Fundamental Aspects of Alloy Smelting in a DC Arc Furnace

312 Pages·2014·4.78 MB·English

by  Rodney Jones

F U N D A M E N T A L A S P E C T S O F
A L L O Y S M E L T I N G I N
A D C A R C F U R N A C E

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Rodney Trevor Jones

PhD

2015

F U N D A M E N T A L A S P E C T S O F A L L O Y S M E L T I N G
I N A D C A R C F U R N A C E

Rodney Trevor Jones

A thesis submitted to the Faculty of Engineering and the Built
Environment, University of the Witwatersrand, Johannesburg, in
fulfilment of the requirements for the degree of Doctor of Philosophy

Johannesburg, 2015

DECLARATION
I declare that this thesis is my own unaided work. It is being submitted to
the Degree of Doctor of Philosophy to the University of the
Witwatersrand, Johannesburg. It has not been submitted before for any
degree or examination to any other university. Much of the information
presented in this thesis was obtained during the course of my employment
in the Pyrometallurgy Division at Mintek in Randburg.

17 June 2015

DECLARATION
I declare that this thesis is my own unaided work. It is being submitted to
the Degree of Doctor of Philosophy to the University of the
Witwatersrand, Johannesburg. It has not been submitted before for any
degree or examination to any other university. Much of the information
presented in this thesis was obtained during the course of my employment
in the Pyrometallurgy Division at Mintek in Randburg.

17 June 2015

ABSTRACT
DC arc furnaces have been applied to a number of smelting processes,
including the reductive smelting of chromite ore fines to produce
ferrochromium, the smelting of ilmenite to produce titania slag and pig
iron, the recovery of cobalt from non-ferrous smelter slags, stainless steel
dust smelting, battery recycling, and nickel laterite smelting.

The recovery of base metals and platinum group metals (PGMs) in a
reductive smelting process is a function of the recovery of iron (which
indicates the extent of reduction). A recovery equation has been
developed that is characterised by a single parameter (K) for each metal
that can either be fitted empirically to the data, or expressed in terms of
the equilibrium constant and the ratios of the activity coefficients
involved.

The DC arc furnace has been modelled electrically as an arc in series with
a layer of slag. The voltage is non-linear with respect to the current.
Equations have been developed (and confirmed by measurement) to
describe how the arc voltage varies as a function of arc length and current.
The voltage distribution across a molten slag bath requires the solution of
Laplace’s equation for a geometry that includes the depression in the
molten slag caused by the impingement of the arc jet. Aspect ratios of the
arc depression were determined photographically.

Equations have been developed for the calculation of the mean residence
time in a continuously-fed batch-tapped furnace, and this has been
illustrated using a novel graphical depiction. The mean residence time is
directly proportional to the tap-to-tap time, and is increased by increasing
the volume of material retained in the furnace between taps.
The ConRoast process treats dead-roasted nickel sulfide or PGM
concentrates by reductive smelting in a DC arc furnace, where an iron-
based alloy is used to collect the valuable metals. This process results in
much lower sulfur dioxide emissions, the ability to accept high chromite
contents, and improved furnace containment. The ConRoast process has
been demonstrated by smelting 50 000 tons of PGM-containing feed
materials at Mintek over a period of operation of about five years.