选煤厂介耗分析论文.doc

第15页
中国矿业大学2011届本科生毕业设计

专业英文翻译

中 国 矿 业 大 学

毕业设计英语翻译

姓 名： 学 号：
学 院： 应用技术学院
专 业： 矿物加工工程 　
翻译题目： 选煤厂介耗分析
指导教师： 职 称： 讲 师

英文原文
An analysis of medium losses in coal washing plants Abstract
A major operating cost in dense-medium separation is in replacement of lost medium solids. The loss of medium solids, being costly, plays a crucial role in determining the economics of any preparation operation. Coal washeries that employ dense-medium cyclones often attempt optimization of the processes by varying the vortex or the spigot diameter and the feed relative density. While these changes help in closer control of the separation process, they also result in medium losses due to changes in the medium split ratio (ratio of the medium flow rate in overflow to underflow). Since medium solids are lost by adhesion to products and as magnetic separator effluent, the effect of the change in medium split ratio on the drain-and-rinse screens and, hence, the magnetic separator circuit needs to be studied. In Tata Steel's coal washeries, at Jharkhand India, which employs primary and secondary dense-medium cyclones in series to produce clean coal, middlings and rejects, reducing the relative density of feed medium, had an insignificant effect on the medium split ratio. On the other hand, changing the cone ratio (ratio of the overflow diameter to the underflow diameter) changed the relative density and the flow rates through the cyclone outlets, thus affecting the performance of the magnetite recovery circuit.
A systematic study through laboratory tests and a detailed plant sampling campaign helped in identifying the causes of magnetite loss. Upon implementation of the recommendations, the magnetite losses decreased, resulting in a saving of approximately US$27,500 per annum. The study also helped in evolving some checkpoints for plant operators for identifying magnetite losses.
Keywords: dense-medium cyclone; magnetite losses; drain-and-rinse screens; magnetic separators

1. Introduction
Dense-medium (magnetite in the case of coal), a slurry/suspension having a relative density intermediate to that of valuable mineral and gangue, is generally used as the medium of separation in most coal preparation plants. The medium, being costly, plays a crucial role in determining the economics of any preparation operation. Dardis (1987) quotes figures of 20–40% of dense-medium plant operating cost being attributable to medium loss for plants engaged in mineral separations employing ferrosilicon as the medium. The figure is 10–20% in the case of magnetite. There are clearly incentives to reduce this loss, and it is often possible to do so with minimal capital expenditure through improved operating procedures and minor changes to plant configuration. This paper draws on the experience from one such study carried out by R&D Tata Steel, Jamshedpur, Jharkhand, India to identify some important operating issues in medium loss in coal washing plants and the factors influencing the loss.
2. Causes of medium loss in dense-medium plants
There are normally only two possible routes by which medium can be lost from the plant:
• adhered to the products of separation, after draining and washing on screens; and
• present in the final effluent from the medium regeneration process, usually magnetic separators, settling cones or other solid–liquid separation devices.
The causes of loss from these sources are as follows:
• forces of attraction between the ore and medium particles, ore porosity, and inefficient washing;
• magnetic separation and classification inefficiencies;
• corrosion and abrasion of the medium, reported for ferro-silicon medium;
• excessive circuit loadings during the addition of fresh medium;
• housekeeping (when the floors are being cleaned and washed off);
• plant downtime (associated with housekeeping);
• medium properties (size, shape, magnetic susceptibility).
There has been much work done over the years, usually by operating plants, to identify and quantify the sources of medium loss and to minimize consumption. The task, however, is complicated by the difficulty of determining an unequivocal medium balance across the plant by sampling process streams. It is rare that a balance thus established, for a relatively short operating duration, reflects quantitatively the actual consumption recorded by the plant over normal reporting periods such as a month or a year.
2.1. Factors affecting losses through drain-and-rinse screens
Napier-Munn et al. (1995), during their investigations of the iron ore washing plants at Mount Newman and Tom Price, found that adhesion loss increases with screen loading. The effect was quite strong, and even moderately loaded screens showed a significant increase in loss (expressed in g/t/m of screen width) over lightly loaded screens.
An increase in operating relative density also led to significant increases in losses. Most of the increase in loss was attributed to the poor drainage characteristics of the higher viscosity medium (Kittel et al., 1987). A small increase in relative density led to a large increase in viscosity and thus poorer drainage characteristics.
The washing arrangement was also found to affect medium losses significantly through drain-and-rinse screens. Of the various washing arrangements, screens with weirs and a vigorous tumbling action reduced the magnetite losses considerably compared to slotted spray bars and screens with flood boxes.

