by Roberto Soldati*, Chiara Zanelli*, Guia Guarini*, Sandra Fazio**, Maria Chiara Bignozzi**, Michele Dondi*16   Febbraio   2018

Porcelain stoneware panels: the behaviour of powders

On production lines for porcelain stoneware panels, the spray-dried powders are dispensed from hoppers and deposited onto a conveyor belt, which then carries them underneath compaction rollers or the press punch according to the compaction technology used.

The process consists of three steps (Fig. 1):
1) Powder flow from the hopper: the quantity of body is dispensed according to the mass flow through the hopper orifice and the speed of movement of the belt. The mass flow depends on the particle size of the spray-dried powder and in particular its coarse fraction. However, there is an even stronger negative correlation with the frequency of irregularly-shaped aggregates: the lower the percentage of these so-called "popcorn" granules, the higher the mass flow.
2) Deposition on the belt: the powders are deposited on the belt according to an oscillatory phenomenon determined by the angle of rest, which results in more or less evident undulations on the surface of the soft layer. The angle of rest depends mainly on the presence of coarse granules and/or irregularly shaped aggregates in the spray-dried powder. If this fraction is greater than 10% by volume, the angle of rest will tend to be greater than 30° and vice versa. Moreover, spray-dried powders with greater flowability (static angles of rest lower than 30°) create denser soft layers (>1 g·cm-3) than less flowable powders (0.9-1 g·cm-3).
3) De-aeration of the soft powder: the gradual compaction performed by the two belts before reaching the compaction rollers results in de-aeration of the soft powder, a process that may be sufficiently energetic to remobilise the top layer of the spray-dried powder. This phenomenon also alternates between a critical point where the agglomerates begin to move due to the air flow and a point of equilibrium where the granules stop moving until the de-aeration pressure builds up again and reaches the critical point.

Table 1. Effect of the intrinsic characteristics of spray-dried powders on their behaviour during deposition.

Given that moisture content has virtually no influence during this stage of the process, a manufacturer wishing to optimise spray-dried powder behaviour during the deposition phase by selecting a powder with suitable characteristics can essentially choose between different degrees of particle size fineness. But even more important than particle size is the "quality" of the spray-dried powder, meaning by this the presence or otherwise of a fraction of coarse aggregate, often formed through coalescence of three or more spherical granules. This generally occurs due to non-optimal management of the spray-dried powder and/or of the subsequent transport and storage stages (granules that either have an excessively high moisture content or have been dampened by dripping condensation or rehumidification operations). The main effects are shown in Table 1. In general, it is advisable to have a finer spray-dried powder and avoid irregularly-shaped aggregates (if possible <5%).

The behaviour of spray-dried powders during pressing is determined by various phenomena involving both the mineral particles making up the body and the porosity between them. These phenomena occur at various scales from a fraction of a millimetre (settling movement of the granules) down to a micron (microporosity of the compact powder after pressing).
The compressibility of the spray-dried powders, expressed in terms of the Carr index (at 40 MPa), varies from 50% to 58%, with most bodies lying between 50% and 55%. It primarily depends on the moisture content of the powders: all comparisons must take account of the degree of moisture and its distribution within the granulometric curve of the spray-dried powder. From this point of view, the differences in behaviour between spray-dried powders with different degrees of fineness may primarily be explained by the different levels of moisture content, which on average are higher in coarser powders.
In the pressing of ceramic bodies, two compaction regimes can be identified. At low loads (<100 kg/cm2), rapid densification can be observed as the specific pressure increases. At high loads (>150 kg/cm2), the increase in apparent density is much slower. The transition between these two regimes is gradual.
The initial situation is that of a layer of soft powder deposited on the belt or in the mould prior to application of the load. More than 60% by volume consists of porosity of various kinds: intergranular macroporosity (P3) makes up 30-31% of the volume of the mould, while the rest consists of intragranular macro and microporosity (Fig. 2). The initial density is equal to the poured density, which generally varies between 0.9 and 1.0 g/cm3.

In the low-pressure regime the granules undergo an initial rearrangement that causes a modest increase in density to more or less the level of tapped density (from 1.03 to 1.12 g/cm3 depending on the rheology). Significantly, the maximum values of tapped density occur for a pressure of approximately 2.5 kg/cm2, corresponding to the apparent yield point of the powders.  Above this point, a much more rapid increase in apparent density is observed up to ~50 kg/cm2 followed by a gradual decrease until the high-pressure regime is reached at around 130 kg/cm2. The apparent density reaches 1.82-2.07 g/cm3 depending on the moisture content of the powders and the deformability of the granules. This process is made possible by the decrease in the dimensions of the intergranular macroporosity, which falls from 30% to <1% at 150 kg/cm2 (Fig. 3). The decrease in P3 is counterbalanced by an increase in P1b, i.e. larger sized microporosity, which peaks at loads of around 50 kg/cm2.
The increase in densification rate at the apparent yield point may be due to the progressive collapse of intragranular macroporosity (P2). The volume of the central cavity and its funnel, determined by image analysis, is estimated at around 15-20% of the granule's total volume. This porosity remains low in percentage terms, perhaps because it is difficult to effectively compress the funnel (P2b), traces of which remain even after compaction under high loads (Fig. 4).
The granules only undergo modest deformation in the low-pressure regime: in the cross-sectional view of the compacted powders we can observe a degree of compression corresponding to an aspect ratio of 1.3   0.2 (at 100 kg/cm2) compared to the initial value of 1.1   0.1. This compression is probably caused by the collapse of the central cavity: a contraction of 13% by volume would result in an aspect ratio of 1.3 (i.e. P1a plus part of P1b).

In the high-pressure regime we observe a moderate increase in apparent density, which increases from 1.82-2.07 g/cm3 (130 kg/cm2) to 1.93-2.14 g/cm3 (300 kg/cm2) and 1.99-2.19 g/cm3 (500 kg/cm2). These values correspond to a porosity of the compacted powders of between 19% and 25% (500 kg/cm2). In this regime, the densification rate is much slower because more work is needed to "close" intragranular microporosity than intergranular macroporosity. The increase in apparent density occurs at the expense of "compressible" microporosity, which in the case of porcelain stoneware has approximate dimensions of between 0.7 and 3 µm. As the load increases, P1b is gradually compressed and is largely converted into the finest microporosity (<0.7 µm), which is therefore "incompressible" (P1a) in the working conditions of porcelain panels. Part of the porosity P1 is in any case closed at high loads because the total porosity of the compact powders decreases.

Deformation of the granules occurs mainly around the "triple points" where the three agglomerates meet. At these points, a residual intergranular porosity is observed in the transition to the high-pressure regime, and in order for this porosity to be closed further plastic deformation of the granules is required. However, this does not result in greater compression of the agglomerates, which maintain an aspect ratio of 1.3   0.2 (at 300 kg/cm2).
A summary of the compaction process is shown in Figure 5.

^* CNR-ISTEC, via Granarolo 64, Faenza, Italy

^** Centro Ceramico, via Martelli 26/A, Bologna, Italy