Rheology and Plasticity II. The National Brick Research Center March 2017 Webinar

Transcription

1 Rheology and Plasticity II The National Brick Research Center March 2017 Webinar

2 Review of shear, flow and rheology Review of rheometer setup Effect of measurement configuration Repeatability of measurements Comparison of Good and Difficult materials Identifying problem materials moisture sensitivity or dewatering behavior Effect of Additives Bentonite Conclusions and future work Outline

3 What is Shear? Shear is the movement (sliding) of one plane or layer of material relative to an adjacent plane or layer due to an applied force. Shear Stress (τ) is the force applied over a unit area. Shear Rate ( ) is the change in velocity of one layer relative to another due to an applied force. Viscosity (η) is the resistance to flow.

4 Shear and Extrusion In extrusion we apply a force that results in flow. For piston extrusion, the ram movement results in shear in the material being extruded. The force required to maintain the shear rate and the resulting flow are characteristics of the material. For auger extrusion the shear rate is determined by the auger speed. The power required to maintain the auger speed and flow rate is analogous to shear stress.

5 Variables related to rheology and plasticity Type of Clay Mineral Amount of Clay Mineral in the mix Water Content Organic Mater Soluble Salts Additives Mixing/Weathering/ Ageing

6 Typical Flow Behavior I There are several characteristic types of flow. For Newtonian fluids, there is a linear relationship between shear stress and shear rate. For dilatant or shear thickening material, the shear stress increases non linearly as the shear rate is increased. This type of behavior is potentially dangerous in extrusion. For shear thinning materials, the shear stress decreases as the shear rate increases. This type of behavior is more common in materials with a high clay content and is thought to be the result of alignment of the platy clay particles.

7 Typical Flow Behavior II For our materials, a minimum stress is required to initiate flow. This minimum stress is known as the yield stress. Yield stress is related to what we call green strength This type of flow is sometimes called Bingham plastic. There is also shear thinning and shear thickening behavior with yield stress.

8 Plastic Limit and Liquid Limit For clay based materials, a minimum amount of water is required for the material to have cohesion and flow. When we add water to a dry raw material (with sufficient clay content), the material goes through several stages. In the first stage the material agglomerates. The balls get larger until we reach something known as the plastic limit Once we achieve the plastic limit, the material has a certain working range until we exceed the liquid limit of the material, the material becomes too soft for stiff extrusion. The plastic limit, working range and liquid limit are a unique to each raw material. A material that has a wide working range of moisture content is called fat while a material with narrow range is called short. The bottom line is that plasticity is a function of moisture content and that this optimum moisture content is a characteristic of the material.

9 Capillar Check Rheometer Capabilities Measure the flow behavior of a raw material while simulating the shear rates that the material would experience in the plant. Look for problem rheologies like dilatency or dewatering. Find the optimum moisture content for a particular raw material. Compare materials or materials from different stockpiles. Study the effect of additives on flow behavior.

10 Simulating shear rate For a given ram speed, we can increase the extrudate speed by restricting the die opening. We are essentially forcing a given volume of material to flow at a faster rate as we reduce the die opening at a given ram speed. Extrusion Speed (ft/min) Ram Speed (mm/min) 3 mm die 4.9 mm die 6 mm die

11 Die Selection To simulate very high shear rates like you would see in the plant, we have to use a small die and fast ram speed. With a small die, the opening is very close to the size of the largest particles. For example the opening in an 6 mesh is 3.4 mm and for a 8 mesh is 2.4 mm. We have had problem with plugging with the smaller die and have seen some strange behavior that may be due to a pile up of larger particles at the die opening. We have standardized on the 6 mm die, but may make a larger one with the understanding that we will get lower shear rates.

