RHEOLOGICAL EFFECT OF DIFFERENT DEFLOCCULATION … Barrachina_493 br_4_2015_Journal_CTM...The rheology describes the deformation of a body under stress impact where bodies in this

Ester Barrachina, Jordi Llop, Maria-Dolores Notari, Diego Fraga, Rafael Martí, Ivan Calvet, Aitor Rey, Teodora S. Lyubenova, Stephan V. Kozhukharov, Vladimir S. Kozhukharov, Juan B. Carda

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Journal of Chemical Technology and Metallurgy, 50, 4, 2015, 493-502

RHEOLOGICAL EFFECT OF DIFFERENT DEFLOCCULATION MECHANISMS ON A PORCELAIN CERAMIC COMPOSITION

Ester Barrachina1, Jordi Llop2, Maria-Dolores Notari2, Diego Fraga1, Rafael Martí1, Ivan Calvet1, Aitor Rey1, Teodora S. Lyubenova1, Stephan V. Kozhukharov3,

Vladimir S. Kozhukharov3, Juan B. Carda1

1 Department of Inorganic and Organic Chemistry, Universitat Jaume I, Castellón de la Plana, España (Spain) Vicent Sos Baynat Blv., 12071 Castelló de la Plana, Spain E-mail: [email protected] 2 Escola Superior Ceràmica de L´Alcora, Castellón, (Spain) Av. Corts Valencianes, 23, 12110 L’Alcora, Castellón, Spain3 Laboratory of Advanced Materials Research (LAMAR) University of Chemical Technology and Metallurgy 8 Kl. Ohridski, 1756 Sofia, Bulgaria

ABSTRACT

The current environmental policy imposes general optimisation of the industrial process. There are two critical steps related to suspensions fluidity in the ceramic spray-drying industry: the emptying of the ball mill and the spray-drying operation. The chemicals producers provide a large number of commercial deflocculants aiming to improve both processes. The present research is focused on the comparison of the rheological effect of four commercial liquid deflocculants on a porcelain ceramic composition in view of different mechanisms of deflocculation. The suspensions prepared have 70 %-solid content and a range of deflocculant concentrations between 0.1 mass % and 0.6 mass %. Each suspension is characterized by measuring its solid content, particle size distribution, pH values, conductivity and rheology, including viscosity at two shear rates and thixotropy calculated from the hysteresis area of the flowability curves. On the basis of the experiments carried out adequate conclusions are presented.

Keywords: spray-drying, rheological properties, deflocculants, viscosity, thixotropy, suspensions, porcelain industry.

Received 10 November 2014Accepted 4 May 2015

INTRODUCTION

At present, the environmental policy forces the industrial companies to improve their anti-pollution systems directly through optimization of the produc-tion process [1 - 4]. In this context, the spray-drying ceramic factories tend to use high solid content suspen-sions in order to reduce the evaporated water volume, increase their production efficiency and reduce energy consumption. As it is shown in Figs. 1 and 2, on the basis of real average data referring to the production

rate, energetic consumption and energy savings levels for a large number of spray-drying Spanish industries we can assume that the solid content increase starting at 68 % will result in energetic savings in the range from 30 % to 50 % [5, 6]. The main way to stand behind the fluidity of ceramic suspensions in industries is to add and apply proper deflocculants and dispersants [7 - 10].

It is well known that the dispersion of ceramic particles is a fundamental step in the industrial ceramic processes aiming to obtain a homogeneous and stable system of individual non-aggregated particles [11]. In

Journal of Chemical Technology and Metallurgy, 50, 4, 2015

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the ceramic industry, chemical additives are usually used to reduce the suspension viscosity, while keeping the solid content as high as possible [12]. Mixtures of sodium tripolyphosphate and metasilicate in a solid state are traditional and widely used materials for suspensions of ceramic paste deflocculation. Recently, this trend has changed in the ceramic sector through mixing liquid organic compounds (mainly those on acrylic acidic polymers base) with traditional materials thus reducing the process costs. However, the use of solid compounds in industries has been limited owing to the health risk involved in their handling [13].

Currently, inorganic substances (silicates, phos-phates and carbonates sodium salts) or organic additives, generally polyacrylates salts of different molecular structures are commonly used in a liquid state. In the ceramic technology they are added from 0.1 to 0.6 mass

percents in order to decrease the dynamic viscosity down to values ca. 300 ± 100 mPa s [14].

