Note: Descriptions are shown in the official language in which they were submitted.
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DEVICE AND METHOD TO CONTROL STEEL PICKLING PROCESSES
Field of invention
The invention consists in a device and in a method to control pickling
processes
for carbon steels, austenitic, ferritic and martensitic stainless steels,
duplex steels
s and special alloys, in which said device aui:omatically manages sampling of
pickling baths and analysing of said samples in order to define (according to
specific conductivity and potentiometric methodologies) critic process
parameters
and to restore desired concentrations of necessary chemicals in the pickling
tanks.
The invention also permits to manage pickling conditions specific for the type
of
~o steel under treatment through definition of remotely activabie operative
procedures
automatically recalling and realising the most apt operating conditions for
pickling
of the specific kind of material under treatment.
State of the art
In the rolling, drawing, extrusion, heat treatment of steel products (such as
plates,
is strips, tubes, rods) oxide layers are formed on tlhe surtace thereof which
must be
removed both to get proper final appearance as well as passivity and
anticorrosive
properties for the final product, and to allow further working.
Said superficial oxide layers are usually eliminated by a chemical treatment
(pickling) based on exposition of the metallic material to the action of one
or more
2o acid baths containing inorganic minerat acid~~ (sulphuric, hydrochloric,
nitric,
hydrofluoric) alone or mixed with one another, at proper dilution and
temperature,
followed by at least one final rinsing in water.
For stainless steels, the usual pickling processes (either by immersion,
spraying or
turbulence) require a mixture of nitric and hydrofluoric acids; such processes
2s entrain very serious ecological problems due ~~lo emission of the reaction
by
products (extremely toxic nitrogen oxides) into the atmosphere as well as of
great
quantities of nitrates into waste water.
Hence, during the recent past a number of alternative "ecological" processes
have
been devised characterised by the elimination of nitric acid.
3o Among such processes, particularly effective air the industrial scale are
those
utilising mixtures of sulphuric or hydrochloric acid, hydrofluoric acid and
ferric ions,
in which the proper concentration of such ions into the pickling bath is
maintained
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through addition of hydrogen dioxide. Some of ;such processes are described in
Italian patents 1,245,594 and 1.255.655 {corresponding to US-A-5 345 383) and
in
European patent application EP-A-0 769 575.
In the traditional pickling technology, the man<~gement of the process usually
s includes an occasional control of the pickling bai;h through manual
titration of the
acidity or measure of the conductivity of the solution and of its iron content
(or of
total metals, through measurement of bath density}; it is also possible to
measure
the content in hydrofluoric acid by means of a specific ion selective
electrode.
Some of these techniques have been utilised in the automation of single
operations in nitric acid based pickling processes of stainless steels.
US patent 4,060,777 (LECO Corp.} discloses the use of ions selective
electrodes
for fluorine and hydrogen ions to measure the concentration of nitric acid {or
other
strong acid) and of hydrofluoric acid in pickling baths containing nitric and
hydrofluoric acids; the electric voltage data gathered by a control circuit
are
~s elaborated by a microprocessor to calculate the concentration of the two
acids and
to adjust relevant concentrations.
JP patent 55040908 (NIPPON Steel Corp.) discloses the determination of the
hydrofluoric acid and of another strong acid (nitric, hydrochloric, sulphuric}
through
the determination with ion selective electrodes of t;he relevant anions after
passing
2o the solution through ion exchange membranes, in order to adjust the acids
concentration.
US patent 5,286,368 (FOXBORO Corp.) measures the concentration of
hydrofluoric acid in a mixture of nitric and hydrofluoric acids through the
complexing ability of trivalent iron ions towards the fluorine ions,
permitting to
2s determine the concentration of the acids in the mixture.
JP patent 072944509 (KAWASAKI Steel Corp.) measures the concentrations of
free hydrofluoric and nitric acids and that of iron ion in a pickling solution
by
measuring the concentration of iron ion by an iron salicylate complex
absorptiometric method, the concentration of free hydrofluoric acid by an iron
3o acetylacetone complex fading absorptiometric mei;hod and the total
concentration
of free acids by neutralising titration method, the concentration of free
nitric acid
being measured by subtracting the concentration of free hydrofluoric acid from
the
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total concentration of free acids.
JP patent 081660003 (MITSUBISHI Heavy Ind. Ltd.) refers to a method for
continuously measuring the iron ion concentration in a pickling solution.
The continuous automatic management of such pickling processes based on nitric
s acid, though better than an occasional manual or automatic control
performed, for
instance, a few times per day, is not essential for the process in temls of
quality of
treated material, because of the functional characteristics of such baths;
particularly, in the pickling of stainless steels, such baths usually have
high nitric
acid concentrations (about 12-15%) and hydrofluoric acid concentration of
about
io 2-5%. The high nitric acid concentration ensures at the same time both high
acidity
and almost constant oxidising power, making it possible to manage the process
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through occasional additions of chemicals. Moreover, the determination of acid
concentration is sufficient to have an adequate control of the pickling
ability of the
bath.
