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We experimentally demonstrate visible light communication (VLC) with PPM modulated 532 nm DPSS laser for link loss upto ~78 dB while supporting bit rates up to 120 kbps and BER better than 1e-6.

Paste backfilling is an incipient underground mine backfill technology in India. It facilitates maximum use of mill tailings with enhanced stability of the underground workings and minimises rehandling of water, as well as provides bulk disposal of mining solid waste. Binder type and dosage plays an important role in paste backfill performance. This paper highlights environmentally friendly utilisation of solid wastes like lead–zinc mill tailings and lead–zinc smelter fuming furnace slag (FFS) as paste backfilling for an underground metalliferous mine. Various experiments were conducted to study the effect of use of FFS as a fractional replacement for ordinary portland cement (OPC) in paste backfilling. The physico-chemical properties of both the lead–zinc mill tailings and fuming furnace slag (FFS) have been examined. In the first set of experiments, raw slag (FFS) was used for paste backfill preparation and experimentation for uniaxial compressive strength (UCS) development, whereas in the second set of experiments, FFS was crushed to -75 μm (80 wt%) and used for the study. Multiple regression analysis of strength development was also conducted up to fifth order. The regression analysis is in accordance with the strength development and justifies the significance of OPC, crushed fuming furnace slag (CFFS) and waste chemistry on the strength gain with curing time. Use of crushed fuming furnace slag as OPC replacement in paste backfill showed encouraging results of strength development in contrast to raw FFS. Also, the economic analysis revealed that the paste backfilling cost per tonne reduced significantly with slag replacement in the binder phase.

An enormous amount of waste materials (tailings, fly ash and slag) are generated during mineral processing, power generation and metallurgical processes. A vast literature on utilising mill tailings for paste backfilling, fly ash and slag as partial replacement to cement is available. However, a detailed review on the application of mill tailings, fly ash and slag as mine paste backfill material has been lacking. This article provides a critical review of these industrial wastes’ utilisation as paste backfill material for backfilling of the underground mine voids. The properties of backfill materials affecting the paste backfill performance, advancements made in geomechanical, microstructural properties, strength prediction, leaching and economic aspects are scientifically reviewed. In addition to the above, the effect of alternative binders, specifically fly ash and slag, modification of paste backfill using alkali activator, superplasticizer, admixture and fibre reinforcement are comprehensively reviewed. The review ended with some future research directions in mine paste backfilling, especially applying natural and artificial additives for early strength development, strength and flowability prediction using statistical and artificial intelligence tools, leachates stabilization and geochemical modeling of leaching. Hence, this review would help in further application of such industrial waste materials for mine paste backfilling.

To maximise ore recovery safely and economically, a proper backfill design of required strength is imperative to facilitate extraction of adjacent stopes. Ordinary Portland cement (OPC) has a direct impact on the strength gained by the paste fill mix and is also a major contributor to the backfilling cost. This has prompted the users to go for partial replacement of OPC with readily available industrial wastes such as fly ash and jarosite to obtain a paste mix of desired strength. This paper highlights the use of a mathematical model to predict the strength requirement (unconfined compressive strength and cohesion) of self-supporting paste fill for secondary pillar extraction of stopes in metal mines. This paper also highlights extensive laboratory investigation carried out for partial replacement of OPC with fly ash and jarosite to map the changes in the strength of paste mix with time. Initially, the percentage of OPC required to achieve the designed strength was evaluated and subsequently, the percentage of OPC in the mix was replaced gradually with fly ash and jarosite. Laboratory results indicated that the strength of the paste mix increases with curing period (7, 14, 28 and 56 days) and the chemical composition of the ingredients also played a major role in strength development. The results were then compared with the required strength predicted from the model to obtain proper paste mix for existing mine’s stope dimension.

The increasing popularity of paste backfilling demands alternative binder optimisation and bulk waste disposal. This study investigates the efficacy of fly ash (FA) as partial replacement of ordinary portland cement (OPC) for paste backfill application in underground mines. The effect of FA addition on uniaxial compressive strength (UCS) and cohesion of paste backfill are demonstrated. The strength development was correlated with microstructural evolution using scanning electron microscopy (SEM)-energy dispersive X-ray spectroscopy (EDS). The study revealed that the rate of strength development in paste backfill slowed down when OPC was substituted by FA. However, the targeted 28 days' UCS of 1.1 MPa for the backfilling stope of lead-zinc mine was achieved with binder dosages of 8 wt% OPC, 7 wt% OPC, 6 wt% OPC, 7 wt% OPC +1 wt% FA and 6 wt% OPC +2 wt% FA. Thus, FA is a suitable binder substitute and up to 25% of OPC (8 wt%) can be replaced with FA. Analysis of microstructural evolution in paste backfill revealed that calcium silicate hydrate (C–S–H) did not develop in OPC-FA binder based paste backfill at its early days’ of curing and gypsum was found only in samples with OPC as sole binder. The multiple linear regression analysis on interaction effects of OPC, curing time, FA replacement percentage for 8 wt% and 5 wt% binder groups indicated that the UCS development is more sensitive towards FA replacement. The obtained results would help in better understanding and design of paste backfill for lead-zinc underground mines.

