TECHNICAL FIELD
This invention relates to a method and apparatus for measuring the deposit forming
propensity of fluids and the effectiveness of deposit inhibitors. More specifically, this
invention concerns a method of measuring the rate of deposition on to a surface of a
piezoelectric microbalance immersed in the fluid where the deposition is driven by an
electrochemically controlled pH change in the vicinity of the microbalance.
BACKGROUND OF THE INVENTION
Deposit formation is a persistent problem in a variety of industrial processes
involving fluids, such as pulp bleaching, sugar production and filtration. The deposits may
remain suspended in the fluid or accumulate on the surface of any material that contacts
the fluid. This accumulation prevents effective heat transfer, interferes with fluid flow,
facilitates corrosive processes, and harbors bacteria.
A primary detrimental effect associated with such deposits is the reduction of the
capacity or bore of receptacles and conduits employed to store and convey the fluid. For
example, in the case of conduits used to convey scale-contaminated water, the impedance
of flow resulting from deposition is an obvious consequence.
However, a number of equally consequential problems arise from utilization of
deposit-contaminated fluid. For example, deposits on the surfaces of storage vessels and
conveying lines for
process water may break loose and become entrained in and conveyed by the
process water to damage and clog equipment through which the water is passed, e.g.,
tubes, valves, filters and screens. In addition, these deposits may appear in, and detract
from, the final product derived from the process, such as paper formed from an aqueous
suspension of pulp.
Furthermore, when deposit-contaminated fluid is involved in a heat exchange
process, as either the "hot" or "cold" medium, scale will be formed upon the heat exchange
surfaces contacted by the fluid. Such scale formation forms an insulating or thermal
opacifying barrier that impairs heat transfer efficiency as well as impeding flow through
the system. Thus, deposit formation is an expensive problem in many industrial fluid
systems, causing delay and expense resulting from shutdowns for cleaning and removal of
the deposits.
Accordingly, there is an ongoing need for the development of new agents that
prevent or inhibit the formation of deposits in fluids and for convenient methods of
measuring the effectiveness of these inhibitors. In addition, as natural inhibitors may
already be present in the fluids of interest, there is a need for effective methods of
characterizing the tendency of industrial and biological fluids as such to form deposits.
For example, the effectiveness of scale inhibitors is manifested by their ability to
suppress crystal growth through blocking active sites of potential centers of crystallization
and preventing the agglomeration of growing crystals.
Common to the above processes is that they occur at the solid-fluid interface.
Therefore, the in situ measurement of the deposition rate in the presence of scale inhibitors
at the solid-fluid interface is of particular interest. Traditional measurements mostly relate
to the change of the bulk properties of a test fluid such as solubility, conductivity, turbidity
and the like following deposit formation. There exist quite a few methods for measuring
deposit growth rate, however, fewer methods exist for conducting the measurements in
situ at the solid-liquid interface.
Methods for measuring deposit growth rate at the solid-fluid interface that utilize a
piezoelectric microbalance are disclosed in U.S. Patent No. 5,201,215, U.S. Ser. No.
09/338,699 and European Patent Application No: 676637 Al. The principle of
piezoelectric mass measurement is based upon the property of a quartz resonator to change
its mechanical resonance frequency f0 proportionally to the mass and viscoelastic
properties of the deposited material. The change in frequency is expressed as follows:

where f0 is the unperturbed resonant frequency of the quartz crystal; N is the harmonic
number; µµ is the quartz shear stiffness, pq is the density of quartz; ps is the surface mass
density of the deposit (mass/area), p is the density of the medium contacting the resonator
and ? is the viscosity of the medium contacting the resonator.
Where the viscoelastic properties of the system are negligible or remain constant
through the measurements, the surface mass density can be measured using a simplified
expression that can be used for the loading causing the resonant frequency change up to
5% (approx. 4.5 mg/cm2):
where C is determined by calibration and is typically equal 1.77 x 10-5 mg/(sec cm2 Hz) for
a 5 MHz quartz crystal.
A method of measuring the rate of calcium carbonate scale onto the surface of the
working electrode of a quartz crystal microbalance mounted in an impinging jet cell, where
the working electrode is polarized using a potentiostat is disclosed in Gabrielli et al.,
"Quartz Crystal Microbalance Investigation of Electrochemical Calcium Carbonate
Scaling", J. Electrochem. Soc, 145, 2386-2396 (1998). This method is, however, specific
to calcium carbonate scale and is impractical for use in certain circumstances as a constant
recirculating flow of test fluid from a reservoir through the impinging jet cell is required.
Consequently, a need still exists for more flexible methods of measuring the deposit
forming capacity of fluids. .
SUMMARY OF THE INVENTION
A variety of species can be precipitated on the surface of a quartz crystal
microbalance, in particular by controlling the fluid pH proximate to the surface of the
microbalance by applying an appropriate current to a working electrode disposed on the
surface of the microbalance. This method can be used to measure the deposit-forming
propensity of any fluid where the solubility of the deposit decreases as the pH of the
solution is increased or decreased.