12 Comparison of a Material with Two Die Sizes Shale - 6mm Die Shale - 4.9mm Die Radial Pressure (kpa) Radial Pressure (kpa) Extrudate Speed (ft/min) Extrudate Speed (ft/min) Low Moisture Content Medium Moisture Content High Moisture Content Low Moisture Content Medium Moisture Content High Moisture Content

13 Pressure Measurement Radial pressure measurement Axial pressure measurement from torque on drive for ram movement The axial pressure measurements had more scatter and are influenced by the amount of material in the barrel of the extruder as would be expected. The average radial pressure measurements in this comparison were vary consistent and resulted in no statistically significant differences. For the highest accuracy measurements, multiple extrusions should be done for each moisture/additive/die/piston speed combination with the uncertainty represented by error bars, but in many cases this not possible due to the additional time involved. For most analysis, we are now standardizing on the 6 mm die and analyzing only the radial pressure results.

14 Pressure Measurement Repeatability Radial Pressure Axial Pressure

15 Repeatability Average Pressure Measurement Radial Pressure Average Axial Pressure Average Radial Pressure (kpa) Radial Pressure (kpa) Run/Sample Run/Sample

16 Materials Comparison Why does one behave well and the other is difficult Good Material to work with? Difficult Material

17 Material Comparison Good Material Difficult Material Radial Pressure (kpa) Good % Moisture Good % Moisture Piston Speed (mm/min) Radial Pressure (kpa) Bad % Moistu Bad % Moistu Piston Speed (mm/min)

18 Material Comparison In this comparison, the bad or difficult material is much more sensitive to moisture content than the good material. At 20% moisture the bad material appeared good, but was unextrudable at the highest piston speed. The flow behavior of the bad was also much more erratic than the good material which is probably due to the coarse end of the particle size distribution.

19 Moisture Sensitivity Radial Pressure (kpa) Piston Speed (mm/min) 18.1% Moisture 17.4% Moisture

20 Dilatent/Dewatering Behavior For this material, an additive was used to help reduce scumming. Some additives can act like flocculants which promote agglomeration and reduce plasticity which can result in dilatant behavior and dewatering. In this example, at the lower moisture content we found evidence of dilatant behavior and dewatering. This tells us that this additive must be used with extreme caution to prevent lockdown or buildup of steam pressure within the column.

21 Effect of Additives Bentonite For this study we extruded a base shale mix and then mixes with 4% and 8% additions of bentonite. Bentonite is commonly used to increase plasticity and extrudability of difficult materials. Addition rates are typically less than 4%, but for this work we used higher levels to make sure that we saw a measurable effect. In this example, we test three addition rates of bentonite (0%, 4% and 8%) and three moisture contents for each (low, medium and high) for a total of 9 extrusions.

22 Bentonite Effect of Moisture After The effect on the average radial pressure required to maintain flow as a function of bentonite addition and piston speed for a single moisture content is shown. In this arrangement of the data, it is seems that the pressure required to maintain flow is increased by the bentonite addition (keeping in mind that there is some variation in moisture content so that this is not technically a direct comparison). Radial Pressure (kpa) % - Medium Moisture 4% Medium Moisture 8% Medium Moisture Piston Speed (mm/min)

23 Effect of Additives Bentonite There are a number of ways to compare the results since we are looking at the radial pressure as a function of additive, moisture content and shear rate. To simplify, the average radial pressure is plotted as a function of the moisture content for a single piston speed here. For the most part, the pressure required for extrusion increased with bentonite addition, and the water requirement was increased due to the additional surface area introduced by the bentonite which makes it difficult to see trends in some cases. The bentonite additions did appear to improve the flow of material.

24 Bentonite Effect of Moisture Before

25 Bentonite Effect of Moisture After

26 Bentonite Addition Observations Bentonite Additions increased water demand of the material and increased the pressure required to initiate and maintain flow. The pressure increase in this case should equate to increased plasticity and green strength.

27 Spring Meeting Information May 9 11, 2017 National Brick Research Center s Spring Meeting Held in conjunction with ACeRs Structural Clay Products Division October 2 4, 2017 Clemson Brick Forum

28 Upcoming Webinars April 26 Color Development and the Use of the Gradient Furnace Part 1 May 31 The Effects of Extrusion Additives June 28 BaCO 3 in the Brick Industry July 26 Color Development and the Use of the Gradient Furnace Part 2 August 30 Brick Glazes and Coatings Making them Fit September 27 NBRC Fall Research Update October 25 The Relationship Between Drying and Permeability November 15 Comparing Particle Size Data Using Different Measurement Techniques