2.2. Losses through the magnetic separators
There is no consensus in the literature as to the contribution which magnetic separator losses make to total medium loss in dense-medium plants. Dardis (1987), for example, claims that magnetic separators account for more than 75% of losses, whereas Mulder (1985) attributes only 18% to this source for the Sishen iron ore dense-medium cyclones. Kittel et al. (1987) reported magnetic separator losses between 2.4% and 24% of the total for the Mt. Newman dense-medium cyclone plant. However, on occasions, when very high viscosity media were used, substantial elevation of the adhesion losses was observed.
Adhesion to coal and the losses in the magnetic separator are the two main routes through which magnetite gets lost in a coal washing plant. In general, magnetic separators seem to contribute 20–40% of this loss, though this proportion will fall where adhesion losses are abnormally high, for example, with porous ores. Magnetic separators are therefore an important, though, not necessarily, a dominant source of medium loss. Since their performance can deteriorate markedly if not operated correctly or properly maintained, they deserve close attention.
Analysis of losses in magnetic separators collected in plant surveys by Rayner (1994) suggests that this could be due to the separator being overloaded, in terms of either its volumetric capacity or, less often, its dry solids capacity. Hawker (1971) and Sealy and Howell (1977) gave loading limits in terms of dry solids feed rate of magnetics and volumetric flow rate of feed slurry, which could not be exceeded without loss of performance.
Dardis (1987) confirmed that the operating variables, which affect magnetic separator performance, include pulp height, magnet position (angle), separation and discharge zone gaps, drum speed, and magnetics to non-magnetics ratio. Lantto (1977), writing from the perspective of a hard rock ilmenite concentrator, explained that the recovery in a magnetic separator was feed quality dependent. He also gave recommendations for various separator parameters.
Based on operating experience at the Iscor iron ore mines, De Villiers (1983) observed that overloading of the magnetic separators was the main cause of magnetic losses. He also gave the separator settings used at the Iscor plants.
3. Investigations at Tata Steel's coal washeries
Tata Steel at Jamshedpur, Jharkhand, India owns captive coal washeries, which supply 60% of coking coal requirements for its integrated steelmaking operations. In the washeries, the ROM coal after being crushed and screened at 0.5 mm, the + 0.5 mm fraction is treated in dense-medium cyclones (called the coarse circuit) and the − 0.5 mm in a flotation circuit (called the fines circuit). The + 0.5 mm coal is fed to the primary cyclones, which produce clean coal at a lower relative density of separation (1.3–1.5). The underflow from the primary cyclones form the feed to the secondary cyclones which in turn produce middlings and rejects at a higher relative density of separation (1.6–1.9). The magnetite recovery circuit is a typical circuit that exists in any coal washing plant and is shown in Fig. 1.