Accordingly, in its principal aspect, this invention is directed to a method of
measuring the deposit forming propensity of a continuously flowing fluid having a bulk pH
of about 1 to about 12 comprising measuring the rate at which deposit forms on a quartz
crystal microbalance having one side comprising a working electrode in contact with the
fluid and the otherside isolated from the fluid, wherein the pH of the fluid proximate to the
microbalance is controlled electrochemically from about 1 to about 14 by applying to the
working electrode a cathodic current of about -0.001 to about -100 mA/cm2 or an anodic
current of from about 0.001 to about 100 mA/cm2 and wherein the working electrode is
coated with or made of a conductive material on which the rate of hydrogen gas evolution
is at least 10 times lower than on a gold under acidic conditions.
In another aspect, this invention is directed to a method of measuring the
effectiveness of deposit inhibitors comprising
a) measuring the deposit forming propensity of a continuously flowing fluid having a
bulk pH of about 1 to about 12 comprising measuring the rate at which deposit forms on a
quartz crystal microbalance having one side comprising a working electrode in contact
with the fluid and the other side isolated from the fluid, wherein the pH of the fluid
proximate to the microbalance is controlled electrochemically at about 1 to about 14 by
applying to the working electrode a cathodic current from about -0.001 to about -100
mA/cm2 or an anodic current from about 0.001 to about 100 mA/cm2 and wherein the
working electrode is coated with or made of a conductive material on which the rate of
hydrogen gas evolution is at least 10 times lower than on a gold cathode under acidic
conditions;
b) adding a deposit inhibitor to the fluid; and
c) re-measuring the rate of deposition on to the quartz crystal microbalance.
In another aspect, this invention is directed to a measurement cell for measuring the
deposit forming propensity of a continuously flowing fluid having a pH of about 1 to about
12 comprising
a) a quartz crystal microbalance assembly having one side comprising a working
electrode for exposure to the fluid and the other side isolated from the fluid;
b) a counter electrode in contact with the fluid for providing a uniform electric field to
the microbalance; and
c) means for stirring the fluid,
wherein the pH of the solution proximate to the microbalance is controlled
electrochemically at about 1 to about 14 by applying to the working electrode a cathodic
current of about -0.001 to about -100 mA/cm2 or an anodic current of from about 0.001 to
about 100 mA/cm2 and wherein the working electrode is coated with or made of a
conductive material on which the rate of hydrogen gas evolution is at least 10 times lower
than on a gold cathode under acidic conditions;
The method and apparatus of this invention is useful for benchtop laboratory work
or, in a portable form, for on-site process control. The method allows reliable and prompt
testing of potential deposit inhibitors in both model and real fluids. It is reproducible,
sensitive and has broader applications than known techniques that suffer from interference
of additional components present in industrial fluids. Specifically, this method allows for
characterizing the ability of deposit inhibitors to prevent deposit growth and when used in
conjunction with conventional chemical tests allows comprehensive characterization of the
properties of deposit inhibitors.
In addition to testing industrial fluids, this method can be applied to biological
fluids to characterize their tendency to form deposits. It has a great potential for medical
applications such as urine tests for susceptibility to kidney stone formation and monitoring
and screening of potential pharmaceutical remedies.
BRIEF DESCRIPTION OF THE ACCOMPAYING DRAWINGS
Figure 1 is a block diagram of an apparatus of this invention.
Figure 2 is a schematic diagram of a preferred configuration of the apparatus of this
invention.
Figure 3 is a top plan view of the quartz sensor of the quartz crystal microbalance
(I).
Figure 4 is a bottom plan view of the quartz sensor of the quartz crystal
microbalance (1).
Figure 5 is a cut away view of the quartz crystal microbalance assembly (15).
Figure 6 is a top plan view of the surface pH measuring module (2).
Figure 7 is a cut away view of the surface pH measuring module (2).
DETAILED DESCRIPTION OF THE INVENTION
This method exploits precipitation of deposits from fluids when the deposit
solubility in the fluid is decreased by raising or lowering the fluid pH. As used herein,
"deposit" means any organic or inorganic material that precipitates from a fluid under the
conditions of pH described herein. This method is applicable to both organic and
inorganic deposits, including salts of metal ions. Representative metal ions include
alkaline earth metals such as sodium calcium, magnesium, and the like and transition metal
ions including cadmium, cobalt, nickel, copper and the like. Typical deposits include
calcium oxalate, calcium carbonate, calcium hydroxide, magnesium carbonate, magnesium
phosphate, calcium uronate, calcium phosphate, cadmium carbonate, cadmium hydroxide,
cobalt hydroxide, nickel hydroxide, copper hydroxide, sodium octanoate, sodium oleate,
and the like.
The rate of deposit formation is measured with a sensitive quartz crystal
microbalance in the vicinity of which the change in fluid pH is generated and controlled
electrochemically. This method utilizes an electrochemical set-up comprising a
measurement cell in which a continuous (constant for a given experimental series) flow of
the test fluid is established relative to the surface of the quartz crystal microbalance
(QCM). The pH of the test fluid is selected such that the deposit-forming salt is initially
soluble in the test fluid but produces deposits when a change of the fluid pH is generated
by applying electrochemical polarization.
