(162v) Controlling of Particle Interactions with Process Pipes in Calcite Mineral Processing to Prevent Depositions

Depositions formed by fouling on the surfaces of process equipment are still a common problem in almost all the industrial fields. To overcome the fouling, either by preventing or cleaning, generally leads to additional operation, maintenance and capital costs and that affects the profitability of the whole process. Deposition of crystals on heat transfer surfaces is common in aqueous systems where solutions are evaporated and heated.[1] Adhesion of particulate materials is an important step in the accumulation of inorganic particles, macromolecules and micro organisms on heat exchanger surfaces. Because the size of such material is less than 1 μm, the interactions are described in terms of colloid chemistry. [2]

An evaporation train in a mineral grinding process has problematic fouling caused by silicates and calcite. Previous thermodynamic calculations have been used to present phase and chemical equilibria in the case of crystallization fouling for an evaporation process [3]. Another fouling mechanism is deposition of particles. By this text studies on colloidal interactions particle and surface is presented.

Process fluids contain often solid particles that can deposit on the heat transfer surfaces. Particle attachment is affected by surface forces and by hydrodynamic forces near the surface. The adhesion of the colloidal particles (10 nm?10 μm) on a surface are controlled by the Lifshitz - Van der Waals interaction forces (LW), Electrostatic double layer interaction forces (EL), Lewis acid-base interactions (AB) and Brownian motion (Br) [4]. In terms of energies the total interaction energy can be written as a sum of the four interaction terms given above:

ΔG^TOT=ΔG^LW+ΔG^EL+ΔG^AB+ΔG^Br (1)

Adhesion of the particles on the surface ?fouling' takes place under those conditions where ΔG^TOT is negative. To obtain individual contributions to the total interaction energy calculated, experimental contact angle and ζ-potential data are required.

Particle size distribution was determined with Beckman Coulter LS 13 320. Based on mass concentrations, particle populations of 0.36 μm and 2.0 μm were found and based on number concentrations, 0.25 μm particles dominated. Comparable particle sizes can be found in electron microscopy analysis.

The elemental composition of the pigment feed and depositions from surfaces of the process were analyzed with SEM-EDS. The amount of impurities was lower than 1 wt-% in the pigment feed but in the depositions of the process units the amount of silicon and manganese were 2?9 wt-% and 1?6 wt-%, respectively.

ζ-potential of calcite pigment particles were measured with Coulter® Delsa 440SX in order to obtain EL interactions. To adjust pH, 0.1 M NaOH and HNO₃ of analytical grade (J.T. Baker) were used. ζ-potential of the calcite pigment is negative and almost constant (-20mV) at pH range from 6.5 to 12. ζ-potential was not measured at lower pH values (<6.5), because in acidic conditions the solubility of calcite increases markedly. ζ-potentials for Stainless Steel AISI 316L were derived from streaming potential measurements [5]. The point of zero charge (pzc) varies between pH 3?5 depending on the cleaning agent and surface topography. The steel surface and calcite particles are both negative and thus have repulsive electrostatic interaction.

Particle (SiO₂, CaCO₃, talc and dolomite) surface energies used in calculations were determined with the Thin layer wicking [6]. The untreated steel surface AISI 316L 2B and coated steel surface energies were determined [7] with the Sessile drop method.

Fouling, i.e. particle attachment on the surface, is thermodynamically favored when ΔG^TOT is negative at the equilibrium distance (when the contact of particle and wall distance is around 2 nm). Even then, colloidal stability can hinder fouling markedly, which is a result of the local maximum in the energy vs. distance diagram. Between the SiO₂ particle and untreated Steel AISI 316L 2B in the conditions of a calcite pigment process an energy barrier (local maximum) occurs that hinders fouling. However, gradual fouling must take place, due to the strong primary minimum of attraction.

A secondary maximum higher than 10 kT should prevent particle attachment for a long period of time, but for smaller particles in the same environment this is lower and do not prevent them to attach. Means to affect the secondary maximum are: to decrease ionic strength or to affect on particle and surface ζ-potentials.

Usually in industrial processes, ionic strengths are much higher than 0.1 M, when electrostatic interactions have only a very small contribution on the total interaction. In the case of fouling, the best situation would be when no primary minimum occurs. This can be achieved by modifying surface materials. Lewis acid-base interactions are repulsive with SiO_xCVD coating and primary minimum in total interaction energy does not occur. Particle attachment should be prevented; in this case, two highly hydrophilic bodies are repulsive.

The dependence of particle chemical composition on adhesion was calculated for the main minerals and impurities in the calcite pigment process. Adhesion of pure calcite on steel AISI 316L surface is much weaker compared to magnesium containing mineral dolomite CaMg(CO₃)₂. Adhesion increases with talc MgSiO₂ and SiO₂, but is strongest with gypsum CaSO₄. Compared to the concentration of magnesium and silicon in the calcite pigment and in the depositions on the surfaces, depositions have much higher amounts of those. This is due to stronger adhesion of magnesium and silicon containing particles on steel surface than with pure calcite.

The effect of surface material on adhesion is shown with SiO₂ particles in the calcite pigment process conditions. With the studied coatings Titanium nitrile has the smallest adhesion on particles. Thus, titanium as a construction material is better than untreated and most of the coated steels.

The adhesion energy between particle and surface was almost linearly dependent on the negative Levis acid-base (AB-) component of surface tension. The total surface tension of coatings varied markedly, but could not explain differences between the adhesion energies.

In the case of crystallization fouling low energy surfaces are less prone to foul. To prevent fouling with modifying surface energies is a challenge because different surface properties are needed in affecting crystallization and particle attachment, which can be even conflicting.

References

[1] Bott TR (1997) Aspects of Crystallization Fouling. Experimental Thermal and Fluid Science 14: 356-360.

[2] Oliveira R (1997) Understanding Adhesion: A Means for Preventing Fouling, Experimental Thermal and Fluid Science 14: 316-322.

[3] Riihimäki M, Muurien E, Keiski RL (2005) Silicate Fouling in Evaporators ? Thermodynamic Equilibrium Modelling of Aqueous Electrolytes. Proceedings of 7th World Congress of Chemical Engineering 10-14 July 2005, Glasgow, Scotland.

[4] van Oss CJ (1994) Interfacial Forces in Aqueous Media. Marcel Dekker, Inc. New York. pp. 440.

[5] Boulange-Petermann L, Doren A, Baroux B, Bellon-Fontaine M-N (1995) Zeta potential measurements on passive metals, Journal of colloid and interface science 171: 179-186.

[6] Wu W, Giese Jr RF & Van Oss CJ (1996) Change in surface properties of solids caused by grinding. Powder Technology 89: 129-132.

[7] Santos O, Nylander T, Rosmaninho R, Rizzo G, Yiantsios S et al. (2004) Modified stainless steel surfaces targeted to reduce fouling?surface characterization, Journal of Food Engineering 64: 63-79.