Comminution circuits for compact itabirites Mining Mineração

In the beneficiation of compact Itabirites, crushing and grinding account for major operational and capital costs. As such, the study and development of comminution circuits have a fundamental importance for feasibility and optimization of compact Itabirite beneficiation. This work makes a comparison between comminution circuits for compact Itabirites from the Iron Quadrangle. The circuits developed are: a crushing and ball mill circuit (CB), a SAG mill and ball mill circuit (SAB) and a single stage SAG mill circuit (SSSAG). For the SAB circuit, the use of pebble crushing is analyzed (SABC). An industrial circuit for 25 million tons of run of mine was developed for each route from tests on a pilot scale (grinding) and industrial scale. The energy consumption obtained for grinding in the pilot tests was compared with that reported by Donda and Bond. The SSSAG route had the lowest energy consumption, 11.8kWh/t and the SAB route had the highest energy consumption, 15.8kWh/t. The CB and SABC routes had a similar energy consumption of 14.4 kWh/t and 14.5 kWh/t respectively.


Introduction
Iron ore grade reduction in Brazilian deposits, mainly in the Iron Quadrangle, results in the need for concentration in order to meet the specifications of the steel industry.This context applies for a large number of mines that nowadays operate with Hematite or high grade Itabirite and who in the near future, will have to beneficiate low grade and compact Itabirite.
The beneficiation route for compact Itabirite included the comminution of all plant feed followed by a desliming and a reverse flotation for production of Pellet Feed.In this kind of circuit, crushing and grinding account for the major operational and capital costs in reason of energy consumption and the large scale of the equipment and buildings.In this context, the study and development of comminution circuits is an important way to make the beneficiation of compact Itabirite feasible.Pereira et al (2010), Vasconcelos et al (2010) and Uliana et al (2012) study the comminution of compacts Itabirites for different mines in the Iron Quadrangle.This work will make a comparison between comminution circuits for compact Itabirites from the Iron Quadrangle.The circuits developed are: a crushing and ball mill circuit (CB), a SAG mill and ball mill circuit (SAB), and a single stage SAG mill circuit (SSSAG).For the SAB circuit, the use of pebble crushing is analyzed, SAB/C route.Figure 1 shows the grinding circuits studied in this work.For each circuit, the energy consumption and design of comminution equipment for a circuit processing of 25 million tons per year (Mty) is analyzed.
The ground product must have 10% of its particles above 0.15mm.Lima et al (2013) show a potential reduction of 15% in the capital costs (CAPEX) and 7% in the operational costs (OPEX) with the use of SAB route, for Itabirites with an Operating Work Index higher than 7,8kWh/t.

Methodology
The study was made with a sample of 2000 tons of a compact Itabirite collected in a Vale mine in the Iron Quadrangle.This sample was not meant to represent any specific mine, but a typical compact Itabirite, with more than 80% above 1 mm and Bond WI exceeding 10 kWh/t.In this way, the results obtained can be used as references for mines that have a large participation of compact Itabirites.Complete information about the tests and sample characterization can be found in Pinto, 2016.
The characterization of sample comprises the following tests: 1 -Bench scale test: 1.1 -Bond Work Index: Determination of Ball Mill Work Index using Bond methodology (BOND 1964).The test considered a 0.106mm screen to close the circuit.The results were used to fit the JKSimet™ software and to predict energy consumption in the grinding circuit using the Bond methodology (BOND 1964).The energy consumption predicted was compared with the energy consumption obtained by pilot tests.
1.2 Drop Weight Test and Abrasion Resistance: Determination of the impact breakage and abrasion breakage parameters (NAPIER-MUNN et al 1999).The results were used to fit the JKSimet™ software.
1.3 Donda Test.Determination of Donda energy consumption parameter, K (DONDA 2003).The results were used to predict the energy consumption in a grinding circuit using the Donda methodology (DONDA, ROSA 2014).The energy consumption predicted was compared with energy consumption obtained by pilot tests 2 -Industrial crushing test to obtain the crushing efficiency for compact Itabirite, in function of the closed side setting (CSS).The test was made in a Metso HP300™ cone crusher with a 25 mm CSS.From the results of this test, the efficiency of crushing for different CSS's was simulated in the JKSimet™ software.
3 -Pilot tests using ball mill and SAG mill circuits to obtain the energy consumption.For CB and SSSAG routes, a simulation was made with the JKSi-met™ software to adequate the grinding product to 10% above 0.15 mm, as aimed in the industrial circuit.The dimensions of the mills and the operational parameters of the pilot tests are shown in Table 1.The design of the industrial circuit does not comprise the primary crusher because this operation was the same for all circuits.For the crushing circuit, 6000 operational hours per year were considered, and for the grinding circuit, 7800 operational hour per year was considered.As the circuit was designed for 25 Mt/y, the mass flow was 4167 tons per hour (t/h) in the crushing circuit and 3205 t/h in the grinding circuit.