Fig. 1. Schematic medium recovery circuit and the sampling points.
The dense-medium and clean coal (middlings or rejects, as the case may be) is laundered to sieve bends and one set of drain-and-rinse screens. The sieve bends and the first section of each drain-and-rinse screen are used to drain medium from the coal; the medium is collected in screen under-pans and returned to the primary cyclone sump via the primary cyclone medium distribution box. The second section of the screens is used to rinse and drain the coal free of adhering medium. The spray water containing the dense-medium rinsed from the coal is collected in the second section of the screen under-pans and returned to the dilute medium sump for subsequent magnetite recovery. The level of magnetite water slurry in the dilute medium sump can be adjusted using the PID (Proportional, Integral, Derivative 3-term controller) loop provided for level control and the modulating splitter actuator. When the slurry levels in the sump rises, the splitter actuator would divert the flow away from the system to maintain balance. The indication loop also generates high and low alarm levels within the control system.
The dilute medium thus collected in the dilute medium sump is pumped to magnetic separators, which produce the recovered magnetite as over-dense medium and a reject tailings circuit. The over-dense medium is returned to the over-dense medium sump and distributed to the dense-medium washing circuits as make up. Magnetic separator tailings are used as product rinsing water.
Considering the overall economics of steel-making, it was thought to reduce the composite clean coal ash at the washeries from 17% to 16% starting April 2003. With a view to achieving 16% clean coal ash, the following changes were made in the coal washing plants:
(a) The relative density of medium in the primary cyclone circuit was reduced from 1.36 to 1.3–1.33
(b) The spigot diameter of the secondary cyclones was reduced from 140 mm to 125 mm.
These changes would have an effect on the medium split ratio (ratio of the medium flow rate in overflow to underflow) and hence an effect on magnetite recovery.
3.1. Effect of reduction of primary relative density on the magnetite recovery circuit
He and Laskowski (1995) studied the changes in medium split ratio by changing the vortex finder diameter and spigot diameter and cyclone inlet pressure at two different medium densities. A total of 27 different vortex finders versus spigot diameter combinations were studied. The tests were carried out with four different magnetite compositions. The studies showed that at a fixed inlet pressure, the relationship between medium split ratio and cone ratio was independent of medium properties. Extending the argument to the change in medium relative density in the primary circuit, it was concluded that there would be no change in medium split ratio due to the reduction in relative density from 1.36 to 1.3–1.33, and hence negligible effect on the medium losses.
3.2. Effect of reducing the spigot diameter in the secondary circuit
Reducing the spigot diameter of the cyclones would indirectly increase the cone ratio, i.e., the ratio of the diameter of vortex finder to the spigot thus affecting the medium split. The flow rate of medium through the overflow would increase and that through the underflow decrease, thus increasing the overall medium split.
Changes in the cone ratio would result in either lower/higher pulp relative density and higher/lower flow rates through the cyclone outlets. Lower pulp relative density will have a negligible effect on the performance of the drain-and-rinse screens. However, lower pulp relative density in the feed to the magnetic separator will inhibit the formation of flocs, which has been identified as the main process step for magnetic separation. According to the “conceptual collection mechanism” model developed by Rayner and Napier-Munn (2000), magnetic separation proceeds through the rapid formation of magnetic flocs or stringers as soon as the feed slurry is exposed to the magnetic field. Size per se of the magnetite particles is not involved in this process, although magnetic susceptibility is. A substantial proportion of the magnetic solids present will become part of these flocs. The residual magnetic solids are scavenged from the slurry during its passage through the collection zone of the separator. This scavenging occurs by solitary particles joining existing flocs, and is therefore a first order rate process relative to the concentration of residual magnetic particles. At low pulp relative density, the rate of flocculation is too slow for useful flocs to form, and thus capture is effectively a single particle process. This causes a distinct disadvantage to small particles, which may be preferentially lost.
Similarly higher pulp relative density would increase the viscosity of the medium coming out through the outlets of the dense-medium cyclone. This in turn would reduce drainage through the drain screens and increase adherence of the medium to the coal samples. This increased adherence of magnetite to coal would directly increase the magnetite loss after rinsing.
Within the capacity of the screens, increase/decrease in medium flow rate would not affect the performance of the drain-and-rinse screens. However, increased medium flow rate to the magnetic separator would reduce the residence time and hence the recovery of magnetics.
Decreased/increased medium flow rate to the magnetic separator would also affect its performance. The pool depth in the magnetic separator needs to be maintained at an optimum for an efficient magnetic separation. This can be done by adjusting the tailings discharge in the magnetic separator.
4. Experimentation
Detailed sampling campaigns were carried out in the magnetite recovery circuit. Samples of clean coal, middlings and rejects, and overflow, underflow and feed to the primary magnetic separator and one secondary separator were collected. These samples were col