On the contrary, the nitric acid free pickling systems, such as those
previously
s cited, found the oxidising properties of the system on the measure of the
ferric ions
(Fe3+) concentration, or better on the control of the Fe3+IFe2+ ratio.
In this case, because of the pickling reaction (1 )
2 Fe3k + Fe0 --~ 3 Fe2+ (1)
in a continuous process for the production of stainless steel strips or in
automatic,
io high productivity plants for rod pickling, the trivalent iron ions
concentration, the
Fe3+lFe~+ ratio and hence the oxidising capacity of the solution tend to
quickly
diminish, continuously and drastically modifying the bath behaviour.
The optimal conditions must be, therefore, continuously adjusted by means of
oxidising agents, such as hydrogen peroxide.
is Moreover, the variation of the trivalent iron concentration indirectly
influences also
the concentration of the free acids present into the bath.
For instance, in a pickling system based on sulphuric acid, hydrofluoric acid
and
ferric salts mixtures, this influence is linked to the following preferred
equilibria:
Fe3~ + n F' --~ FeFn~3-n~+
2a Fe2+ + S042~ ~ Fe~~04
Hence, during the oxidation/reduction reaction of the couple Fe3+IFe2+
liberation of
respectively sulphuric acid and of hydrofluoric. acid will occur from relevant
complex salts, thus modifying the bath composition.
A process control through occasional analytic: measures, followed by large
zs additions of chemicals to restore the best pickling conditions, causes,
therefore,
too ample variations of the bath parameters with adverse consequences on the
product quality and on the process costs.
On the other hand, frequent manual contrnls and relevant composition
adjustments are time consuming and costly, sincE; this requires a large amount
of
3o personnel to ensure a satisfactory control frequency (e.g, a control per
hour).
The criticity of nitric acid free pickling processes is obviously linked to
the total iron
amount dissolved per time unit, to the number of pickling tanks to be
controlled, to
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the number of materials requiring different operative conditions and to the
practical
capability to ask for frequent manual additions of acids into the tanks.
The management of pickling processes for :stainless steels such as those
previously cited for continuous pickling plants of stainless steel strips or
for high
s productivity automatic plants for rod processing, iproved to be critic for
the quality
of the final product; it can also be non economic without the use of an
automatic
system for sampling, controlling and dosing of the reactants.
The control device and the method according to i:he present invention require
the
utilisation of specific skilfulness and analytical methods for a proper
management
to of such processes.
Summary of the invention.
It is an object of present invention a control device. for nitric acid free
pickling baths
comprising means to take a sample of the bath to be analysed; means to analyse
said sample in order to measure a number of parameters according to specific
is conductivity methodologies {to fnd out the concentration of hydrofluoric
acid, of
the sulphuric acid or ofi another inorganic sl;rong acid) and potentiometric
methodologies (to find out the concentrations of trivalent and bivalent iron)
as well
as to measure the redox potential value of sand sample and its temperature;
restoring means, apt to calculate, according to the above measured values, the
zo quantity of correction chemicals {preferably hydrofluoric acid, sulphuric
acid and
an oxidising agent) to be added to the pickling bath in order to restore at
the
desired level the value of said parameters and to actuate at least a device to
add
into said pickling bath said quantities of correction chemicals.
Preferably, the measured parameters are the concentration of sulphuric acid,
that
2s of hydrofluoric acid and those of bivalent and trivalent iron ions.
It is a further object of present invention a control method for controlling
nitric acid
free pickling baths, which comprises at least the steps of:
- tacking a sample of a pickling bath;
- measuring the concentration in said sample of a pickling bath of the acids,
of the
3o bivalent iron and of the trivalent iron;
- measuring the redox potential and the temperature of said sample of a
pickling
bath;
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- restoring at preset levels the values of said measured concentrations in
said
pickilng bath by adding calculated quantity of correction chemicals to the
pickling
bath.
List of Figures
The invention will now be described with reference to a non limiting
embodiment
shown in the enclosed figures where:
Fig. 1 schematically shows a plant comprising an analysis device according to
the
invention;
Fig. 2 shows a simplified scheme of an analysis device according to the
invention;
m Fig. 3 schematically shows the analysis vesss~l CA of fig. 2, comprising a
conductivity measuring system and a preferred embodiment of the rinsing means
of the vessel itself and of the measure electrode;
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Fig. 4 schematically shows the analysis vesaei CA of Fig. 2, comprising a
potentiometric measuring system and a preferred embodiment of the . rinsing
means of the vessel itself and of the measure electrode.
