Feasibility analysis for granulation of silver using Granshot method

Introduction

Processing of metals into desired shapes and sizes is an ancient process which is used in the manufacture of utensils, tools, and weapons. In modern civilization, processing of metals is not a difficult task, as we have different latest techniques that enable us to convert a metal into desired shapes as per requirements. Granulation and atomization of metal is still a challenging task, in which metal lumps and scrap are converted into fine grains of required size.^1–3 Gas atomization is one of the methods used to get fine powder from metals.^3–6 In granshot process, the metal is converted into granules by simple processing using a continuous casting machine with some modifications. ⁷ The molten metal that flows from the crucible is quenched in water and will solidify in an instant, forming granules of that particular metal.^{8, 9} The technique developed for granulation is very old and is still in practice for processing heavy metals. This technique was mainly used to get granules from scrap. In 1960, some modifications were made to the granulation process. Although the technique was the same, the apparatus had been modified; currently, water is sprayed over the stream of molten metal and the metal disintegrates into small particles. These small particles or granules are transferred into the cooling chamber for further cooling. ¹⁰ The latest modification recorded in the granulation process was in the year 1987, when potential energy was used to energize the flow of metal from the furnace in the form of a stream and then disintegrated into granules just after quenching in water. ¹¹ This type of granulation process is still in practice these days successfully. All of the above-mentioned granulation process was developed for ferrous metals and yield was not considered as the objective in these inventions. In the present study, granulation of silver was done using the same granshot process with some modifications in the process. Highly pressurized argon (Ar) gas is used to flush the superheated metal through the orifice and the metal granules are collected in the water bath.^{12, 13} A schematic flow chart of the process is shown in Figure 1.

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Figure 1.

Schematic diagram of Granshot process.

The use of argon gas enables the quick flush out of the superheated metal into the water bath before solidification, thereby improving the yield of the process. In this study, the process of granulation is explained by experimentation of the granulation of silver. In addition to yield, shape and size are the other main factors considered in this study.

Material and methodology

Silver (Ag)

In this experimentation, granulation is carried out on silver metal. Silver is an extremely soft, ductile, and malleable metal used in the jewelry industry. The detailed mechanical properties of silver are listed in Table 1.

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Table 1.

Mechanical properties of silver used in experimentation.

S. no.	Property of metal	Value
1	Phase	Solid
2	Melting point	1234.93 K (961.78 °C, 1763.2 °F)
3	Boiling point	2435 K (2162 °C, 3924 °F)
4	Density	10.49 g/cm3
5	Density at melting point	9.320 g/cm3
6	Thermal expansion at 25 °C	18.9 µm/(m.K)
7	Thermal conductivity	429 W/(m.K)
8	Young’s modulus	83 GPa
9	Shear modulus	30 GPa
10	Bulk modulus	100 GPa
11	Poisson ratio	0.37
12	Brinell hardness	206–250 BHN

Experimentation

Large lumps of silver metal are cut into small pieces to accommodate them in the crucible. Then these pieces are entered into the crucible and the temperature of the control system is set at a temperature greater than the melting point of silver. The temperature should be set little high during superheating so that the viscosity of silver achieves more than 2.2 cP. The superheated silver is allowed to flow through the orifice located at the bottom of the crucible at argon pressure >2 Bar. The molten metal flowed through the orifice and stream of metal was quenched in the water bath. When the metal came into contact with water, it became spherical in shape and formed into granules, which were settled at the bottom of the water tank. After cooling, water was drained out and the silver granules were collected from the tank. The collected granules are shown in Figure 2. The detailed process parameters used in the process are shown in Table 2.

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Figure 2.

(a) Experimental setup for granulation process. (b and c) Granules collected in granulation tank.

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Table 2.

Parameters selected for experimentation.

S. No.	Parameters	Value
1	Metal lumps weight	4.108 kg
2	Induction furnace power in kW	12 kW
3	Melting time	9.38 minutes
4	Stand by time	10 minutes
5	Orifice diameter	1mm
6	Melting temperature	962 °C
7	Super heating temperature	1070 °C
8	Pouring pressure	2 Bar with turbo pressure

Results and discussions

Physical characteristics

The granshot process for granulation of silver is successfully completed. The acquired granules are perfectly spherical and have diameters of range 1.2–3.8 mm. The efficiency of the process is also highly impressive, that is, 99.9%. The details of the experimental results obtained are shown in Table 3.

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Table 3.

Results observed from granulation.

S. No.	Results	Value
1	Weight of silver granules	4.06 kg
2	Diameter of granules	1.2–3.8 mm
3	Shape	Spherical

Scanning electron microscopy and energy-dispersive X-ray spectroscopy

Scanning electron microscopy (SEM) was employed to study the surface of the granules of silver. The main objective of SEM is to check the shape and surface texture of granules. In Figure 3(a) and (b), the irregular shape of the granule was observed under SEM and it was found that the shape of this granule is not even close to spherical shape. The irregularity in shape may be because of its improper quenching or striking of the metal droplet against the wall of the water tank in a semisolid state. The surface of this granule contains blowholes. During solidification, gas and vapors try to escape the metal when it is in semisolid state, which results in the formation of blowholes on the surface. The granules shown in Figure 3(c) and (d) are very close to spherical shape. This indicates that it gets the desired shape just before get quenched and then gets solidified immediately. The surface of this granule (c and d) is clearer compared to the previous one (a and b). Though the surface of this granule is not so smooth, it is free from blow holes and voids on the outer surface.

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Figure 3.

SEM images of granules. (a and b) Irregular granule with blowholes on the surface. (c and d) Spherical granule.

Figure 4 depicts the EDS mapping of silver granules. From the EDS image, it is indicated that the major portion of the granules consists of silver. The processing of metal does not affect the phase of metal. The detailed state of elements present in granule is shown in Table 4. It shows that the majority of stakes take silver.

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Figure 4.

Energy-dispersive X-ray spectroscopy (EDS) of silver granule.

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Table 4.

Elemental state in granule recorded by EDS.

S. No.	Element	Weight	At. weight
1	O K	6.02	29.95
2	Ni K	1.18	1.61
3	Ag L	92.79	68.45

Conclusion

The article includes the process of silver metal for the granulation process. On the basis of experimentation and characterization, the following conclusions were made.