As the definition of lime is generic it is important to examine the physical composition of lime materials to see if there are common factors which influence their flow properties. The presence of clays, which have a tendency to readily adsorb water molecules and dissolved ions, may be a factor in some limes being cohesive. The actual CaCO3 levels were established by thermal gravimetric analysis, TGA, and these
were compared with the declared values stated by the fertiliser manufacturers as provided to Fertmark. The Fertmark standard method is by titration which does not definitively measure the calcium carbonate in a sample but rather measures the total acid neutralising effect of the sample (NZ Fertiliser Quality Council Codes of Practice, 2004). Acid neutralising hydroxides associated with clay are possible impurities which may adversely affect flow, and differences between the TGA and declared values may indicate their presence. TGA also provides information on the thermal decomposition of both carbonates and hydroxides. Impurities were identified using x-ray diffraction analysis.
Thermal gravimetric analysis
Thermal gravimetric analysis identifies the materials comprising the test matter by the profile of the thermal decomposition and the temperatures and amplitude of the thermal peaks. It is known that most of the limestone should comprise the calcite form of CaCO3, with additional moisture, silicate and clay impurities. Calcite is a
rhombohedral crystal, and is the most stable polymorph of calcium carbonate below 50°C (Mackenzie et al, 1970). Accordingly the thermal decomposition of calcite to CaO was taken as the actual percentage of CaCO . Impurities were identified from
profiles of material decomposition, however, as these materials occur in small amounts, and are often close in thermal decomposition range, actual composition is difficult to confirm and complementary data was obtained by x-ray diffraction (Montoya et al. 2003).
Differential thermal analysis
A Q600 thermal analyser (TA Instruments, New Castle, Delaware, USA) was used to provide simultaneous differential thermal analysis, DTA, differential scanning calorimetry, DSC, and heat flow analysis of the 10 limes studied. The analysis comprises measuring changes in percentage mass as a proportion of initial total mass with temperature change over a preset range. In this study the range was; 40°C – 1,200°C at a ramp rate of 20°C/ minute. Simultaneously a DSC, which measures the endothermic and exothermic reactions as they occur in temperature change in °C/mg and, the heat flow as energy per gram in W/g of material. For an example of a printout from this machine see Figure 3.4.
Figure 3.4: Supreme Lime sample run showing thermal decomposition of hydroxides of 4.401% of mass from 371.84°C and decomposition of carbonate of 38.25% by mass % CO2
X-ray diffraction
The second part of the investigation was to identify the actual composition of the impurities using XRD and to establish the proportional composition of the limes. These results, along with the nature of the parent limestone may indicate links between mineralogy, the composition of impurities and the proportions of each impurity found in each limestone.
Powder cohesion
The SMS cohesive index and Hausner ratio are both indicators of the cohesiveness; and flow properties of the powder being tested and can be determined relatively quickly; they enable comparisons between batches of input or product rather than quantitative measures of design or flow variables.
The SMS cohesive index requires a powder testing machine made by Stable Micro Systems,(Godalming, Surrey, UK), and is determined by proprietary software from measurements of the force required to turn a paddle through a sample of material; the calculation requires the additional inputs of loose poured bulk density and Hausner Ratio.
Cohesion and caking were investigated using a Powder Flow Analyser by Stable Micro Systems, SMS, Godalming, Surrey, UK. Hausner ratios were measured by manual tap testing samples with 100 taps, with 10 replicates and comparing the tap density to a loose pour density averaged over 10 replicates, coefficient of variation, which is the standard deviation over the mean was between, 0.3% and 1.1%.
The SMS cohesive index and Hausner ratio are indicators of the flow properties of the powder being tested and can be determined relatively quickly; they enable comparisons between batches of input or product rather than quantitative measures of design or flow variables. The SMS cohesive index is determined by measuring the force required to turn a paddle through a sample of material and calculating the work done from the area under the curve of a plot of Force versus Distance travelled by the paddle through the powder; this work is converted to an index which measures cohesiveness. However, for definitive answers to powder flow solutions and estimation of hopper design parameters, a shear testing regime based upon powder flow mechanics is required. Table 3.2 compares values of flow properties of Hausner ratio and Stable Micro-Systems powder flow analyser, cohesive index.
Table 3.2: The SMS Cohesive Index; alongside the Hausner Ratio, and flow-ability.
Cohesive Index (CI)
Flow Behaviour
Hausner Ratio Flow-ability
>19 Hardened/Extremely
Cohesive > 1.4 Very difficult
19 – 16 Very cohesive 1.25 - 1.4 Difficult
16 – 14 Cohesive 1.1 -1.25 Medium flowing
14 – 11 Easy flowing 1.0 - 1.1 Free flowing
<11 Free flowing