In the enclosed Figures, corresponding elements will be identified with same
reference.
Detailed description of the invention
Fig. 1 schematically shows a plant comprising an analysis device according to
the
invention, comprising:
~ a plurality of pickling tanks V (V1, .... , Vn);
io ~ an analysis device A (later on described 'with reference to the
simplified
scheme of Fig. 2) which, in the herein described embodiment, includes a
couple of analysis devices (A1, A2) simultaneously working on different
parameters;
~ a plurality of reservoirs S (S1, S2, S3) each containing a solution at a
given
is concentration of one of the correction chemicals (a strong mineral acid,
preferably sulphuric acid, hydrofluoric acid and an oxidising agent,
preferably
but not necessarily hydrogen peroxide) to be added into one of the tanks V;
~ a plurality of permanent recycling piping, connecting tanks V to the
sampling
inputs l (Fig. 2) of the analysis device A;
20 ~ a plurality of piping to feed the correction chemicals, connecting
reservoirs S to
tanks V;
~ addition means enabling the analysis device A to control the addition into
tanks
V of correction chemicals contained in reservoirs S.
For simplicity, in Fig, 1 components not interesting for the present
description,
2s such as valves, pumps, actuators, filtering and rinsing means, known per
se, as
well as other, if any, circuitry components are omitl:ed.
The analysis device A comprises (Fig. 2) means to pick up from a vessel V a
sample of the pickling bath; means to analyse it to measure, according to
specific
conductivity and potentiometric methodologies, the preset parameters (the
strong
~o mineral acid, for instance sulphuric acid, and the hydroflcroric acid
concentrations,
as well as the ones of trivalent and bivalent iron), the redox potential and
the
temperature of said diluted sample; means to calculate the amounts of
correction
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chemicals to be sent from reservoirs S to tanks V to adjust said parameters
and
means to actuate the devices at the output of reservoirs S to send into the
pickling
bath the calculated amounts of said correction cheimicals.
Hereinafter, ~'sufphuric acid" will means any strong mineral acid.
s Since the time necessary for measuring the concentration of sulphuric and
hydrofluoric acids is shorter than that necessar)r for the measure of iron
ions
concentration (just some minutes vs about 30 minutes), the analysis devices
(A1,
A2) are preferably divided, each one being specialised in only one of said
analysis
(measure of sulphuric acid and of hydrofluoric acid, respectively of iron ions
to concentrations).
The analysis devices (A1, A2) can be managed by a logic unit of higher level,
not
shown in the figures, which can be placed "in loco" or in a remote site,
connected
to the analysis devices (A1, A2) through bi-directional transmission means,
known
per se.
is Alternatively, said analysis devices (A1, A2) can be of the same model and
comprise the analytical means apt to measure the concentration both of the
acids
(sulphuric and hydrofluoric) and of the iron ions.
In such a case, the device according to the invention could also work in case
of
malfunction of one of the analysis devices (A1, A2).
2o Fig. 2 shows a simplified scheme of an analysis device A {A1, A2) of Fig.
1,
comprising in combination relationship:
~ a sampling module C, the sampling inputs of which 1 (11, ...., In) are in
sequence connected to the permanent recycling piping among the pickling
tanks V (V1, ..., Vn; Fig. 1 } and the analysis device A; at least a reservoir
{not
2s shown), in which the bath sample to be analysed is loaded, is provided
inside
the sampling module C;
~ a reagent storage DR, containing the chemicals for the analyses;
~ dosing means D (D1, D2) apt to draw the amounts of chemicals necessary to
the analyses and to transfer the same into the analysis vessel CA, part of the
3o dosing means D being apt to draw with low accuracy (from about 2 to about
5%) high quantities of chemicals, the remaining dosing means being apt to
draw with high accuracy (about 0,1%) small quantities of chemicals; in Fig. 2
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the dosing means D with low and high acc~,rracy are respectively grouped in
two different functional units (D1, D2).;
~ an analysis vessel CA, containing the measure electrodes (generically named
EM in Fig. 2), receiving from sampling module C the bath sample to be
s analysed, from dosing means D the chemicals necessary for the analysis and
from a reservoir W (not shown) the water (preferably having a conductivity
lesser than 100 microsiemens) necessary to dilute said sample to a desired
dilution ratio; in Fig. 2 further elements (such as stirrers) present in
analysis
vessel CA are omitted, not being part of present invention;
io ~ a logic unit UL, controlling and managing the analysis procedures,
acquiring
and elaborating the information from measure electrodes EM and actuating
means to send inta the pickling bath the solutions of the correction chemicais
contained in the reservoirs S (Fig. 1 ).
in a preferred but not limiting embodiment the dosing means of functional unit
D1
is are peristaltic pumps with constant delivery, while the dosing means of
functional
unit D2 are syringes in antacid material (e.g. PES) operated by an electric
stepping motor.
