settling +
FiltrationCr 0.1 to 1.0 0.05 to 0.5Cu 0.2 to 1.5 0.05 to 0.5Ni 0.5 to 2.5 0.25 to 1.5Cd 0.2 to 1.5 0.05 to 1.0Zn 0.2 to 1.5 0.05 to 1.0
Expected Effluent Quality (mg/L) for Total Metals
NaOH
Mix Tank Clarifier Filter
Hydroxide Precipitation
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
Hydroxide Precipitation of Soluble Metals
Things that Make Life More Difficult
Many Metals are Amphoteric
Double Solubility Curve pH Must beCarefully Controlled
Hydroxide Precipitation of Soluble Metals
Things that Make Life More Difficult
Many Metals are AmphotericDouble Solubility CurvepH Must be Carefully Controlled
Presence of Chelating Agents will Interferewith Ability to Precipitate Metals
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
A Big Problem in Wastewater Treatment
Commonly Used Chelating Agents
Ammonia, NH3
Phosphate
EDTA
Citrate
Cyanide
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 Sulfate3. Use a reducing agent to convert the heavy metal from the ion back to the metal
Sodium Borohydride
1. Precipitate at high pHpH > 10 (may also add CaCl2)
Chelates
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
Ni(NO3)2(aq) + Na2S(aq) --> NiS(s) + 2 NaNO3(aq)
CdCl2(aq) + Na2S(aq) --> CdS(s) + 2 NaCl(aq)
Cu(NO3)2(aq) + Na2S(aq) --> CuS(s) + 2 NaNO3(aq)
Sulfide Precipitation of Soluble Metals
Precip + Precip +Settling + Settling +Filtration Filtration
Cr 0.05 to 0.5 0.01 to 0.20Cu 0.05 to 0.5 0.01 to 0.25Ni 0.25 to 1.5 0.05 to 0.50Cd 0.05 to 1.0 0.01 to 0.25Zn 0.05 to 1.0 0.01 to 0.25
OH OH + S
Ca(OH)2
Mix Tank Clarifier FilterMix Tank
FeS
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
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
Hydroxide Precipitation of Soluble Metals
Things that Make Life More Difficult
Metals are AmphotericDouble Solubility CurvepH Must be Carefully Controlled
Presence of Chelating Agents will Interferewith 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)
Batch TreatmentUsually 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
NaO
H
NaO
H
EffluentEffluent
SludgeSludge
Continuous (Flow-Through) Treatment
Usually Used For Larger Flows
Addition of OH Controlled by pH Meter / Controller
NaOH
Storage React
FlocculationClarifier
pH
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
Continuous Treatment
Sludge is Recirculated from Sludge Hopper of Clarifier to the Influent or Reaction Tank
Sludge Recirculation
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
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.
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 metalfrom the chelate (Sulfide)
precipitated sludge
Chemical ClarificationChemical Clarification
Precipitation MethodsPrecipitation Methods
Prepared ByMichigan Department of Environmental Quality
Operator Training and Certification Unit