The primary reason to use sodium silicate is to add silicate but not sodium ions. Nevertheless, since sodium silicate is a soluble compound, its application provides an excellent way to add silicate ions to suspensions. In fact they are generally added to remove unwanted flocculating cations such as magnesium and calcium cations. Consequently, the effect of sodium silicate refers to pH variation of the suspension used and a specific adsorption of negatively charged deflocculant anions on the clay positive edges being the driving force of electrostatic nature.

Organic deflocculants like polyacrylates and phos-phonates have to be added if the flocculating cations amount in the suspensions is not adequate. They interact with the coating flocculating cations and particle surfaces and hence increase the effective electrostatic charges. The polyacrylates are a chemical class of acrylate polymers obtained through polymerization of acrylic acid esters and salts, whereas the phosphonates are organic com-pounds containing C-PO(OH)2 or C-PO(OR)2 groups, where R stands for alkyl or aryl units [15], respectively.

Nowadays, three basic mechanisms are known and can act either individually, or in combination, in depend-ence of the deflocculant used [16, 17]. They are:

• Electrostatic action: cation exchange affecting the thickness of the electrical double layer of the raw materials particles;

• Steric hindrance: steric repulsion by the introduc-tion of functional groups which act as spacers among the raw material particles;

• Cation capture: binding of interfering cations by complexing.

The rheology describes the deformation of a body under stress impact where bodies in this context mean solids, liquids or gases. The rheological behaviour of suspensions is focused on viscosity which can be de-scribed as the physical property of a liquid to resist a shear-induced flow. It may be influenced by the follow-ing independent factors: (i) physico-chemical nature of the investigated substance; (ii) temperature variations; (iii) pressure in the compressed fluids; (iv) shear rate; (v) time and history of the dispersion and (vi) electrical field applied. Suspensions for which the latter phenomenon is typical are those whose flow behaviour is strongly influenced by the magnitude of the electric fields acting

Fig. 1. Correlation between the production rate, expressed in kg sprayed powder per kg evaporated water and energy consumption (expressed in kWh expensed energy for each kg dried powder product for spray-drying process performed in typical industrial regime .

Fig. 2. Energy consumption reduction for Spray-drying operation performed in optimised industrial conditions, due to the increased solid content of the suspensions used.


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upon them [18 - 20].The more common rheological behaviours are

schematically illustrated in Fig. 3 together with the cor-responding flow and viscosity curves.

Each liquid which is presented by a straight line of a slope (α) starting at the origin of the flow curve and whose viscosity is independent of the shear rate (γ) can be considered a “Newtonian liquid” (Fig.3, curve 1). The case where the apparent viscosity ηapp of the pseu-doplastic liquid (Fig.3, curve 2) decreases drastically with shear rate increase from low to high levels refers to a non-Newtonian liquid. Liquids whose viscosity increases with shear rate increase under certain condi-tions of stress (Fig. 3, curve 3) show dilatant behaviour, i.e. these are non-Newtonian liquids as well. It can be assumed that the difference between the pseudoplastic liquids and the typical plastic ones is that the former possess an additional yield point (Fig.3, curve 4) being also non-Newtonian liquids. [11,19, 21 - 23].

The aim of the present research is to compare the rheological effect of four commercial liquid defloccu-lants exhibiting preferential action mechanisms [24 - 31]

on a porcelain ceramic standard composition in view of the different mechanisms of deflocculation.

EXPERIMENTAL

Commercial Deflocculants Four commercial deflocculants of different particle

dispersion mechanism are selected and their basic prop-erties are specified in Table 1.

The deflocculants investigated are marked by A, B, C and D aiming clarity. Compositions A, B and C contain sodium silicate with polymers or phosphonates, whereas D contains only phosphonates. Besides, compositions A and B act according to the mechanism of electrostatic action (due to the silicate anions) and steric hindrance (rendered by the polymeric moieties), whereas C defloc-culant action is based on the electrical charges of the silicate anions combined with the complexant effect of the phosphate groups. D deflocculant stabilizes the suspensions by complexant caption ability. Besides, sup-plemental beneficial effect is also expected for phosphate containing compositions C and D since the phosphate

Fig. 3. Common flow behaviour of Newtonian (1) and Non-Newtonian liquids (2, 3, and 4) according to [21] 1 – Newtonian liquid; 2 – Pseudoplastic liquid, 3 – Dilatant liquid, 4 – Plastic liquid.