Again in a preferred embodiment, the analysis device A also comprises means
(hereinafter described with reference to Figures 3 and 4) permitting to rinse
the
zo analysis vessel CA and the measure electrodes EM after each measure with
water
and after a given number of measures with a chemical solution (preferably but
not
necessarily 10-20% hydrochloric acid}, thus permitting to keep in optimal
conditions the measure electrodes EM, to have reliable analytical data, to
reduce
to a minimum the maintenance interventions and i:o highly enhance the
electrodes
2s life.
To ensure a constant quality of the final product, each type or family of
materials to
be pickled must be treated according to standard and characteristic parameters
(hydrofluoric and sulphuric acids concentration, trivalent and bivalent iron
ions
concentration, ratio between trivalent and bivalent iron ions, hydrogen
peroxide
3o concentration, temperature of the sample to be analysed, and so on); in a
preferred embodiment of the invention, the parameters characterising each
working step as well as those concerning the operation of the analysis device
A,
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which all permit to perform different analyses on pickling baths relating to
the
specific working step, are grouped into operating procedures biuniquely
correlated
with the material itself and stored in the logic unit UL, which are when
necessary
recalled according to the material to be pickled.
s Preferably but not necessarily, an operating procedure comprises at least
the
following information:
~ the order and the kind of the analyses to be performed;
~ the prefixed values of the parameters for the pickling bath;
~ the magnitude of the admissible deviations with respect to the prefixed
values,
io beyond which the logic unit UL actuates said means to send into the
pickling
bath the solutions of the correction chemicals contained in reservoirs S,
~ the dilution ratios with water in the analysis vessel CA of the pickling
bath
sample to be analysed.
The proper operation of the analysis device A can be advantageously checked
is periodically and automatically; to this end, in a preferred embodiment of
the
present invention a further operative autocalibration procedure is stored in
the
logic unit UL which activates after a given number of analyses and comprises
the
functional steps of drawing from a container (preferably but not necessarily
located
in the reagent storage DR) a fixed amount of a standard solution having a
known
2o composition, of transferring it into the analysis vessel CA, of analysing
it, of
comparing the obtained analytical results with the known composition and of
activating alarm signals if the deviation between obtained analytical results
and
known concentrations is larger than a desired value.
According to an embodiment of present invention, not shown in the figures, the
2s logic unit UL can be connected to a central operative post andlor to a
logic unit of
higher level, by which it can be controlled and managed; as above said, this
logic
unit of higher (suet can be placed "in situ" or be remote.
In particular, at each change of working activity, the central post operator
can
modify the operative procedure performed by one or more of the logic units UL,
3o activating the one pertaining to the activity to be initiated; the operator
can also
recall from one or more of the logic units UL an operating procedure, modify
it and
have it to be performed by the logic units UL andlor inputting a new operative
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procedure storing it in the logic units UL.
The analytical methods, which are utilised in the analysis of the pickling
baths, will
now be described to better understand the described details, which are part of
present invention.
s a) Conductivity determination of hydrofluoric acid and of sulrJhuric acid
(or of other
acid strong with respect to hydrofluoric acid)
This determination is based on the principle that, in an aqueous solution
formed by
a mixture of a weak acid such as hydrofluoric acid and of a stronger acid such
as
sulphuric acid, the solution conductivity is practically equivalent to the one
of the
Io strong acid at the same concentration; the method also exploits (in a stage
subsequent to a first conductivity measure on a bath sample duly diluted to
measure the sulphuric acid concentration) the high affinity of hydrofluoric
acid for a
metal canon present in the solution as a salt of known concentration. The salt
anion most preferably come from a strong acid (e.g, nitric or hydrochloric
acid) so
zs that the reaction forming fluorocompiexes of the metal cation and
hydrofluoric acid
will generate a significant increase of conductivity due to the formation of
an
equivalent amount of fully dissociated strong acid, measured by a second
conductivity measure.
For instance:
2o n HF + Fe(N03)3 --~ FeFn~3'"~+ + n HN03
Such conductivity increase is, therefore, proportional to the concentration of
hydrofluoric acid which, after a proper calibration, can be quantitatively
measured.
Such salts can be, for instance, ferric nitrate, fE:rric chloride, aluminium
nitrate,
aluminium chloride; in a preferred embodiment of the invention a solution of
ferric
2s nitrate*9H20 is utilised, at a concentration of 750 ~~/I.