The methane retention mechanism in coal seams is markedly different from those of conventional gas reservoirs. Methane remains mainly as physically adsorbed molecules on micropore surface. Chemical and petrographic compositions of coal are the measures of maturity and type of organic matter that control the methane sorption characteristics of the coal. 99% of Indian coal occurrences are contributed by lower Gondwana sequences housed in two major geologic formations, younger Raniganj and older Barakar. The Raniganj Formation is best exposed in Raniganj Sub-basin and Barakar Formation is best exposed in Jharia Sub-basin of Damodar Valley. Present work attempts a systematic investigation on comparative account of methane sorption characteristics of coals from Raniganj Formation of Raniganj Sub-basin and Barakar Formations of Jharia Sub-basin in relation to their chemical composition and petrographic makeup. Chemical analyses shows that moisture, ash, volatile matter and fixed carbon varies between 2.5 and 4.6%, 10.0–27.2%, 38.8–40.2% (dmmf) and 59.8–61.2% (dmmf), respectively for Raniganj coals and, 0.5–1.1%, 16.7–32.9%, 20.7–22.0% (dmmf) and 78.0–79.3% (dmmf), respectively for Barakar coals. Carbon content is distinct for the suites of coal, 79.2–85.4% and 85.6–92.0% for Raniganj and Barakar coals, respectively. The vitrinite reflectance for the Raniganj coals ranges 0.53–0.72% and the Barakar coals ranges 1.09–1.23%. Based on the chemical composition and vitrinite reflectance value Raniganj coals belongs to high volatile bituminous type, whereas Barakar coals belongs to high to medium volatile bituminous type. Such variation in composition and maturity is mainly attributed to the variation in precursor organic matter as well as the basinal and thermal history of the sub-basins under consideration. H/C atomic ratio of the Raniganj and Barakar coals varies between 0.65 and 0.80 and 0.51–0.72 and O/C atomic ratio varies between 0.05 and 0.13 and 0.01–0.07, respectively. Coals of both the Raniganj and Barakar formations are mostly of kerogen Type-III with Raniganj coals falling in wet gas maturity stage approaching early-thermogenic methane generation whereas Barakar coals falling in condensate gas stage approaching peak-thermogenic methane generation. The Langmuir volume ranges from 9.3–21.8 cc/g (daf) for Raniganj coals and 21.1–29.1 cc/g (daf) for Barakar coals. Sorption capacity for the set of coals shows a strong rank dependency and increase with corresponding increase in rank down the stratigraphic column. Methane sorption capacity shows positive relationship with carbon content and vitrinite reflectance, and negative relationship with moisture content, ash and volatile matter. Moisture effect is more prominent in low rank Raniganj coals. The adsorption capacity shows a strong positive relation with vitrinite content and a moderate negative relation with inertinite content for both the Raniganj and Barakar coals, which may be attributed to dominancy of micropores in vitrinites with rank enhancement. The multiple regression analysis shows that the moisture is the main predictor of the VL, and the interaction of moisture with ash and reflectance mainly control the sorption capacity. A predictive model equation is developed for determination of sorption for Damodar basin coals from carbon, ash and moisture data.

Solid-to-water proportion decides the effectiveness of paste backfill in terms of transportation characteristics during mine backfilling. This article highlights various laboratory tests conducted to determine the optimum solid-to-water ratio. Also, numerical simulation was carried out using computational fluid dynamics technique (ANSYS FLUENT) to understand the slump lifting process and variation in volume of the paste with time. The optimum slump and spread for lead–zinc mill tailings paste were in the range 190– 200 mm and 330–340 mm respectively. The optimum water content in the paste fill for this study was found to be 23 wt%. Results show that the solid percentage is inversely related with slump and spread. Also, an optimum slump lifting speed needs to be maintained for accurate values of slump and spread.