Table 1. Basic description of the tested deflocculants marked with indexes A, B, C and D.

Deflocculant Composition Mechanism

A Silicate (min. 90 %mass) with polymer

Electrostatic Action / Steric Hindrance

B Silicate (min. 90 %mass) with polymer

Electrostatic Action / Steric Hindrance

C Silicate (min. 90 %mass) with phosphonates

Electrostatic Action / Complexant Effect

D Phosphonates Complexant Effect


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compounds are known to have corrosion protective capability [32, 33]. The latter provides active corrosion inhibition by passivation of the steel ball milling and spray-drying industrial equipment (such as pipelines and other facilities). All deflocculants investigated in the present research are supplied by Zschimmer & Schwarz international Company [17]. The analysis of the derived deflocculant-containing suspensions was performed by systematic pH and electrical conductivity measurements.

Suspensions preparationThe suspensions prepared consisted of a standard

porcelain spray-dried powder mixed with water (a solid content of 70 mass %) and different types of deflocculant additives (A, B, C or D) of amounts ranging from 0.1 mass % to 0.6 mass %. They were milled for 7 minutes in a planetary alumina ball mill to achieve particles size less 63 µm. This industrial milling requirement refers to all particles in case of porcelain pastes preparation.

Electrolytical and rheological measurements of the suspensions obtained

All tested parameters were measured immediately after the preparation of each paste in order to avoid any ageing impact on the suspensions prior to the respective investigation. In each case the solid phase fraction was quantified from the difference between the wet and dried weight of a sample slip. The particle size distribution was measured by a LS 13 320 Laser Diffraction Particle size analyzer of Beckman Coulter. These measurements were performed in order to control the variation of rheological properties driven by the size distribution modification. The latter is attributed to the dependence of the suspen-sions fluidity on the deflocculation rate.

The electrolytical parameters were tested by a Eutech pH6+ and a Eutech Cond7, product of Thermo Fisher Scientific Company. The rheological variables of

each suspension were obtained using a traditional control rate mode (i.e. RC-mode) comprising seven subsequent stages. Haake Viscotester 550 was used. The results obtained are summarised in Table 2.

The first rheometrical cycle was used to adjust the testing time of all suspensions. The viscosity and thixotropy values were derived on the ground of the second cycle. The apparent dynamic viscosity was studied at two different shear rates: a low one of100 s-1 and a high one of 1000 s-1, respectively. The lower shear rate simulates the emptying of the industrial ball mills, while the higher one resembles the rate used during the spray-drying operation. The magnitude of thixotropy was determined by MCR- rheometer (Haake Viscotester 550). Its values provided the determination of the hys-teresis area enclosed between the flowability curves (shear stress versus shear rate) of the second cycle [21]. Besides, two typical theoretical models were used for curve adjustment analysis, namely: the Bingham plastic and the Herschel-Bulkley model [14,18]

RESULTS AND DISCUSSION

Analysis of the commercial deflocculants The conductivity of the deflocculants investigated

is in the range from 35 mS cm-1 to 50 mS cm-1, being the deflocculant with complexant effect (index D), the one that had the highest pH value as shown in Fig. 4. All sodium silicate containing deflocculants (A, B and C) reveal strong alkalinity, have pH values in the range above 12, while the purely phosphonated one (D) shows a slightly lower 10.5 pH value.

Analysis of ceramic suspensionsAccording to the results obtained the average value

of the solid content is equal to 70.08 ± 0.15 mass %. This value corresponds to the constraints imposed by the

Cycle subsequence Shear Rate (s-1)

Time (s)

First cycle 0-1000 120 0-0 1

1000-0 120 Stop 0 120

Second cycle 0-1000 120 0-0 1

1000-0 120

Table 2. Control rate mode programme.


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high industrial requirements. Furthermore, the standard deviation is narrow enough providing the comparison of the properties of the different suspensions prepared.