The electrochemical polarization is controlled by applying anodic or cathodic
current to the working electrode of the QCM. Current control is particularly advantageous
compared to potential control because it does not require the use of reference electrodes
and compensation for fluid resistance.
The QCM is a piezoelectric resonator connected to a measuring and driving circuit.
The resonator is a quartz crystal plate with electrodes on its sides used for the connections.
One of the resonator sides (the fluid side) with its electrode (the working electrode) is
immersed in the test fluid and the other side (the contact side) is isolated from the fluid to
avoid shunting the resonator through the fluid.
The choice of material for the working electrode is critical, as not all conductive
materials are suitable under all test conditions. For example, under the acidic conditions
required for the calcium oxalate tests described herein, steady production of hydrogen gas
bubbles on a gold working electrode effectively hinders calcium oxalate precipitation as a
result of the bubbles partially blocking the working electrode and stirring the near
electrode fluid, thereby preventing the necessary pH increase. The surface of the QCM
coated with a conductive material may serve as either an anode or cathode, depending on
the targeted change in the surface pH.
A suitable electrode material for a given test may be determined empirically by
measuring the volume of evolved hydrogen gas, or by observing the size and number of
bubbles lifting off from the electrode surface under the electrochemical polarization
required to precipitate the deposit from the test fluid.
For purposes of this invention the working electrode may made of or coated with
any chemically stable, conductive material on which the rate of hydrogen evolution is at
least 10 times lower than on gold cathode under the required conditions of solution pH and
electrochemical polarization in an acidic solution. Other considerations in selecting the
electrode material include simplicity of handling, cost, and resistance to dissolution during
the test. For a detailed description of the dependence of exchange current density for the
hydrogen evolution reaction on the strength of the metal-hydrogen bond formed in an
electrode reaction see D. Pletcher and F. Walsh, "Industrial Electrochemistry" Blackie
Academic and Professional, London, 1993.
Preferred conductive materials include gold, titanium, silver; lead; cadmium;
diamond-like thin film electrodes with or without implanted ions; silicon, silicides of
titanium, niobium and tantalum; lead-selenium alloys; and mercury amalgams, and the
like.
Under test conditions where anodic current is applied to the electrode, the
conductive material must withstand the resulting oxidative conditions. Preferred electrode
materials for this application include titanium, diamond-like thin film electrodes, silicon,
silicides of titanium, niobium, tantalum, and the like.
In an embodiment of this invention, the pH measurement is accomplished using a
surface pH-measuring module (2) comprising a mesh electrode (27) laid over a flat-tip
combination pH electrode (26) as shown in Figs. 6 and 7. The mesh should be as thin and
dense as possible and made of the same material as the microbalance working electrode.
Once subjected to the same test conditions, the mesh electrode allows approximating the
surface pH conditions near the microbalance surface.
In another embodiment, the pH measurement is accomplished using a microtip
combination electrode such as those available from Microelectrodes, Inc., Bedford, NH. A
microtip combination electrode is a miniaturized conventional pH electrode based on
selective diffusion of protons though hydrogen ion sensitive glass, and the determination of
potentials between the internal electrolyte and a silver/silver chloride reference electrode.
In another embodiment, the surface pH measurement is accomplished using a pH
electrode fabricated on the fluid side of the quartz crystal microbalance (1). The working
electrode (16) of the microbalance (1) has definite dimensions. The geometry of the
electrochemical diffusion layer near the edge of this electrode presumes that the
electrochemically induced pH change occurs not only in the vicinity above the electrode
but also near the electrode edge in the lateral direction in the same plane. Therefore, if a
small size pH sensor is placed near the edge and in the same plane as the microbalance
working electrode the measurement of the near electrode pH is possible.
Thin-film metal oxide electrodes are preferred for such pH measurements. These
materials are prepared by reactive sputtering of metals selected form the group of tungsten,
platinum, palladium, ruthenium, and iridium metal targets in argon-oxygen atmospheres to
produce a thin film several micron thick directly on the quartz crystal substrate (17).
As described above, the rate of precipitation of a deposit is measured by
precipitating the deposit on to the QCM in the vicinity of which a change in fluid pH is
generated and controlled electrochemically. Control of the pH is accomplished using a
surface pH measuring module. The module serves to establish the dependence of the near
electrode pH on applied electrochemical polarization at given test conditions.
A slow scan of current from a near zero to a sufficiently large current densities
(typically about 30 mA/cm2) is used to determine the current ranges producing the desired
increase or decrease in pH. Consequently, the current is controlled in this range during the
scale deposition. Once the appropriate current is established for a certain class of fluids,
this value is used in all further experiments without performing additional pH scans. For
example, calcium oxalate at bulk pH 2.5 requires current density of 4.5 mA/cm2 in a model
1 mM solution and 5 mA/cm2 in pulp mill bleaching effluents; calcium carbonate at bulk
pH 7.8 requires current density of 4.5 mA/cm2 in pulp mill bleaching effluents.
