Journal of Supercritical Fluids 13 (1998) 369–374

Fine particles preparation of Red Lake C Pigmentby supercritical fluid

Yong Gao *, Tshomba Kays Mulenda, Yi-Feng Shi, Wei-Kang YuanUNILAB Research Center of Chemical Reaction Engineering, East China University of Science and Technology,

200237 Shanghai, People’s Republic of China

Received 19 June 1997; received in revised form 25 November 1997; accepted 22 December 1997

Abstract

Fine material preparation for several kinds of pigments (Red Lake C, C.I. Pigment Blue and C.I. Pigment Yellow)was investigated under supercritical conditions. In the experiments, acetone was used as solvent and carbon dioxideas anti-solvent were applied in the recrystallization processes. To obtain the ultrafine particle products, a series of5–500 mm nozzles were used to disperse the solution out from the crystallizer. Experimental results showed thatoperations under supercritical conditions were able to provide very small particles, e.g. less than 1 mm in diameter,very narrow size distribution and spherical morphology. Further study also showed that the particle size could becontrolled by temperature, pressure and nozzle size. © 1998 Elsevier Science B.V.

Keywords: Pigments; Recrystallization; Supercritical CO2; Ultrafine particles

1. Introduction stance which requires the operation to be carriedout at higher than its critical temperature (Tc) and

In industrial pigment production, the size and critical pressure (Pc). SCFs have some uniquedistribution of particle products which determines properties, such as higher solubility compared withtheir quality and application is usually considered gas, higher molecular diffusivity compared withan important problem within the industry. In liquid, pressure-dependent density, and so on [1,2].traditional processes, the preparation of fine par- Now many chemical engineers are applying themticle materials is carried out by mechanical grind- to discover new technology or to develop newing. With the increasing applications of ink and process. SCFs have been applied in separation,dye, especially in printing superfine fiber, the prep- chemical reaction and fine material preparationaration of fine pigment particles (<1 mm in diame- processes. For example, the rate of a liquid-phaseter) with good light properties is highly important. enzyme catalysed reaction can be enhanced by

Supercritical fluid (SCF) is a less common sub- performing the reaction in supercritical CO2 [3].Researchers have succeeded in dissolving proteinsin supercritical carbon dioxide [4] and obtained* Corresponding author. Tel: +86 2164252720;

fax: +86 2164253528; e-mail: scf_group@yahoo.com organic and inorganic powders from supercritical

0896-8446/98/$19.00 © 1998 Elsevier Science B.V. All rights reserved.PII S0896-8446 ( 98 ) 00074-6

370 Y. Gao et al. / Journal of Supercritical Fluids 13 (1998) 369–374

fluid mixtures [5–8]. The GAS (gas anti-solvent) 2. Experimentalprocess is an important method applied for prepa-ration of ultrafine particles [9,10]. In the GAS An apparatus for investigating fine particle prep-

aration was established and the flow sheet of theprocess, a high pressure gas is dissolved in theliquid solvent to make a volumetric expansion of system is shown in Fig. 1.

In the experimental investigation, the apparatusthe liquid solvent which lowers the equilibriumsolubility and precipitation of the dissolved com- can be operated in continuous or batch modes. In

the batch operation, a tubular recrystallizer (D=pound then occurs. The expanded liquid solventhas higher diffusivity and lower viscosity than the 25 mm, H=250 mm) was first filled with the satu-

rated solution by the pump (prepared by pigmentnormal liquid and a purer particle product withless solvent inclusion can be expected in this pro- and acetone before each test) and then the stop

valve was shut. When the saturated solution wascess. In the GAS process, particle size and particlesize distribution are influenced by the volumetric heated until the planned operation temperature

was reached, carbon dioxide was passed throughexpansion. After precipitation, the growth of par-ticles is caused by liquid expansion. The applica- a compressor (model G2Z-5/400) with a flow rate

of 5 m3/h and added to the recrystallizer. Totion of a nozzle may be able to alter the particlediameter, size distribution and morphology in the prevent carbon dioxide condensation, carbon diox-

ide from the compressor was heated before enter-process.In this paper fine material preparation of several ing the recrystallizer. When the pressure in the

recystallizer reached the designed value, the valvekinds of pigments was studied under supercriticalconditions, including Red Lake C, Pigment Yellow between the recrystallizer and the collector was

opened and the fine particles were obtained in theand Pigment Blue. In our experiments, acetonewas used as solvent and carbone dioxide as the collector. In this operation mode, the pressure in

the collector was reduced to atomospheric pres-anti-solvent. To obtain the finer particle products,a series of 5–500 mm nozzles were used to disperse sure. In the continuous experiments, a mass flow

meter (model E5850) was installed to measure thethe solution out from the recrystallizer. The effectsof temperature, pressure and nozzle size on Red carbon dioxide mass rate of the different processes

and a pressure controller (model YT-2) was usedLake C particle size were investigated anddiscussed. to assure a constant carbon dioxide supply to the

Fig. 1. A schematic of the experimental system.