To ensure a sufficiently linear dependence of conductivity from the variation
of
acids concentration, the sample dilution must be attentively evaluated as a
function of the concentration of the acids present in the bath to be analysed;
as a
non-limiting example, for sulphuric acid concentrations up to 200 gll and for
3o hydrofluoric acid concentrations up to 60 g/l, dilution ratios from 1:100
to 5:100,
and preferably 4:100, are deemed to be acceptable.
Another variable essential for the obtainment of reliable results (which must
be
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managed by the logic unit UL of the analysis device A) is the sample
temperature
after dilution with wafer; in fact, in the industry the water temperature can
have
considerable variations (usually between +5 and +40° C) according to
the weather,
to the water source and to the holding time in resen~oir W.
s It is apparent that a conductivity measure is greatly influenced by the
temperature,
and usually such a problem is overcome by means of an automatic compensation
system incorporated into the measure device; in the present case, the
automatic
compensation can correctly adjust only the effect on the first conductivity
measure
(determination of the sulphuric acid concentration) but not on the second one
io (determination of the hydrofluoric acid concentration) performed after
addition of
ferric nitrate as the solution composition is changed and its dependence from
the
temperature is, in fact, different before and after the ferric nitrate
addition.
This critic problem is solved with an analysis device A according to the
invention,
in which the logic unit UL takes into account the conductivity variation due
to the
is addition of a volume v3 of the ferric nitrate solution, depending on the
sample
temperature.
The amount of ferric nitrate utilised during the titration must be such to
ensure a
full complexing of the hydrofluoric acid; in the con:~idered system, for
hydrofluoric
acid concentration less than 60 gll the ratio between the volume v3 of a
solution of
2o ferric nitrate*9H20 at 750 gll and the volume v1 of the bath sample must be
higher
than 0,5 and preferably 1.
As a non limiting example the following operating procedure is given along
with
relevant computations for a sample dilution of 4:100 in volume:
~ filling of the analysis vessel CA, by means of dosing means D2, with a given
2s water volume v2, having a conductivity of less than 100 microsiemens to
obtain
a dilution ratio of 4:100;
~ picking up from the sampling module C (by means of dosing means D2) of a
given volume v1 of the pickling bath sample to be analysed;
~ start stirring the solution;
30 ~ first conductivity measure (L~);
~ addition of a given volume v3 = v1 of a solution of ferric nitrate*9H2~ at
750
gll;
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~ stirring of the solution and measuring of its temperature T;
~ second conductivity measure (Lz).
The logic unit UL acquires the data L~, Lz, T and automatically find the
concentration of the acids through the foNowing callculations:
s ~ sulphuric acid concentration (gll): a ~ Liz + b ~ I_~ -c
~ hydrofluoric acid concentration (g/l): a~ ~ bz + ~~, ~ 8 - c~
where:
a, b, c, a,, b,, c~ are coefficients of the quadratic e<~uations;
8=Lz_L~_~
to ~=c2+{c3~T);
cz, c3 are constants depending on the quantity of ferric nitrate*9H20 added to
the
diluted sample before the second conductivity measure.
In this example:
a = 0,0066; b = 5,015; c = 6,98
is a~ = 0,0120; b~ = 2,8$1; c~ = 3,81;
c2 = 9,632; c3 = 0,297.
Fig. 3 shows the characteristics of the conductive y cell CC, which specific
form
allows to minimise the negative effects due to the high viscosity of the
solution and
to facilitate the rinsing of the measure platinum plagues.
zo Said conductivity cell CC comprises a hollow body B, in glass and having a
substantially cylindrical shape, containing two blackened platinum plaques EL;
at
the lower and upper parts of the hollow body B there are holes (F1, F2)
letting the
sample to be analysed to circulate inside the hoilov~ body B.
Preferably, the hollow body B has a diameter of about 20 mm (and anyhow
2s comprised between about 17 and 23 mm) and <~ height of about 40 mm {and
anyhow comprised between about 35 and 45 mm); the EL plaques dimensions are
about 10 x 5 mm (and anyhow between about 8 x 12 mm and about 3 x 7mm), the
distance from one another being about 15 mm (~~nd anyhow between about 12
and 18 mm).
~o To avoid polarisation of electrodes EL, the mea;>ure electric circuit (not
shown)
connected to the conductivity cell CC must work at high frequency (between 25
and 40 kHz).
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b) Determination of bivalent iron
The bivalent iron determination can be made through potentiometric analysis,
by
potassium permanganate titration according to thE; classic methodology.
