Abstract
Engineers have known for some time that the efficiency of filtering iron ore concentrate slurry can be significantly improved by pre-heating the slurry ahead of the filters. However, heating such large quantities of dense slurry by conventional means has proven to be inefficient and high-maintenance. Recently though, submerged combustion was considered by Altos Hornos de Mexico SA (AHMSA), an integrated steel producer in the state of Coahuila, Mexico, as a means of pre-heating their iron ore concentrate. The company toured many iron ore facilities in Brasil and Labrador, reviewing their heating processes, and concluded that submerged combustion could provide a competitive advantage through operational efficiency and lower capital cost than steam boilers.

Inproheat was retained by the company to design and manufacture a SubCom slurry heating system to heat 403 m3/h of iron ore concentrate slurry from 25°C to 60°C. The 60GJ/h SubCom system consists of five natural gas combustion chambers mounted in an iron ore concentrate storage tank. The system features a heat recovery unit whereby the exhaust gases are used to preheat the incoming slurry stream, giving the system an overall thermal efficiency of 95%. As a result, the steel company expects to realise significant improvements in filtering efficiency which will enable them to increase the capacity of their pelletising plant without installing additional filters.

Introduction
This paper describes the design process and expected outcomes of a slurry heating system, using submerged combustion technology, at an integrated steel plant in Mexico. The purpose of heating the iron ore concentrate slurry prior to pelletising was to increase the filtration rate and cake moisture, enabling the pelletising plant to increase production without increasing the number of filters. The benefits of heating the slurry prior to filtering has been shown through independent studies, but has not been considered until now because of the

high cost of heating the slurry using conventional heating technologies, such as a boiler / heat exchanger system, or direct steam injection. The two above-mentioned conventional heating technologies are high capital cost systems due to the ancillary systems needed to support them, relatively low thermal efficiencies achieved and maintenance required.

Boilers operate at thermal efficiencies of around 75% to 85% in optimal conditions and require regular maintenance to ensure safety, due to the high-pressure operation. The thermal efficiencies of boiler systems tend to decrease with time as the heat exchangers foul. Furthermore, boiler systems require condensate return systems to maintain a reasonable efficiency; adding more components requiring maintenance. Steam and condensate piping must be insulated for heat conservation and personnel protection, adding to the cost.

Direct steam injection systems are less complicated and more efficient than a boiler and heat exchanger system since the steam is used to directly heat the liquid. However, direct steam dilutes the heated slurry, which is counterproductive prior to filtering and offsetts the benefits of heating the liquid.

Submerged combustion is a proven technology in several heavy industrial applications, and perhaps most notably in the liquid natural gas (LNG) industry where it has been used for over 50 years to heat seawater for the vaporisation of LNG. Relatively unknown in the mining industry, submerged combustion has the advantage of being able to heat any type of non-combustible liquid to near boiling point at thermal efficiencies approaching 100%, and without significant maintenance requirements. Submerged combustion is a process in which the products of combustion are exhausted through the liquid-to-be-heated. The energy transfer occurs directly between the liquid and the hot gases, resulting in high efficiencies with no fouling issues.

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