Test conditions and representative electrode materials for a variety of scale-forming
salts are summarized in Table 1. In Table 1, "Bulk pH" refers to the test solution pH,
"Proximate pH" refers to the pH proximate to the working electrode of the QCM and
"Current density" refers to the current density applied to the working electrode in order to
generate the proximate pH.
In a preferred aspect of this invention, the deposit is calcium carbonate.
A preferred working electrode material for measuring the calcium carbonate scaling
capacity of a solution is gold.
In another preferred aspect, the calcium carbonate solution has a bulk pH of about 7
to about 8 and the pH of the solution proximate to the microbalance is controlled
electrochemically at about 10.5 to about 12 by applying to the working electrode a
cathodic current of from about -4.5 to about -6 mA/cm2.
In another preferred aspect, the deposit is calcium oxalate.
A preferred working electrode material for measuring the calcium oxalate scaling
capacity of a solution is silver or titanium. A more preferred electrode material is titanium.
Use of titanium allows accumulation of calcium oxalate deposit with less hydrogen
evolution than a silver working electrode. Moreover, the titanium coating is more
chemically and electrochemically resistant and allows for a reverse of current that makes a
QCM an anode. Such a reverse is not recommended for a silver-coated QCM since it leads
to immediate corrosion of the metal layer. A reverse of current allows for an
electrochemical cleaning of the QCM working electrode after a deposition experiment.
Reversing the current to make the QCM an anode results in rapid dissolution of calcium
oxalate deposited on the surface of the QCM thus making it possible to clean the system
between experiments without disassembling it. This option is of special importance for
on-line applications of the technique.
In another preferred aspect, the calcium oxalate solution has a bulk pH of about 1.5
to about 2 and the pH of the solution proximate to the microbalance is controlled
electrochemically at about 2 to about 5 by applying to the working electrode a cathodic
current of from about -5 to about -30 mA/cm2.
Embodiments of the apparatus of this invention is shown in Figs. 1 and 2. The
apparatus consists of a quartz crystal microbalance (1), a surface pH measuring module
(2), counter electrode (3), means (4) for controlling and measuring bulk fluid temperature,
fluid flow and pH, and a data acquisition and control unit (5) connected to an external
computer (6).
The data acquisition and control unit (5) executes experimental procedures and
relays the experimental data to an external computer (6). The computer software controls
the experiment setup and data acquisition, and processes and plots the data. The
programmed parameters are: applied current, temperature, stirring rate, time of the
experiment and readings per minute. Other parameters are programmed via the software
operating the apparatus from a computer. The external computer processes and stores the
experimental data while displaying the test parameters and the deposition graphs
(deposited mass and rate) in real time. It also micrors the actual current in the system and
frequency of the microbalance. The collected data can also be compiled and analyzed
immediately using commercially available data analysis software. The temperature and
stirring rate are programmed separately via respective devices (the temperature controller
and digitally controlled stirring means).
The measurement cell (7) is configured in a two-electrode arrangement using the
working electrode (16) of the quartz crystal microbalance (1) and counter electrode (3).
The measurement cell is made of chemically resistant plastic, preferably transparent (e.g.,
PVC or/and acrylic).
The counter electrode (3) is electrolytically connected to the bulk fluid and capable
of applying a uniform electric field to the working electrode of the microbalance (1). The
counter electrode (3) is manufactured from graphite or other resistant materials readily
apparent to those of skill in the art such as platinum, stainless steel, and the like.
In principle, the apparatus of this invention can utilize any electric source capable
of supplying to the working electrode a polarization of suitable magnitude, polarity and
stability. The electrical conditions established in the circuits can be controlled and
measured using the equipment commonly used by those skilled in the art.
A steady flow of the bulk liquid past the quartz crystal microbalance (1) is
accomplished using a suitable stirring device such as an impeller, a mechanical paddle
stirrer, or a magnetic stirbar. In the preferred embodiment, a magnetic stirbar (11) and
digitally controlled stirplate (9) are used. By "steady," a relatively constant flow is
intended. That optimal flow rate can be readily determined in an experiment on a model
fluid. The preferred rate for the unit displayed in Fig. 2 is 400 rpm.
The temperature of the fluid is controlled using any suitable thermal regulating
means including, but not limited to, a cooler or heater (12) disposed in the bulk liquid. The
temperature of the bulk liquid is measured by a thermocouple (13) connected to a
controller. The temperature of the bulk liquid as measured by the thermocouple can be
maintained constant or be varied, as much as is practical to simulate the desired system.
The apparatus of this invention is operated as a batch system as shown in Fig. 2. In
Fig. 2, number (10) represents a position in the measurement cell (7) at which either a
surface pH measuring module (2) or the quartz crystal microbalance assembly (15) can be
attached to the measurement cell (7). A removable cover (14) for the measurement cell
contains apertures for the cooler or heater (12), temperature sensor (13) and counter
electrode (3).
