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