The operative sequence requires:
s ~ pouring into the analysis vessel CA a given water volume v2, through the
overflow pipe TP, to obtain a dilution ratio >_ 1:;~0;
~ picking up from the sampling module C (by means of dosing means D2) a
given volume v1 of the pickling bath sample to be analysed, and addition of
said sample into the analysis vessel CA;
Io ~ acidification of the diluted pickling bath sample by means of addition
into the
analysis vessel CA (by means of dosing means D1 ) of a given non-critical
amount of a solution of a strong acid, e,g, a sulphuric acid solution 1:1 bw;
~ potentiometric titration, having a preset Tina! point or with an automatic
search
of the final point with a 0,1 N potassium permanganate solution added into
Is analysis vessel CA by means of dosing mean's D2;
~ emptying and rinsing analysis vessel CA.
c) Determination of trivalent iron
The trivalent iron is measured by iodometric titration, utilising however some
specific attention to permit the use of an automatic device and the obtention
of
2o reliable and reproducible results.
Said determination requires the following operatin~~ sequence:
~ pouring into the analysis vessel CA a given water volume v2, through the
overflow pipe TP, to obtain a dilution ratio >_ 1:;i0;
~ picking up from the sampling module C (by means of dosing means D2) a
25 given volume v1 of the pickling bath sample to be analysed, and addition of
said sample into analysis vessel CA;
~ start of stirring;
~ addition into analysis vessel CA (by means of dosing means D1 ) of a given
non-critical volume of a lanthanum nitrate solution having a known
3o concentration;
~ waiting for 30 s without stirring;
~ addition into analysis vessel CA (by means c~f dosing means D1 ) of a given
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non-critical volume of a hydrochloric acid solution at 1:1 voi;
~ addition into analysis vessel CA (by means of dosing means D1 ) of a given
non-critical volume of a potassium iodide solution, at a concentration for
instance of 1 kg/l;
s ~ waiting for 5 min without stirring;
~ start of solution stirring;
~ potentiometric titration with 0,1 N sodium thiosulphate (added by means of
dosing means D2) of the iodine liberated by the reaction of trivalent iron
with
potassium iodide;
io ~ emptying and water rinsing of analysis vessel C;A.
For this automatic analysis, a most prominent aspect is the use of lanthanum
nitrate; in fact, the addition of a salt including a cation able to complex
the fluorine
ion linked to the iron ion is essential for the quantitative analysis of the
ferric ion
through iodometric analysis.
is This analysis can be manually performed utilising a solution of calcium
chloride;
however it was proved that calcium chloride cannot be utilised for the
automatic
titration of trivalent iron, because of the subsE;quent precipitation of
calcium
fluoride and of calcium sulphate, which tend to continuously foul the
electrodes in
analysis vessel CA, giving rise to significant errors and complex upkeeping.
On the
2o contrary, it was found that lanthanum salts can qu~~ntitatively release the
ferric ion,
generating powdery and non-sticking lanthanum fluoride precipitates, thus
permitting the automatic management of the process with high reliability and
very
limited upkeeping.
This same result can also be achieved by adding to the system a complexing
2s agent for the iron ion, which however can quantitatively release it during
the
subsequent reaction with potassium iodide; complexing agents such as EDTA can
be fit for this purpose.
The potentiometric system, schematically illustrated in Fig. 4, comprises a
measure electrode E (inert to the working environment) immersed in analysis
3o vessel CA and a reference electrode R (preferably in glass, of the type
AgIAgCI)
positioned outside said analysis vessel CA and in contact with the solution
under
measurement through a saline bridge, comprising an electrolyte (contained in a
CA 02353387 2001-06-O1
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14
tank SR) which is made to continuously pass through a porous septum SP placed
at an extremity of a small plastic tube T.
The continuous passage of the electrolyte through the septum SP is intended to
consent the electric continuity; to avoid the contact between septum SP and
the
s hydrofluoric acid of the pickling bath and to continuously renovate the
electrolyte.
In a preferred embodiment, the measure electrode E is made from a body in
antacid material bearing at one extremity a platinum plaque P, one of whose
surfaces, mirror finished, faces downwards, thus preventing the salts deriving
from
the reaction products to be deposed on the meas~.Ering face of plaque P,
fouling it.
to Advantageously, to the electrolyte (preferably GSM potassium chloride) can
be
added a 10% solution of glycerine (of another compatible product having a
viscosity at 20°C comprised between 1,15 and 1;45 centipoise, inert
with respect
to the working environment and functionally equivalent) to enhance the
viscosity
and reduce the flow speed, thus permitting a better autonomy of the
potentiometric
is system for a given volume of tank SR.
d) Determination of h r~oaen peroxide
The determination of the free hydrogen peroxide in nitric acid free pickling
processes such as the ones here described, is necessary in the treatment of
ferritic and martensitic steels for the control of finis,hinglpassivating
baths generally
2o utilised as the last operation before final rinsing; usually said baths
comprise
sulphuric acid (20-60 g/l), hydrogen peroxide (3-1CE g/l) and sometimes
hydrofluoric
acid.
