wrd ot chemical clarification metals precipitation 445210 7

8/18/2019 Wrd Ot Chemical Clarification Metals Precipitation 445210 7

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Chemical ClarificationChemical Clarification

Precipitation MethodsPrecipitation Methods

Prepared By

Michigan Department of Environmental Quality

Operator Training and Certification Unit


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Some things (chemists use term

compounds) dissolve in water –these are said to be soluble.

Some things (compounds) don’t dissolvein water – these are said to be insoluble.


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A precipitation reaction is areaction in which soluble ions in

separate solutions are miedtogether to form an insoluble compound that settles out ofsolution as a solid. !hat insoluble

compound is called a precipitate.


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Precipitate "ormation

soluble insoluble


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Precipitation of Metals from Wastewater

Metals Precipitation!ransforms dissolved contaminants into an insolublesolid# facilitating the contaminant$s subse%uentremoval from the li%uid phase b& sedimentation and 'or filtration.

suall& accomplished b&•p* Ad+ustment•Chemical Coagulation•"locculation

•Separation


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Hydroxide Precipitation

Most Soluble *eav& Metals ,ill Precipitate,hen p* is -aised to a iven Point

Cu(/01)2(a%) 3 2 /a0*(a%) Cu(0*)2(s)

3 2 /a/01(a%)

p* is -aised b& Addition of *&droide


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Sodium *&droide#/a0*Caustic Soda

Calcium *&droide#Ca(0*)2

*&drated 4ime



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Sodium *&droideCaustic Soda# Soda 4&e

Available in 4i%uid or 5r& "orm

4i%uidCommercial Strength

67 8 # 91 8 /a0*

0ften Stored at 27 867 8 Cr&stalli:es at 61 5eg. "

5r&;;.; 8 /a0*


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Calcium *&droide

*&drated 4ime# Sla=ed 4imeAvailable in 5r& or 4i%uid "orm

5r& "orm Available in


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Sodium *&droide

*&drated 4ime

4ess >pensiveSolids Settle "aster *igher Sludge Solids Conc

Sludge >asier to 5ewater

BS

!a=es 4onger to /eutrali:eMore Complicated "eed S&stem

A 4ot More Sludge



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Process ,ater Chemical!reatment

..

>%uali:ation !an= !reatment !an=

!reated ,ater Clarifier

"ilter Press

Sludge


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Cu(NO3)2(a) ! 2 NaOH(a) ""# Cu(OH)2(s) ! 2 NaNO3(a)

Ni(NO3)2(a) ! 2 NaOH(a) ""# Ni(OH)2(s) ! 2 NaNO3(a)

2 $%NO3(a) ! 2 NaOH(a) ""# $%2O(s) ! 2 NaNO3(a) ! H2O(l)


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Precipitation of Metals from Wastewater

!he Solubilit& of Metals is 5ependent on p*


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Cadmiump* .7

Copperp* ?.

Chromiump* 9.6

/ic=elp* 7.?

Dincp* 7.

"rom Chart (E


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/eed to 5etermine 0ptimum p* for ,aste

Ma& or Ma& /ot be Close to


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Metal Precipitation

Precipitation of more than one metal from the

same solution

0perate in Stages

Ad+ust p* for >ach Metal in Stages

5raw 0ff Sludge# or !ransfer

Supernatant


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Precip 3 Precip 3 Settling Settling 3

"iltrationCr 7. to .7 7.76 to 7.6

Cu 7.2 to .6 7.76 to 7.6/i 7.6 to 2.6 7.26 to .6Cd 7.2 to .6 7.76 to .7Dn 7.2 to .6 7.76 to .7

&xpected &ffluent 'uality (m%) for *otal Metals

NaOH

Mix *an+ Clarifier ,ilter



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*&droide Precipitation of Soluble Metals

!hings that Ma=e 4ife More 5ifficultMan& Metals are

Amphoteric

Metal h&droides thatre@solubili:e if the p*

increases or decreasesfrom the optimumprecipitation value


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!hings that Ma=e 4ife More 5ifficultMan& Metals are

Amphoteric

5ouble Solubilit& Curve

p* Must be

Carefull& Controlled


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!hings that Ma=e 4ife More 5ifficult

Man& Metals are Amphoteric

5ouble Solubilit& Curvep* Must be Carefull& Controlled

Presence of Chelating Agents will nterfere

with Abilit& to Precipitate Metals


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ChelatesCan’t 4ive ,ith Gem# Can’t 5o ,ithout Gem

An agent that will form a compound with a heav&metal ion# with the intended purpose of =eeping

the metal in solution.