371Y. Gao et al. / Journal of Supercritical Fluids 13 (1998) 369–374

Table 1Operation conditions

Material T ( K) P (Mpa) V (g/min) Nozzel size (mm)

Red Lake C 313–423 6.0–25.0 0.1–5.0 5–500Pigment Blue 15 313–423 6.0–25.0 0.1–5.0 5–500Pigment Yellow 1 313–423 6.0–25.0 0.1–5.0 5–500

Table 2recrystallizer without obvious pressure fluctua-Physical properties of Red Lake Ctions. The pressure in the collector during the

filtration was kept lower than that in the recrystal- Mean particle size (mm) 10–30lizer and then the carbon dioxide flowed to the

Specific surface area (m2/g) 7–110absorber was recycled to the compressor. In thispH 10% slurry 6.5–8.0process, a metering pump was used to continouslyParticles shape Needles or rods

supply the saturatured solution to the recrystal- Specific gravity (g/cm3) 1.65–2.11lizer. During the experiments, the precipitatedpowder remained in the collector. After an experi-mental run, the particle product could be removed

3. Results and disscussionfrom the collector after reducing the operatingpressure to the atmospheric. Considering the

Particle preparation by mechanical grinding andaggregation of the pigment particles, an organicregular recrystallization was also done for compar-solvent is usually used to disperse them in theison with that by SCF. All the prepared particlescollection.were measured by using a scanning electron micro-When the saturated solution and CO2 werescopy (SEM) (model S250 MK 3, Cambridge).continuously fed into the recrystallizer, the expan-Particle diameter, size distribution and morphol-sion of the solution occured with an anti-solventogy were determinated though the SEM photos.effect (GAS process) and newly nucleated particlesThe results are shown in Figs. 2–7.were formed. In the flow process of a mixture,

The photos of Red Lake C are shown in Figs. 2–which includes solution, particles and CO2 to the4. For the traditional mechanical grinding, thenozzle, the particles continued to grow. Due toprimary particles are seen like thin pieces, withthe post-expansion of the bulk fluid at the nozzletheir size larger than 1 mm and their size distribu-exit, the temperature of the collector was usually

lower than that of the recryallizer. In the collector,almost all formed particles would precipitate andacetone would be separated with CO2 in anabsorber filled with activated carbon and zeolite.In the continuous operation CO2 could be recycledback though the compressor. The experimentalconditions are listed in Table 1.

The materials used in the experiments weresupplied by Shanghai No.5 Dye Factory. RedLake C is also called Pigment Red (53:1), itschemical formula being Ba(C17H12N2O4ClS)2 ofthe molecular weight of 888.6. At 25°C the satu-rated concentration of Red Lake C in acetone is0.2 g/100 ml. The physical properties of Red Lake

Fig. 2. Red Lake C powder produced by mechanical grinding.C in industrial production are presented in Table 2.

372 Y. Gao et al. / Journal of Supercritical Fluids 13 (1998) 369–374

Fig. 3. Red Lake C powder recrystallized in a saturated Fig. 6. Pigment Yellow powder recystallized in a saturatedsolution. solution.

Fig. 4. Red Lake C powder produced by supercritical fluid.

Fig. 7. Pigment Yellow powder produced by supercritical fluid.

Fig. 8. Diameter distribution of Red Lake C particles.

Fig. 5. Pigment Yellow powder produced by mechanicalgrinding. smaller and narrower in size distribution (shown

in Fig. 8). Compared with mechanical grinding,the particles obtained by regular recrystallizationtion relatively wide. In the supercritical CO2 pro-

cess, the particles that were formed under the are usually too large to meet the industrial require-ments. The results for Pigment Yellow particleabove conditions are basically spherical, much

373Y. Gao et al. / Journal of Supercritical Fluids 13 (1998) 369–374

Fig. 11. Influence of nozzle diameter on particle size.Fig. 9. Influence of temperature on particle size.

chances for molecular nucleation and promotesthe crystal growth process. Therefore, larger pig-ment particles are formed at higher operationtemperatures (shown in Fig. 9). It is understanda-ble, according to the experimental data of RedLake C at different temperatures, that the opera-tional temperature should be kept at slightly higherthan the critical temperature of CO2.

When the operating pressure is increased, theCO2 solubility in the solution increases, as thehigher supersaturation concentration of Red LakeC is attained. So once a catastrophic nucleationoccurs in the recrystallizer, a lot of smaller pigmentparticles will be formed in the process (shown inFig. 10). For the same supercritical fluid, the higherpressure will be suitable for preparing smallerparticles and the reasonable operation pressureFig. 10. Influence of pressure on particle size.can be determined according to the required par-ticle size.

It was found from the experiments that the sizemeasurement are shown in Figs. 5–7, where thesame phenomena are also observed. The size distri- and structure of the nozzle play an important role

(shown in Fig. 11). Though the experimental databution of the Red Lake C particles is seeminglymuch better than that of Pigment Yellow (Fig. 8). is not very satisfactory when plotted as a straight

line y=0.58415+0.011140x (where x stands forWith further analysis of the experimental dataof Red Lake C, the change in particle sizes with the mean particle diameter, y for the nozzle diame-

ter), it is obvious that the precipitation of largertemperature, pressure and nozzle diameter can beobtained, as shown in Fig. 9–11. particles occurs when larger diameter nozzles are

used. Following the above analysis, the pigmentWith an increase in temperature in the recrystal-lizer, the collision movement of pigment molecules crystals will further grow up in the flow process

until they leave the nozzle, so a suitable size nozzlein the solution is accelerated and this increases the

374 Y. Gao et al. / Journal of Supercritical Fluids 13 (1998) 369–374

must be carefully chosen for preparing fine pigment Referencesparticles via supercritical fluid.

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