The analytical methodology and the operative sequence utilised for the
determination of the hydrogen peroxide are the same utilised for the
determination
2s of bivalent iron in the pickling baths.
e) Determination of the redox potential
The device according to the invention measure, before the determination of
bivalent iron, the solution redox potential on the diluted pickling bath
sample
utilising the potentiometric system already described; the thus obtained value
is
3o very near (~ 20 mV) to the redox potential measured in the bath before its
dilution.
The obtained value is compared with a range of values (usually comprised
between 200 and 550 mV) stored into the logic unit UL to be utilised as a
first
CA 02353387 2001-06-O1
WO 00!33061 PCT/EP99/09367
signal of the correct operation of the system: if the measured value is
outside of
said range, the logic unit UL of the analysis device A stops the analysis
procedure
and sends an alarm. The calibration of the potentiometric system is made at a
given frequency (say, once per week) by redox potential measure on a standard
s solutian of known potential (usually 468 mV).
As already said, the logic unit UL of an analysis device 1 according to
present
invention, after measuring the desired parametE;rs on the pickling bath sample
under analysis, calculates the amount of each of the solutions at known
concentration of the correction cherriicals (sulphuric acid, fiydrofluoric
acid and
to oxidising agent) contained in reservoirs S, said chemicals being
opportunely
added to the pickling bath to restore the desired composition values and
actuates
addition means {such as, for instance, dosing pumps or electrovalves) at the
output of reservoirs S to send into the pickling bath said calculated amounts
of the
correction chemicals.
is Being known the plant characteristics (volume of tank V, delivery of each
adding
means; preset concentration values for said correction chemicals,
concentration of
said chemicals, and so on) to have the correct amount of correction chemicals
added to the pickling bath, the logic unit UL mast just calculate the
actuating
period of said addition means.
2o Studies and experiments of this Applicants did chow that, to bring back to
the
desired values the concentrations in the pickling bath of sulphuric acid, of
hydrofluoric acid, of trivalent iron ion and of the oxidising reagent, the
logic unit UL
must actuate each of the addition means regulatiing the addition into the
pickling
bath of the sulphuric acid, hydrofluoric acid and oxidising reagent solutions,
for a
2s period of time s (in seconds) given by the following expression:
s = K ' {vo - vm) ' vb~~P
in which:
s = actuating time {seconds);
K = factor inversely proportional to the concentration of the correction
chemicals
30 (I/g);
vo = given concentration for the specific corrective chemical (g/I);
vm = concentration of said specific corrective chemical resulting from the
analysts
CA 02353387 2001-06-O1
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lb
(g/l);
Vb = volume of tank V;
p = delivery of the addition means (I/s).
To bring back to the desired value the ratio F; between the concentration of
s trivalent and bivalent iron ions, the logic unit UL calculates the actuating
period s1
{in seconds) of the addition means sending into the pickling bath the
oxidising
reagent solution, by:
~ calculating B~ = A ~ R, in which A is the concE:ntration (gli) of the
bivalent iron
ion resulting from the titration with permanganate, R is the desired ratio
to between the concentration of, respectively, the trivalent and the bivalent
iron
ions, and B~ is the theoretical concentration of i;he trivalent iron ion;
~ comparing B, with the measured concentration B of the trivalent iron ion
(gll);
~ if B >_ B~ (the measured concentration of the trivalent iron ions is greater
than
that of the bivalent ones) the logic unit UL does not act;
is ~ if B < B, {the trivalent iron ions concentration is less than the
measured one)
the logic unit UL calculates the actuating period s1 of the addition means.
regulating the addition to the pickling bath of 'the oxidant reagent solution,
by
means of the formula
s1 =K~K~~C/p
2o in which:
~ s~ = actuating period (s);
~ K factor inversely proportional to the concentration of the correction
chemical
(ilg),
~ K~ = factor proportional to the tank volume V (I),;
2s ~ C = (B~-B)/R = amount of bivalent iron ion to bE; oxidised to restore the
desired
value for iron ion concentration (gll);
~ p = delivery of the addition means (Ils).
Alternatively the bath can be managed in function of the ratio R between
trivalent
iron and bivalent iron according to the following calculation:
30 ~ Calculation of the total iron T = A + B
where A is the concentration of Fe2+ obtained from the permanganometrie
analysis
and B is the concentration of Fe3+ obtained from the iodometric analysis.