*ighl& 5esired in Plating Solutions

A


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Commonl& sed Chelating Agents

Ammonia# /*1

Phosphate

>5!A

Citrate

C&anide


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Methods of Addressing the Problem

H. se a precipitating chemical that stealsthe metal from the chelate

• Sulfide

• Sodium 5imeth&ldithiocarbamate (5!C)

2. Add a material which the chelate prefers over the metal to be precipitated.

"errous Sulfate1. se a reducing agent to convert theheav& metal from the ion bac= to themetal

Sodium


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Sulfide Precipitation of Soluble Metals

>tremel& 4ow Metal Solubilities

0ften Preferred for ,astewaters

Containing Chelating Agents

More /egative >nvironmental mpacts

More Costl& (About !wice J as for 0*)

Ma& Produce *2S in 4ethal Concentrations


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/i(/01)2(a%) 3 /a2S(a%) @@I /iS(s) 3 2 /a/01(a%)

CdCl2(a%) 3 /a2S(a%) @@I CdS(s) 3 2 /aCl(a%)

Cu(/01)2(a%) 3 /a2S(a%) @@I CuS(s) 3 2 /a/01(a%)

Sulfide Precipitation of Soluble Metals


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C (0*)


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Precip 3 Precip 3Settling 3 Settling 3"iltration "iltration

Cr 7.76 to 7.6 7.7 to 7.27

Cu 7.76 to 7.6 7.7 to 7.26/i 7.26 to .6 7.76 to 7.67Cd 7.76 to .7 7.7 to 7.26Dn 7.76 to .7 7.7 to 7.26

0* 0* 3 S

Ca(0*)2

Mix *an+ Clarifier ,ilter Mix *an+

"eS


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Sodium 5imeth&ldithiocarbamate(5!C or S55C)

/ot p* sensitive –

an& p* above 1

Controls chelated waste streams

/on amphoteric

0rganic sulfur compound

Ber& toic

ndustrial biocide

Agricultural insecticide


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Sodium pol&thiocarbonate (P!C)!hio@-ed

Much less toic than 5!C

678 less sludge than 5!C# lime or iron chemistriesK

-e%uires lower dosing of flocculent chemistr&for li%uid'solid separation

!reated waters pass toicit& tests for /ationalPollutant 5ischarge >limination S&stem (/P5>S)discharges

* d id P i it ti f S l bl M t l


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!hings that Ma=e 4ife More 5ifficult

Metals are Amphoteric5ouble Solubilit& Curvep* Must be Carefull& Controlled

Presence of Chelating Agents will nterferewith Abilit& to Precipitate Metals

*&droide Sludges Ma& be 5ifficult to 5ewater

Metal "inishing Sludge is Classified as a *a:ardous ,aste b& "ederal L State 4aw (-C-A)

< t h ! t t


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Continuous ("low@!hrough) !reatment

suall& sed "or 4arger "lows

Addition of 0* Controlled b& p* Meter ' Controller

/a0*

Storage-eact

"locculation

Clarifier

p*


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Sludge -ecirculation


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Continuous !reatment

Sludge is -ecirculated from Sludge *opper ofClarifier to the nfluent or -eaction !an=

Sludge -ecirculation


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-eview – Metal Precipitation

Sodium *&droide is ha:ardous to handle in thatit is both NNNNNNNNNNNNN and NNNNNNNNNNNNNN.toic corrosive

2. 4ist three options for precipitating two metalswith different optimum p*.

a.

b.

c.

0ptimi:e for 0ne Metal

"ind a Compromise p*

0perate as a !wo@Stage S&stem


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1. Metal h&droides that resolubili:e if the p*increases or decreases from the optimumprecipitation value are said to be

NNNNNNNNNNNNNNNNNN.amphoteric

H. Ammonia# phosphates# and >5!A are

NNNNNNNNNNNNNNNagents used in platingsolutions.

6. 5escribe the problem that ma& occur when

precipitating metals from wastewater where thematerials described in OH are present.

chelating

Chelates hold metals in solution# not allowingthem to precipitate.


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. 4ist 1 wa&s a wastewater treatment plant

operator ma& be able to overcome the

problem in O6 that is caused b& the materials

listed in OH.

a.

b.c.

9. -ecirculation of NNNNNNNNNNNNNNNNN ma& bebeneficial in that it provides EseedF materialfor the precipitation process.

Precipitate at high p*

Add a metal to tie up chelateAdd a chemical that steals metalfrom the chelate (Sulfide)

precipitated sludge


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wrd ot chemical clarification metals precipitation 445210 7

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