Initially, optimal pH conditions are determined with the surface pH measuring
module (2) installed. Then the surface pH measuring module is removed and the quartz
microbalance assembly (15) is installed for measurement of scale forming capacity. In the
first case, the mesh electrode (32) of the surface pH module (2) is connected to the same
electrochemical system and subjected to the same test conditions as the working electrode
(16) of microbalance (1). Both surface and bulk pH in the system are measured using the
surface pH measuring module (2) connected to a pH-reading circuit in the data acquisition
and control unit (5).
Preferred configurations of the quartz crystal microbalance (1) and quartz crystal
microbalance assembly (15) are shown in Figs. 3-5. In this embodiment, the mass sensing
element of the piezoelectric microbalance is an AT cut quartz crystal (17) with evaporated
electrodes (16), (18), and (19). Electrode (16) is the working electrode as it is immersed
into the tested fluid during measurements.
Working electrode (16) wraps around the edge of the top or fluid side of the crystal
(17) to its bottom side to form contact (18) as shown in Figure 4. The bottom side of the
quartz crystal (17) includes contact (18) and second contact (19). Contacts (18) and (19)
provide electrical connections with the microbalance operating circuit in the data
acquisition and control unit (5) by way of connecting wires.
The quartz crystal microbalance assembly (15) is shown in Fig. 5. The quartz
crystal (17) is sealed in an aperture of a plastic cylinder (25). A retainer ring (20) and at
least one
O-ring (21) ensure that only the top side of the quartz crystal (17) is exposed to the test
fluid and that the bottom side of the crystal is isolated from the fluid. Spring-loaded
contacts (22) operatively connect the connecting leads (23) and (24) respectively to the
working electrode (16) and contact (19). The leads are then connected to the microbalance
operating circuit in the data acquisition and control unit (5).
The top or fluid side of the preferred surface pH measuring module (2) of this
invention is shown in Figs. 6 and 7. In this embodiment, a flat-bottom, upside-down
mountable combination pH electrode (26) is fixed coaxially in a plastic cylinder (27) in
such a way that the flat sensing surface of the electrode is in direct contact with a mesh
(27) made of the same material as the microbalance electrode (16) and attached to the top
of the plastic cylinder.
Maximizing the number of apertures per inch in mesh (27) leads to a better
simulation of the microbalance working electrode. Variation of this parameter can be
achieved by using mesh as thin and dense as practically available.
As shown in Fig. 7, the flat mesh (27) is connected to the electrochemical circuit in
the data acquisition and control unit (5) using a wire (28) passing through an eccentric
channel (29) in the plastic cylinder (27). Both the wire (28) and pH electrode (26) are
tightly sealed in the corresponding channels of cylinder (27). The pH electrode (26) is
connected to the pH reading circuits in the data acquisition and control unit. With the
thickness of mesh (27) less than the electrochemical diffusion layer the device allows
measuring the pH near the polarized electrode surface, thus simulating the environment
near the surface of the working quartz microbalance. The same polarization is applied to
the mesh electrode (27) of the surface pH module (2) and the working electrode of the
quartz microbalance (1).
The foregoing may be better understood by reference to the following Examples
that are presented for purposes of illustration and are not intended to limit the scope of this
invention.
EXAMPLES
The use of the method and apparatus of this invention to measure the effectiveness
of calcium oxalate and calcium carbonate scale inhibitors in Kraft pulp bleach plants is
described below. It is understood that the following is illustrative of applications of this
invention and is not intended to be limiting.
Pulp produced by the Kraft process is normally bleached in a multistage sequence
to obtain the desired brightness and strength. The main objectives of pulp bleaching are to
increase the brightness of the pulp and to make it suitable for the manufacture of printing
and tissue grade papers by removal or modification of lignin and its degradation products
of the unbleached pulp. The bleaching of chemical pulp is accomplished by a series of
treatments involving chlorine dioxide, caustic, hydrogen peroxide, and other bleaching
agents. The bleaching of chemical pulp normally begins with the first stage chlorine
dioxide. The bleaching of pulp is done through chemical reactions of bleaching agents
with the lignin and coloring matter of the pulp under different conditions of temperature,
time, concentration, and pH.
Calcium oxalate and calcium carbonate scale present a difficult problem to control
that can impair the performance of stock pipelines, washing filters, filtrate tanks, refiner
plates, and heat exchangers. The formation of these deposits is a result of relatively high
concentration of calcium oxalate in process equipment operating in the pH range of 2-8
and calcium carbonate in process equipment operating in the pH range of 9-11. The
removal of these materials is difficult and results in costly downtime.
The source of calcium oxalate in pulp bleaching operations is ultimately due to the
wood. Calcium is introduced into the pulp mill principally from the wood, although some
calcium may also be introduced into the process from the mill's fresh water and cooking
liquor (sodium hydroxide and sodium sulfide). Oxalic acid is formed during pulping and
bleaching but is also present in native wood. Carbon dioxide from the air reacts with
alkaline mill solutions producing carbonate and hydrocarbonate ions that creates potential
for calcium carbonate deposition. The precipitation of calcium oxalate and calcium
carbonate is strongly dependent upon changes in temperature and pH.
