8/13/2019 pH Measurement in Finishing Application

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Titanium Dioxide (TiO 2), “white pigment,” is thelargest volume pigment manufactured globally.Annual production equals approximately 3,555x 10 6 metric tons. Marketed as a fine powder,this product is used as a decorative or protec-tive coating and coloring for plastics, fibers,paper, rubber, glass, cement, glazes, porcelainenamels, printing inks, and cosmetics.

Titanium Dioxide “Finishing” is the final step inthe production of high purity TiO 2. Selectivecoating of raw, purified TiO

2 with inorganic

oxides optimizes dispersability, dispersionstability, opacity, gloss, and durability. Theactual finishing process variables and coatingsused are specific to the final product applica-tion and intended market sector.

The initial finishing stage disperses the basepigment in water with phosphate, silicate, ororganic dispersants. The suspension is milledand classified to remove oversized particles.Selective precipitation through controlledchanges in pH and temperature coats small

quantities of colorless hydrous oxides (P 2O5,SiO 2, Al2O3, TiO 2, and ZrO 2) onto the dispersedparticles. Once coated, the pigment is filtered,washed, and dried. Specific surface treatmentsare also added at this point, depending uponend product requirements.

Accurate, reliable measurement of pH inpigment manufacturing applications bringsunique challenges and frustrations, leading tohigh maintenance labor and material replace-

ment costs. Harsh process conditions are notconducive to in-line pH measurement.

The finishing process is often characterized byhigh temperatures and extremely acidicconditions. Due to the batch nature, tempera-tures are often cycled over broad ranges.Entrained solids, fine particulate, and high saltcontent comprise the suspension.

Oxygen AICI 3

Sulfate process

Refined and coated pigment

Scrapiron

Copperas

FeSO 4 . 7H2O

Nuclei

Dilute H 2SO 4

Dry-milled, uncoatedpigment

Chlorine

Dilute H 2SO 4

Nuclei(Blumenfeld)

Coating additions,eg, AL 2(SO 4)3 followed

by pH adjustment

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Chloride process

In situ seeds

CarbonChlorine

TiCI4

limenite

Dry andgrind

Digest

Dissolveand reduce

Clarify

Crystalize

(cool)

TiO(SO 4) containing

solution Fe 2+

Hydrolyze(boil)

Filter

TiO2 Pulp

Leach

Filter

and wash

Slag

Dry andgrind

Digest

Dissolve

Clarify

TiO(SO 4) containingsolution Fe 2+

Hydrolyze(boil)

Filter

TiO2 Pulp

Leach

Filter andwash

Mineral rutile(sand) or

synthetic rutile

Chlorinate

Purify

Oxidationreactor

Pigmentcollection

TiO2

Neutralize

Condenseand store

H2SO 4

Acidfiltrate

Inorganicadditions for

crystal growthcontrol

Calcine

TiO 2

Dry mill<

v

^ ^

v

v

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Organic addition(optional)

PIGMENT FINISHING

Dispersein water

Wet mill

Coat

Filter, washand dry

Grind in fluid-energy mill

8/13/2019 pH Measurement in Finishing Application

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Common problems associated with these measurementsinclude reference junction poisoning, abrasion of the pHmeasuring glass, sensor drift, probe breakage, and probefouling. This results in constant cleaning, calibration, andultimately, frequent probe replacement.

Pictured above, a titanium dioxide application equipped with the AutoClean System.

Endress+Hauser’sAutoClean System pro-

vides an ideal solution tothe challenges presented inTitanium Dioxide Finishing.In conjunction with a CPA463 automated, retractablepH holder constructed oftitanium or Kynar (PVDF),the AutoClean Systemautomatically retracts,cleans, and reinserts thesensor into the processwithout operator interven-tion. Sensor health isassessed diagnosticallythrough the CPM 152 pH/ ORP Analyzer. Measure-ment outputs are transmit-ted via dual 4-20 mAoutputs. General instru-ment fault relays andremote inputs providesystem status and allowremote control through theDCS.

An integral ball valve built into the holder providesisolation of the sensor from the process during cleaning,calibration, and routine sensor replacement. Cleaningcycles, programmed into the local controller, automati-cally initiate a cleaning cycle either at fixed intervals,pre-selected times, or upon a sensor alarm diagnostic.

Pressure sensors incorporated into the holder providefeedback regarding holder position (inserted/retracted).Interlocks prevent reintroduction of the holder insertionshaft with the sensor removed. A retraction securitylock insures process pressure does not force thesensor from the process, should loss of system airoccur. An external injector integrated into the systemallows introduction of rinse water and cleaning agentsinto the wash/rinse chamber of the holder.

The recommended pH sensor is a liquid filled, 12 mm,pH sensor with 100 ohm RTD and zirconium reference.A pressurized reservoir insures continuous electrolyteflow through the junction, minimizing reference foulingand prolonged sensor life.

The net benefit to plant operations is realized in in-creased instrumentation reliability, enhanced confidencein the accuracy of the measurement point, and de-creased operating and maintenance costs. Althoughsignificant savings are achieved in material replacementcosts, the greater savings come from reduced mainte-

nance requirements.

(Editor’s Note: The following savings in material and maintenance labor are typical of an AutoClean System installed in titanium dioxide finishing applications, and are based on average industry labor rates.)

Savings: Year 1 $ 18,162.25Savings: Year 2 $ 32,951.25Savings: 5 Years $149,967.25 TotalROI (months) 9.77

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