In general, the particle size distribution remains unchanged with the addition of different quantities of deflocculant to the suspensions. Unique deviations are observed in the range of deflocculant content from 0.1 mass % to 0.2 mass % as demonstrated in Fig. 5. At this deflocculant concentration the suspensions prepared by adding A and D shows a rather distinguishable particle size due probably to agglomeration phenomena. In comparison, the compositions with addition of B and C are much more stable during the milling process. The average value of particle size obtained is ca. 15.6 ± 0.2 μm as illustrated in Fig. 5. Furthermore, a supplemental insignificant effect of decrease of the average particle size is observed at 0.2 mass % content of each defloc-culant used. Regarding the electrolytical parameters, the electrical conductivity obtained in each suspension is directly proportional to the percentage of the defloc-culant content and the values range is from 5.1 mS/cm to

6.1 mS/cm as expected with the addition of electrolytes (Fig. 6).

Fig. 7 shows the correlation dependence between the defloculant content and the pH values of the suspensions obtained. It is found that the suspension pH increases proportionally from 8.5 to 10.25 for suspensions A, B and C. Unlike them D deflocculant’s content does not affect the suspension pH value which stays equal to 8.5. This fact is a consequence of the presence of complex-ing agents of functional groups in composition D. These moieties have free valences in their electronic shells, able to form electron pair bonds with free multivalent cations without modifying either [H+], or [OH-] [17].

The tests carried out at low shear rate (100 s-1) re-lated to the fluidity during the emptying of the ball mills are illustrated in Fig. 8. As expected, prior to reaching the deflocculant content excess, the apparent dynamic viscosity is inversely proportional to the addition of de-floculants A and B. It is worth adding that they have the highest viscosity at 0.1 mass %. The optimal rheological behaviour at low shear rate is reached at 0.4 mass % of

Fig. 4. Electrolytical properties of the used deflocculants.

Fig. 5. Correspondence between the deflocculant content and particle size distribution mean values of the resulting suspensions.

Fig. 6. Electrolytical properties of the investigated defloc-culant containing slips.

Fig. 7. Dependence between the deflocculant addition and the derivative suspension pH value.


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D deflocculant. The apparent dynamic viscosity shows a tendency to decrease at deflocculant contents below 0.2 %. It achieves values lower than 300 mPa.s at 0.6 % content in the case of composition D (Fig. 8).

Fig. 9 illustrates the viscosity behaviour at a high shear rate (1000 s-1). It is evident that all four commercial liquid deflocculants have similar apparent dynamic vis-cosity values, between 300 mPa.s and 400 mPa.s. Only composition D tends to decrease the suspension apparent dynamic viscosity below 300 mPa.s after reaching 0.4 wt % content. Referring to the thixotropy behaviour, deflocculants A, B and C show a narrower rheological stability region (between 0.2 mass % and 0.4 mass %) in comparison to that of deflocculant D. The latter shows a decrease of this value because of the higher deflocculant content incorporated in the suspension (Fig. 10).

Fig. 11 shows the flow curves of suspensions loaded with 0.4 mass % of deflocculant adjusted to the Bingh-man plastic model. Suspensions with deflocculants B and D show the smallest slopes in the tested range of shear

rate (<100 s-1) although the lowest yield point indicated in Table 3 refers to deflocculant D.

On the other hand, a very clear relationship is outlined if the Herschel-Bulkley model is applied to these flow curves. This is demonstrated in Fig. 12. Table 4 sum-marises the data obtained for Herschel-Bulkley model regression line of the tested deflocculants.

Fig. 8. Correlation between the deflocculant content and the derivative suspension viscosity, determined at low shear rate - 100 s-1.

Fig. 9. Correlation between the deflocculant content and the derivative suspension viscosity, determined at high shear rate - 1000 s-1.

Fig. 10. Correlation between the deflocculant content and the resulting thixotropy of the investigated suspensions.

Fig. 11. Flow curves at 0.4 mass % deflocculation content with Bingham plastic model.

Deflocculant

index

Yield point

τ0 (MPa)

Slope

µp (MPa·s)

Correlation

factor

A 11.580 560,5 0,9996

B 10.070 305,6 0,9992

C 12.570 391,7 0,9996

D 6.897 336,4 0,9987

Table 3. Bingham plastic regression line of tested defloc-culants.


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If criterion “n” of deviation from a Newtonian fluid approaches a unit, then the Herschel-Bulkley flow equation transforms into Bingham plastic model. In fact this is the case as seen from Table 4. Hence, it can be concluded that the plastic model is applicable to the suspensions tested.