In the batch system, the microbalance and counter electrode are installed in the
proximity to each other (from about 1 to about 3 cm). The length of the counter electrode
is such that it passes through the cell top to bottom but still allows for stirbar rotation. A
high-density graphite counter electrode (Perkin-Elmer, Oak Ridge, TN) is used. The
apparatus is designed to be used with 1-L samples. The measurement cell is made of
Plexiglas. The quartz microbalances used are 5 MHz silver- or titanium-coated polished
quartz crystals (Maxtek, Inc., Torrance, CA). In calcium carbonate tests, gold-coated
crystals can also be used. A digitally controlled 400S Stirrer and Teflon-coated 62-mm
Spinstar stirbar (VWR, Chicago, IL) are used. The systems are connected to data
acquisition and control unit.
The data acquisition and control unit is a microprocessor-controlled electronic
instrument that incorporates circuits operating the quartz crystal microbalance and
electrochemical galvanostat. The unit is connected to an external computer. The computer
software controls the experiment setup and data acquisition and processing. The
programmed parameters are: applied current, temperature, and stirring rate of the solution.
The bulk pH of the solution is adjusted as fit for each type of the tests. The computer
software processes and stores the experimental data while displaying the test parameters
and the deposition graphs (deposition amount and rate) in real time. In a preferred
apparatus, an Omega CN76000 microprocessor-based temperature controller is integrated
into the apparatus to maintain the desired temperature of the solution.
The measurements for calcium oxalate are performed at 25 °C and bulk pH 2.5-2.7.
The measurements for calcium carbonate are performed at 25 °C or 70 °C and bulk pH 7.8-
8.0. Stirbar rotation speed of 400 rpm is maintained.
The scaling capacity of a solution is evaluated by the deposition rate observed at
preset time intervals as well as by the total deposited mass accumulated by the end of the
test. The percent inhibition is calculated as follows:
% Inhibition =
100% x (Total deposit w/o inhibitor - Total deposit with inhibitor) / Total deposit without
inhibitor.
In Tables 1-2, Samples A and Al is a terpolymeric organic acid of different degree
of polymerization. Sample B is an alkaline solution of an acrylic polymer with a small
amount of sulfur-containing inorganic salts. Sample C is a mixture of an acrylic polymer
with an inorganic phosphorous salt. Composition D is a carbohydrate-based inhibitor.
Sample E is a phosphonate polymer. Compositions A-D are available from Ondeo Nalco
Chemical Company, Naperville, IL.
Example 1
Screening Of Calcium Oxalate Scale Inhibitors Using Model Solutions
A 1-mM (128-ppm) test solution of calcium oxalate is prepared as follows.
Sodium oxalate (0.268 g) and calcium chloride dihydrate (0.294 g) are separately dissolved
in 35 ml of 0.1 N HC1. The solutions are diluted to 100 ml each with deionized water,
mixed under intense stirring, and the mixed solution is diluted to 2 L volume with
deionized water with 0.1 N HC1 added as needed to adjust the pH to 2.6. This solution is
used as the control. The rest of the solutions contained a potential inhibitor. The results are
given in Table 2.
Example 2
Dependence of Mass Accumulation of Calcium Oxalate Scale on the Applied Current
Density Applied on Titanium Coated QCM
The results are given in Table 3. A 1-mM (128-ppm) test solution of calcium
oxalate at bulk pH 2.4 is used in this test. After calcium oxalate is deposited on the
surface of the QCM, the current is reversed (applied current density +2.0 mA/cm2). Rapid
mass loss due to dissolution of calcium oxalate is observed, with no damage to the QCM.
Example 3
Screening Of Calcium Oxalate Scale Inhibitors Under Acidic Conditions in Model
Solutions
4-mM (128-ppm) supersaturated solutions of calcium oxalate at bulk pH 1.8 are
used in this test. The inhibitors (20-ppm concentration based on the dry weight) are added
prior to mixing equimolar calcium chloride and sodium oxalate solutions. An estimate
obtained with a surface pH probe used consecutively indicates that the applied current of
22 mA/cm2 leads to a pH increase to 2.5-3.0. The results are given in Table 4.
Example 4
Screening Of Calcium Oxalate Scale Inhibitors Using Mill Water
The test solutions contain 1-mM (128-ppm) of calcium oxalate added to mill
water. Sodium oxalate (0.107 g) and calcium chloride dihydrate (0.118 g) are separately
dissolved in 400-ml of mill water. 0.1 N HC1 is added to the samples to maintain pH at
about 2.5. The solutions are mixed and used as the control without an inhibitor, or with
inhibitors for screening.
The system is flushed with deionized water immediately after the analysis. After
each analysis of model solution or mill water, the surface of the crystal is cleaned of the
deposit with 0.1 N HC1 (5-10 min) and washed with deionized water. The results are given
in Table 5.
