Gravity methods

In the previous chapters our attention was paid mainly to the study of the figure of the earth and with this purpose in mind we represented the total field as a sum of the normal and secondary fields. In this chapter, we will discuss a completely different application of the gravity method, related to exploration geophysics, in which the gravitational field is measured in order to study lateral changes of the density near the earth s surface. By analogy, we also represent the gravitational field as a superposition of two fields ... 

In general, minerals which accumulate under placer conditions must be hard and abrasion-resistant to survive. The same properties are important if minerals are to be concentrated by gravity methods of separation, based on the difference in their densities. Although heavy, soft minerals can be concentrated in gravity plants, losses are high. Gold is an exception it is not a hard mineral, but it is cohesive and although battered out of shape, does not slime to a fine mud. 

The results from the XRF analysis are relatively similar to the published results of Ben-David (9,10). Ben-David analyzed 324 Tyrian shekels using Caley s specific gravity method (11), but this method is highly inaccurate. Furthermore, it is not clear exactly what coins were measured, as there is no tabulation of the data. Ben-David did not indicate the number of coins, or which specific coins from the hoard that was measured, making a direct comparison impossible. However, the general trend that Ben-David s analysis shows is replicated in our XRF data. 

Figure 3. Comparison of the XRF data (weight percent) with the results by Ben David using Caley s specific gravity method.

Hammerschmidt discovers hydrates as pipeline plugs provides Hammerschmidt equation discovers thermodynamic inhibitors 1941 Katz et al. begin -values and gas gravity methods to predict hydrate mixtures... 

Section 4.2 deals with the most useful hydrate equilibria—calculations of temperatures and pressures at which hydrates form from gas and free water. In this section, two historical methods, namely, the gas gravity method (Section 4.2.1) and the Kvs, value method (Section 4.2.2), for calculating the pressure-temperature equilibrium of three phases (liquid water-hydrate-vapor, Lw-H-V)1 are discussed. With the gas gravity method in Section 4.2.1.1, a method is given for limits to expansion, as for flow through a valve. In Section 4.2.2 a distribution coefficient (KVSi) method is provided to determine whether a component prefers residing in the hydrate or the vapor phase. These methods provide initial estimates for the calculation and provide a qualitative understanding of the equilibria. A statistical... 

In this section two prediction techniques are discussed, namely, the gas gravity method and the Kvsi method. While both techniques enable the user to determine the pressure and temperature of hydrate formation from a gas, only the KVSI method allows the hydrate composition calculation. Calculations via the statistical thermodynamics method combined with Gibbs energy minimization (Chapter 5) provide access to the hydrate composition and other hydrate properties, such as the fraction of each cavity filled by various molecule types and the phase amounts. 

Even with such limitations, the A vsi-value method represented a significant advance in hydrate prediction ability. It was conceived prior to the determination of the hydrate crystal structures and it is a fine representation of the intuitive insight that characterizes much of Katz s work. The Kysi-value method was the first predictive method, and it was used as the basis for the calculations in the gravity method, so it is logical that the A si-value method should be more accurate. 

Both the gas gravity method and the Kysi-value method enable the estimation of three-phase (Lw-H-V) equilibrium between quadruple points Qi and Q2 for mixtures as well as for simple natural gas hydrate formers such as those in Table 4.2. 

For concentration (separation of gangue minerals) several methods can be applied, depending mainly on the composition of the ore. They include ore sorting, gravity methods, flotation, magnetic, and electrostatic separation. 

Gravity methods can also be applied for both scheelite and wolfhunite. The usual equipment consists of spirals, cones, tables, and a sink-float. 

The decrease in particle size enabling a quantitative dissolution during digestion. This is especially necessary for concentrates gained by gravity methods. 

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