The suspension indexed C shows the highest dy-namic viscosity followed by that with index A when

the viscosity curves recorded with a low shear rate are considered. Slips with deflocculants A and B show an intermediate behaviour. The sample indexed D has the lowest dynamic viscosity value as shown in Fig. 13.

CONCLUSIONS

The present research is devoted to the comparison of the rheological effects on a standard porcelain ceramic composition provided by different mechanism of defloc-culation. Four commercial liquid deflocculants of suit-able capabilities for industrial applications were used.

It is found that the defloculants indexed A, B and C show identical behaviour referring to the tests performed aiming to determine the particle size distribution, the electrical conductivity, the pH values, the apparent dynamic viscosity at a low and a high share rate. The same is valid for the thixotropy behaviour observed. The results obtained are expected since the basic building ingredient of the deflocculants is a silicate with (A,B) and without (C) a polymer base.

Deflocculant indexed D having a complexant action shows the lowest pH value, the highest conductivity and clear deviations in regard to the other tests results obtained. Consequently, on the basis of the acidic na-ture of the phosphonates, structuring incorporates, and complexant effect proposed, it can efficiently inhibit any corrosion of pipes and containers in the respective in-dustrial facilities. Additional investigations are required in respect to the corrosion behaviour of the suspensions used as some phosphates are good corrosion inhibitors.

It is found that the deflocculant’s type does not affect the particles size distribution of the suspensions. This is valid for all concentrations with the exception of 0.1

Deflocculant

index

Yield point

τ0 (MPa)

Fluid consistence

K (MPa·sn)

Deviation from a

Newtonian fluid

n

Correlation

factor

A 10.430 731,7 0,9447 0,9998

B 9.250 430,1 0,9292 0,9995

C 12.990 337,7 1,0310 0,9997

D 5.286 600,3 0,8805 0,9995

Table 4. Herschel-Bulkley model regression line of tested deflocculants.

Fig. 12. Flow curves at 0.4 mass % deflocculation content with Herschel-Bulkley model.

Fig. 13. Viscosity curves obtained from the investigated suspensions with 0.4 mass % deflocculation content.


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mass % . It is considered that the quantity in the latter case is not enough to mill properly the paste.

Deflocculant D exhibits a superior performance if an optimum viscosity is required during the industrial mill process, i.e. in the case of a low shear rate. At high shear rates during the ceramic spray-drying process defloc-culant B providing charges and steric effects shows the lowest viscosity at ca. 0.2 mass % - 0.4 mass % content, whereas deflocculant D tends to decrease viscosity at 0.4 mass % and higher concentrations. It is shown that deflocculant D has lowest thixotropy. The other defloc-culants exhibit a narrower range of rheological stability which may hamper the fluidity of industrial suspensions due to ageing processes.

All ceramic suspensions prepared have a standard porcelain composition and a plastic liquid behaviour as they show a yield point and their viscosity decreases at shear rate increase. Deflocculant D provides the opti-mal industrial working conditions because its addition leads to the lowest yield point among the compositions investigated.

The experiments carried out provide to conclude that all four deflocculants tested can be used to stabilize ceramic porcelain suspensions of a standard porcelain composition. The deflocculants indexed as A, B and C act mainly electrostatically determining thus the corresponding deflocculation mechanism, but sodium silicate only is not enough to reach the optimum rheo-logical conditions. Hence, a huge content of sodium silicate (90 mass % or more) has to be mixed with polymers or phosphonates to improve the rheology of the ceramic suspensions. Thus results very close to those obtained by the complexant mechanism can be obtained. The latter mechanism is more suitable to reduce viscosity and thixotropy as well as to preserve the industrial facilities.

Acknowledgements The chemical company Zschimmer & Schwarz

España S.A. is highly appreciated for the technologi-cal advices and supply of the deflocculants tested in the present research. The authors are grateful to the Spanish Government for the financial support of this work through the RETO INVESTIGACIÓN (ENE2013-49136-C4-2-R).

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RHEOLOGICAL EFFECT OF DIFFERENT DEFLOCCULATION … Barrachina_493 br_4_2015_Journal_CTM...The rheology describes the deformation of a body under stress impact where bodies in this