The experimental data summarized in Tables 2, 4, and 5 clearly distinguish more
and less effective inhibitor compositions. Thus, composition A decreases the rate of
deposition dramatically and is the most effective in both mill water and in model solutions.
Example 5
Screening Of Calcium Oxalate Scaling Capacity of Mill Waters Acquired from Various
Hardwood DO Process Stages
The results of the use of the apparatus for screening of the oxalate scaling capacity
of actual mill waters acquired from various DO process stages are given in Table 6. Two
sets of mill water, taken before and after the change in the operational procedure at the
mill, were analyzed. The tests were performed on the original solutions and on the same
solutions spiked with 1-mM calcium oxalate.
Example 6
Screening Of Calcium Carbonate Scaling Capacity of Mill Waters Acquired from
Hardwood DO Process Stage (Gold Coated QCM)
The test results of the calcium carbonate scaling capacity of DO filtrate are given in
Table 7. The pH of the sample is adjusted to 8.0 and 100 ppm of sodium carbonate were
added, and current density 5.0 mA/cm2 was applied. The samples contained scale
inhibitors A and E at different concentrations. The test demonstrates the effect of the
inhibitor concentration on the scaling capacity of the solution.
Example 7
Screening of Calcium Carbonate / Magnesium Carbonate Scale Inhibitors using Gold
Coated QCM
The pH of the sample (magnesium oxide slurry diluted 1:400) was adjusted to 8.0,
and current 4.5 mA/cm2 was applied. The results are given in Table 8.
Although this invention has been described in details for the purpose of illustration,
it is to be understood that such details are provided solely for that purpose and that
numerous modifications, alterations and changes can be made therein by those skilled in
the art without departing from the spirit and scope of the invention except as it may be
limited by the claims. All changes which come within the meaning and range of
equivalency of the claims are to be embraced within their scope.
WE CLAIM
1. A method of measuring the deposit forming propensity of a continuously
flowing fluid having a bulk pH of about 1 to about 12 comprising
measuring the rate at which deposit forms on a quartz crystal
microbalance (1) having a top side comprising a working electrode (16) in
contact with the fluid and the other side isolated from the fluid, wherein
the pH of the fluid proximate to the microbalance (1) is controlled
electrochemically, via a galvanostatic system, at about 1 to about 14 by
applying to the working electrode (16) a cathodic current of about -0.001
to about -100 mA/cm2 or an anodic current of from about 0.001 to about
100 mA/cm2 and wherein the working electrode is coated with or made of
a conductive material on which the rate of hydrogen gas evolution is
insufficient to hinder precipitation of scale onto the working electrode
under the electrochemical polarization required to precipitate the deposit
from the fluid.
2. The method as claimed in claim 1, wherein the working electrode (16) is
made of or coated with one or more conductive materials selected from
gold, titanium, silver; lead; cadmium; diamond-like thin film electrodes
with or without implanted ions; silicon, silicides of titanium, niobium and
tantalum; lead-selenium alloys; and mercury amalgams.
3. The method as claimed in claim 1, wherein the deposit is selected from
the group consisting of calcium oxalate, calcium carbonate, calcium

hydroxide, magnesium carbonate, magnesium phosphate, calcium
uronate, calcium phosphate, cadmium carbonate, cadmium hydroxide,
cobalt hydroxide, nickel hydroxide, copper hydroxide, sodium octanoate
and sodium oleate.
4. The method as claimed in claim 1, wherein the pH of the fluid proximate
to the surface of the microbalance (1) is measured using a surface pH-
measuring module (2) comprising a mesh electrode (32) laid over a pH
electrode (26) wherein the mesh (27) is made of the same material as the
working electrode (16).
5. The method as claimed in claim 1, wherein the pH of the fluid proximate
to the surface of the microbalance (1) is measured using a pH-measuring
device (7) selected from a microtip pH electrode and a pH electrode
fabricated on the surface of the quartz crystal microbalance (1).
6. The method as claimed in claim 1, wherein the deposit is calcium
carbonate.
7. The method as claimed in claim 6, wherein the working electrode (16) is
made of gold.
8. The method as claimed in claim 7, wherein the fluid has a bulk pH of
about 7 to about 8 and wherein the pH of the fluid proximate to the
microbalance is controlled electrochemically at about 10.5 to about 12 by
applying to the working electrode a cathodic current of from about -4.5 to
about -6 mA/cm2.
9. The method as claimed in claim 1, wherein the deposit salt is calcium
oxalate.
10.The method as claimed in claim 9, wherein the working electrode (16) is
made of silver, titanium, diamond like thin films, silicon, suicides of
titanium, niobium and tantalum.
11. The method as claimed in claim 10, wherein the fluid has a bulk pH of
about 1.5 to about 2 and wherein the pH of the fluid proximate to the
microbalance is controlled electrochemically at about 2 to about 5 by
applying to the working electrode a cathodic current of from about -5 to
about -30 mA/cm2.