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I7
~ Calculation R = BIA
~ Compare R (present ratio) with R1 (pre-fixed ratio)
~ If R > R1 the logic unit UL does not make any addition of oxidizing product
~ If R < R1 the logic unit UL calculates the actuating period s1 (in seconds)
of the
s addition means regulating the addition the pickling bath of the oxidizing
product
solution according to the following formula
s1=K.K~~C/p
where
C = A-[(A+B)/(R1+1 )] = amount of bivalent iron t~~ oxidize to restore the
present
io ratio R to the prefixed value R1
s1 = actuating period (s)
K = coefficient, inversely proportional to the volume of the tank V (I)
P = delivery of the addition means (Ils).
Fig. 3 schematically shows an exploded view of the analysis vessel CA of Fig.
2,
is comprising a measure system of conductivity type and a preferred embodiment
of
the rinsing means of analysis vessel CA and of measure cell CC.
In Fig. 3 it is possible to see:
~ the conductivity measure cell CC used for conductivity measure;
~ the analysis vessel CA;
20 ~ the overflow TP, mobile, the position of which (controlled by the logic
unit UL)
consents to set the liquid level in the analysis vessel CA, and to empty the
same vessel;
~ rinsing means (F, U) controlled by the logic unit UL, enabling the rinsing
of
analysis vessel CA and of the conductivity measure cell CC.
25 Fig. 4 schematically shows an exploded view of the analysis vessel CA of
Fig. 2,
comprising a potentiometric measure system as well as a preferred embodiment,
similar to the one in Fig. 3, of the rinsing means of analysis vessel CA and
of the
measure electrodes.
In Fig. 4 can be seen:
30 ~ the potentiometric system, comprising the mea ure electrode E, the
reference
electrode R, positioned outside of the analysis vessel CA, and the saline
bridge
which in turn comprises an electrolyte conta~~ined in tank SR, continuously
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18
passing through a porous septum SP placed at one extremity of a small plastic
tube T;
~ the analysis vessel CA;
~ the mobile overflow TP, the position of which (;controlled by the logic unit
UL)
s consents to set the liquid level in the analysis vessel CA, and to empty the
same analysis vessel;
~ rinsing means (F, U) controlled by the logic unit UL, enabling the rinsing
of
analysis vessel CA, of the electrode E extremity and of the porous septum SP.
in the preferred embodiment shown ~in Figures 3 and 4, such rinsing means
to comprise a plurality of slits F placed along the upper edge of the analysis
vessel
CA and a nozzle U apt to rinse with a water spray the extremity of the measure
electrode E and the porous septum SP, respectively the conductivity measure
cell
CC; in Figures 3 and 4, can also be seen the lid CP for the analysis vessel CA
and means MS supporting the electrode E, the small tube T of the
potentiometric
is system, the conductivity measure cell CC and the small tubes (not
explicitly
indicated in Figures 3 and 4} connecting the dosing means D (D1, D2) with the
analysis vessel CA; lid CP and supporting means MS will not be described, as
known per se and anyhow not pertaining to present: invention.
Preferably, the analysis vessel CA, the measurf: electrode E and the porous
zo septum SP (respectively the analysis vessel CA and the conductivity measure
cell
CC) are water rinsed after each analysis and w~~shed with a chemical solution
after a given number of analyses.
To rinse said components with water after each analysis the logic unit UL
performs
in sequence the following steps:
2s ~ fully emptying analysis vessel CA;
~ pouring in said analysis vessel CA a large amount of water through slits F;
~ filling with water analysis vessel CA up to havE; the tip of electrode E and
the
porous septum SP, respectively the conductivity measure cell CC immersed;
~ emptying analysis vessel CA;
30 ~ further rinsing the tip of electrode E and the porous septum SP,
respectively
the conductivity measure cell CC by spraying on them some water through
ryozzle U;
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19
~ emptying analysis vessel CA and preparing it for the subsequent analysis.
To wash after a given number of analyses with a chemical solution (preferably
10-
20% hydrochloric acid) the analysis vessel CA, the tip of electrode E and the
porous septum SP (respectively the analysis vessel CA and the conductivity
s measure cell CC), the logic unit UL fills with water analysis vessel CA
through slits
F up to have the tip of electrode E and the porous septum, respectively the
conductivity measure cell CC immersed, picks up from a tank (preferably but
non
necessarily placed within the reagent's storage DR) an amount of product
(preferably hydrochloric acid) necessary for said chemical washing and send it
into
to analysis vessel CA; after a given period of time the logic unit UL empties
analysis
vessel CA and rinse it with water, to eliminate any trace of the chemical
solution.
Moreover, when not working, analysis vessel CA is filled with water through
slits F
and nozzle U, to avoid any fouling andlor damaging of the electrode E tip, of
the
porous septum SP, and of the conductivity measure cell CC.
is It is possible for an expert to modify and improve, as suggested by
ordinary
experience and by the natural technical evolution, the device for the control
of
pickling baths according to present description, still remaining within the
scope of
present invention.