Results
Pereira et al (2010), in a characterization for Compacts Itabirites of Serra Azul, obtained an impact resistance (Axb) between 33 and 157 and a Bond WI between 8.7 kWh/t and 12.0 kWh/t.
Particle size distribution for the feed of industrial crushing test and pilot grinding tests are shown in Figure 2. The SAG mill pilot test and crushing test, used the same sample, without crushing.For ball mill pilot test, the sample was crushed until 12.5mm.
The results of the bench scale tests, are shown in Table 2.The mass balance of the crushing circuit for CB route, with the size refer-ence and equipment design is shown in Table 3.The crushers reference of unit capacity was obtained in the manufacture's manual (SANDIVIK 2011).
The energy consumption of crushing plants (secondary to quaternary) was 2.0 kWh/t.Table 4 compares the energy consumption and the mill design obtained from the pilot tests using Donda and Bond methodologies for the routes studied.The product of the grinding has 10% of particles above 0.15mm for all circuits.Mill diameter and length were obtained using the total power requirement and Austin equation for SAG Mill (AUSTIN 1990) and Rowland equation for Ball Mill (ROWLAND 1986).

Table 4
Grinding Circuits -Energy Consumption and Design.The amount of slimes, material below 0.010 mm, in the product of the grinding is an important parameter for the grinding circuit.Slimes are preju-dicial for the beneficiation circuit and must be removed from the circuit before flotation, so that, a large amount of slimes reduce the global mass recovery of the plant.
Table 5 compares the amount of slimes obtained in the grinding product of pilot test for each circuit analyzed.

Conclusions
The SSSAG route had the lowest energy consumption, 11.8 kWh/t and the SAB route had the highest energy consumption, 15.8 kWh/t.The CB and SABC routes had a similar energy consumption of 14.4 kWh/t and 14.5 kWh/t respectively.
The energy consumption in the SS-SAG route was lower than the ball mill consumption, 12.4 kWh/t, even with a new feed size without previous crushing in the SSSAG.
The energy consumption provided by the Donda methodology was close to that obtained in tests on a pilot scale for the CB and SAB/C routes.In contrast, the energy consumption provided by the Bond methodology was lower than the results obtained on a pilot scale for all routes studied.
Use of pebble crushing led to a reduction of 18.8% in the SAG mill power consumption for the SAB/C route, without increase in power consumption in the ball mill.
The amount of slimes in the grinding product was very close in all routes, ranging from 13.6% in the SAB/C route to 14,7% in the SSSAG route.With these results, it is not possible to clearly see a tendency of major slimes generation for any routes.

Figure 2
Figure 2 Feed of Pilot and Industrial Tests.

Figure 3
Figure3shows the crushing efficiency in function of size, for different

Table 1
Pilot Grinding Tests Parameters.

Table 3
Crushing Circuit -Mass Balance and Design.

Table 5
Grinding Product Slimes.For this route, two pebble crusher of 600kW must be considered, with an additional energy consumption of 0.2kWh/t. *