12. A method of measuring the effectiveness of deposit inhibitors comprising
a) measuring the inorganic scale forming propensity of a
continuously flowing solution having a bulk pH of about 1 to
about 12 comprising measuring the rate at which deposit forms
on a quartz crystal microbalance having a top side comprising a
working electrode in contact with the solution and a bottom side
isolated from the solution, wherein the pH of the solution
proximate to the microbalance is controlled electrochemically,
via a galvanostatic system, at about 1 to about 14 by applying
to the working electrode a cathodic current of about -0.001 to
about -100 mA/cm2 or an anodic current of from about 0.001 to
about 100 mA/cm2 and wherein the working electrode is coated
with or made of a conductive material on which the rate of
hydrogen gas evolution is insufficient to hinder precipitation of
scale onto the working electrode under the electrochemical
polarization required to precipitate the deposit from the fluid;
b) adding a deposit inhibitor to the solution; and
c) re-measuring the rate of deposition on to the quartz crystal
microbalance.
13. A measurement cell (7) for measuring the deposit forming propensity of a
continuously flowing solution having a pH of about 1 to about 12
comprising:
a) a quartz crystal microbalance assembly (15) having a top side
comprising a working electrode (16) for exposure to the fluid
and a bottom side isolated from the fluid; or
b) a surface pH-measuring module (2) for exposure to the fluid,
the pH-measuring electrode assembly comprising a mesh
electrode (32) laid over a pH electrode (26) wherein the mesh

(27) is made of the same material as the working electrode (16)
of the microbalance;
c) a counter electrode (3) in contact with the fluid for providing a
uniform electric field to the microbalance; and
d) means for (9,11) stirring the fluid, wherein the pH of the
solution proximate to the microbalance is controlled
electrochemically at about 1 to about 14 by applying to the
working electrode a cathodic current of about -0.001 to about -
100 mA/cm2 or an anodic current of from about 0.001 to about
100 mA/cm2, wherein the working electrode (16) is coated with
or made of a conductive material on which the rate of hydrogen
gas evolution is insufficient to hinder precipitation of scale onto
the working electrode (16) under the electrochemical
polarization required to precipitate the deposit from the fluid
and wherein optimal pH conditions are determined with the
surface pH module (2) installed and scale forming capacity is
measured with the quartz crystal microbalance assembly (15)
installed.
14.The measurement cell as claimed in claim 13, comprising means for
(12,13) monitoring and controlling the temperature of the fluid.
15.The measurement cell as claimed in claim 14, wherein the means for
(12,13) monitoring and controlling the temperature of the fluid are
immersed in the fluid.
16. The measurement cell as claimed in claim 13, wherein the quartz crystal
microbalance assembly (15) or surface pH measuring module (2) is
disposed horizontally in the measurement cell (7) and the counter
electrode (3) is disposed vertically and downstream from the quartz
crystal (1).
17.The measurement cell as claimed in claim 13, wherein the working
electrodes (16) of the quartz crystal microbalance assembly (15) is made
of or coated with one or more conductive materials selected from gold,
titanium, silver; lead; cadmium; diamond-like thin film electrodes with or
without implanted ions; silicon, silicides of titanium, niobium and
tantalum; lead-selenium alloys; and mercury amalgams.
18.A measurement cell for measuring the deposit forming propensity of a
continuously flowing solution having a pH of about 1 to about 12
comprising:
a) a quartz crystal microbalance assembly having a top side
comprising a working electrode for exposure to the fluid and a
bottom side isolated from the fluid;
b) a surface pH-measuring module for exposure to the fluid, the
pH-measuring electrode assembly comprising a mesh electrode
laid over a pH electrode wherein the mesh is made of the same
material as the working electrode of the microbalance;
c) a counter electrode in contact with the fluid for providing a
uniform electric field to the microbalance; and
d) means for stirring the fluid, wherein the pH of the solution
proximate to the microbalance is controlled electrochemically at
about 1 to about 14 by applying to the working electrode a
cathodic current of about -0.001 to about -100 mA/cm2 or an
anodic current of from about 0.001 to about 100 mA/cm2 and
wherein the working electrode is coated with or made of a
conductive material on which the rate of hydrogen gas evolution
is insufficient to hinder precipitation of scale onto the working
electrode under the electrochemical polarization required to
precipitate the deposit from the fluid.

A method and apparatus for measuring the deposit forming propensity of a continuously flowing fluid having a bulk
pH of about 1 to about 12 comprising measuring the rate at which deposit forms on a quartz crystal microbalance (1) having a top
side comprising a working electrode (16) in contact with the fluid and a bottom side isolated from the fluid, wherein the pH of the
fluid proximate to the microbalance is controlled electrochemically at about 1 to about 14 by applying to the working electrode (16)
a cathodic current of about -0.001 to about -100 mA/cm2 or an anodic current of from about 0.001 to about 100 mA/cm2 and wherein
the working electrode (16) is coated with or made of a conductive material on which the rate of hydrogen gas evolution is at least 10
times lower than on a gold cathode in acidic solution.