Removal of Phosphorus, Sulphur and Arsenic from

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70wt% sulphur, 67wt% arsenic, and about 30wt% phosphorus were removed. ... sulphur. Such a combination of elements can lead to harmful effects on the environment .... Arsenic belongs to group V of the periodic table same as phosphorus.
The 8th Pacific Rim International Congress on Advanced Materials and Processing Edited by: Fernand Marquis TMS (The Minerals, Metals & Materials Society), 2013

REMOVAL OF PHOSPHORUS, SULPHUR, AND ARSENIC FROM FERRONICKEL AND NICKEL ALLOYS H. Chehade, Y.D, Yang, P. Wu, M. Barati and A. McLean Department of Materials Science & Engineering, University of Toronto, 184 College Street, Toronto, Ontario, Canada M5S 3E4 Tel: 416 978 1292, Fax: 416 978 4155, E-mail: [email protected] Key words:Used catalyst, ferronickel, nickel alloy, removal of phosphorus, sulphur and arsenic Abstract In the petroleum industry catalysts are often used to accelerate the reaction rate and remove the hazardous sulphur and arsenic from crude oil. A large quantity of spent catalyst material is generated every year and this represents a relatively cheap source of nickel supply. In order to use this nickel for the production of stainless steel or nickel alloys, the phosphorus, sulphur and arsenic in the material must be removed. The aim of this project is to remove these impurities from the nickel-based alloy produced from recovery of spent catalyst without incurring a significant loss of other valuable elements. Slag refining of nickel alloy was carried out in a 10 kw induction furnace. The effects of an oxidizing slag, a weak reducing slag and a strong reducing slag were examined. It was found that a strong reducing slag containing 70wt% CaC 2 and 30wt% CaF 2 was most suitable for the simultaneous removal of phosphorus, sulphur and arsenic without incurring a significant loss of valuable metal elements. With this slag, more than 70wt% sulphur, 67wt% arsenic, and about 30wt% phosphorus were removed. Introduction The petroleum industry relies on solid catalysts to produce high grade clean transportation fuels by facilitating hydrocarbon transformations. The catalysts are composed of a combination of metals, metal oxides, and metal sulfides. For example, diesel fuels with low sulphur content are produced through hydrotreating of sulphur containing oil streams using a Ni-Mo-Co catalyst on Al 2 O 3 Matrix. The flexibility of catalytic chemistry allows refineries to adjust their product specifications to produce a variety of products that range from heavy oils and residues to light and middle distillates and feedstock for petrochemicals[1,2]. However all catalysts have a confined life span. The catalyst used in the petroleum industry deactivate with time. Usually, the catalyst is regenerated and reused for a number of cycles after which the activity of the catalyst decreases to a minimum and is later discarded as a solid waste. The solid waste is then termed β€œSpent Catalyst”. The decrease in activity of catalyst is attributed to three factors. First, contamination by different chemicals, second, damage of phase structure and dispersion and finally, coke formation during the treatment processes[3,4]. The discarded solid must be treated for a number of reasons. There are a number of factors (environmental, economical, and health related issues) that control the refining of spent catalyst industry[5]:

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1- The market demand of world for catalysts is around 180,000 tons/year. According to recent studies, this value is estimated to increase by 4.4% per year due to an increase in the processing of heavier oil stocks containing higher concentrations of sulphur and asphaltene and also due to an increase in demand for clean fuels with ultralow sulphur. Furthermore, the rapid deactivation of the catalysts during petroleum hydrotreating processes and the lack of a reactivation process to enhance its life cycle and increase its reuse are driving refining initiatives. 2- Spent catalysts are classified as hazardous wastes since they are composed of organic/inorganic residues and other toxic compounds, such as arsenic, phosphorus, and sulphur. Such a combination of elements can lead to harmful effects on the environment and human health and thus require immediate processing. For example, a spent catalyst when in contact with water can result in the release of toxic gases such as HCN gas. 3- Governments are banning the disposal of spent catalysts without appropriate treatment. There are strict regulations that prohibit companies from such acts that are also unacceptable from the environment-protection and society-responsibility points of views. On the other hand, such solid wastes contain high amounts of precious metals that can be recoverable. The discarded spent catalyst contains valuable metals such as V, Co, Ni and Mo. These elements are usually used as alloying additions in the steel making industry and thus the spent catalyst is considered to be an appropriate additive in making a variety of steel products. However, the high concentration of Phosphorus (P), Sulphur (S) and Arsenic (As) limits its uses to scrap in Nickel and Copper smelters. P, S and As are considered as troublesome elements from the point of view of metal quality and the environmental concerns associated with their metallurgical processing. For example their accumulation in scrap steel is detrimental since they increase temper brittleness, decrease toughness, reduces corrosion resistance property and impairs weld ability of the steel and its alloys. As such there is a need to develop new processes that can remove P, S and As to a concentration below the final product specification limit. Current projects are centered on the idea of selective removal of P, S and As by hydrometallurgical techniques. However, there hasn't been any reference to successful proposals in the literature for the simultaneous removal of P, S and As to acceptable levels which is of interest in this project. Principles of Dephosphorization, Desulphurization and Dearsenization Recovering metals from secondary resources, for example spent catalysts, generally requires a chemical process to capture the metal in the desired chemical format. Thermodynamic principles establish the feasibility of a chemical reaction under certain operating conditions while kinetics determines the overall rate at which the reaction will proceed. Recovery of the Ni-Mo-V-Fe-Co metals from the alloy requires the removal of contaminant elements P, S and As. Dephosphorization The dephosphorization reaction is traditionally carried out under oxidizing conditions using CaO based fluxes during the refining of liquid steel. The reaction is given by:

Where,

5

3πΆπ‘Žπ‘‚(𝑠) + 𝑃2 (𝑔) + 2𝑂2 (𝑔) = πΆπ‘Ž3 𝑃2 𝑂8 (𝑠)

726

(1)

βˆ†πΊ1 π‘œ = βˆ’2313750 + 556.47𝑇 (J/mol)

(2)

The equation shows that in order to achieve good dephosphorization, the temperature should be low and the oxygen partial pressure should be high. The dephosphorization reaction should be conducted under controlled oxygen partial pressure so as to achieve maximum phosphorus removal without losing the valuable elements. Dephosphorization reaction under reducing conditions can be conducted using calcium containing fluxes. The reaction has the form:

Where,

3πΆπ‘Ž(𝑙) + 𝑃2 (𝑔) = πΆπ‘Ž3 𝑃2 (𝑠)

βˆ†πΊ3 π‘œ = βˆ’653460 + 144.01𝑇 (J/mol)

(3) (4)

According to the equation, good dephosphorization under reducing conditions can be achieved with high temperature and very low oxygen partial pressure Desulphurization Desulphurization reaction is traditionally conducted under reducing conditions where the reactions are given by: 1 πΆπ‘Ž(𝑙) + 2𝑆2 (𝑔) = πΆπ‘Žπ‘†(𝑠) (5) Where, (6) βˆ†πΊ5 π‘œ = βˆ’548100 + 103.8𝑇 (J/mol) And 1 1 πΆπ‘Žπ‘‚(𝑠) + 2𝑆2 (𝑔) = πΆπ‘Žπ‘†(𝑠) + 2𝑂2 (𝑔) (7) Where, (8) βˆ†πΊ7 π‘œ = βˆ’79898 βˆ’ 14𝑇 (J/mol)

Dearsenization The standard free energy of formation of Ca 3 As 2 was measured by D.J. Min and N. Sano [6]. The arsenic removal reaction is given by:

Where,

3πΆπ‘Ž(𝑙) + 2𝐴𝑠(𝑙) = πΆπ‘Ž3 𝐴𝑠2 (𝑠)

βˆ†πΊ9 π‘œ = βˆ’723800 + 172.8𝑇 (J/mol)

(9) (10)

According to the above equation good dearsenization also requires reducing conditions Oxygen Partial Pressure Consideration Taking desulphurization reaction as an example, with S and O in dissolved state, the equilibrium constant is given by [7]: 𝐾7 =

π‘Ž[𝑂] π‘Ž(πΆπ‘Žπ‘†) π‘Ž[𝑆] π‘Ž(πΆπ‘Žπ‘‚)

727

(11)

and the equilibrium sulphur distribution between slag and metal is given by: 𝐿𝑆 =

𝑓[𝑆] (𝑀𝑑%𝑆) = 𝐢𝑠 [𝑀𝑑%𝑆] π‘Ž[𝑂]

(12)

From the above equation, the equilibrium sulphur distribution depends on the sulphur capacity of slag which is also related to the free oxygen O2- ions and oxygen activity in metal [7].Usually the activity of oxygen is given by the deoxidation equilibrium in steel, however if the slag is characterized by a higher oxygen potential, in other words it has for example higher β€œFeO” content than the equilibrium a FeO / a [O] , the unstable oxide (in this case FeO) should dissolve and raise the ambient oxygen activity at the interface between slag and metal [7]. This should decrease the sulphur distribution and thus weaken desulphurization [7]. Thus controlling the oxygen partial pressure in the slag is an important consideration for the success of such a process. Designing Slags for Impurity Removal from Hot Metal Designing slags with high refining capacity has been a major focus of the Ferrous Metallurgy group at the University of Toronto. A large number of publications[8, 9, 10] are dedicated to characterizing the effect of different slags on the desulphurization and dephosphorization reactions in the steel and ferronickel industry. Red Mud During the extraction of Aluminum from Bauxite ore, about 2-4 tonnes of waste slag is generated for every one tone of Aluminum produced [11]. One of these by products is termed red mud [11]. Recently the idea of reusing red mud as an input slag for pyrometallurgical refining processes has been receiving increasing attention [11]. There is a great interest in using fluxes for hot metal pretreatment that produce high efficiency, low cost and environmentally friendly processes [11]. The chemical composition of a typical red mud from the waste slag of Aluminum extraction process is given in Table 1. Yang et al [11] studied the treatment of liquid steel with waste materials from the aluminate industry. It was shown that the by-products from aluminum production showed good desulphurization performance and as such are ideal reagents and environmentally friendly flux for hot metal pretreatment [11].

CaO RM1

29.65

Table I. Chemical Composition of Red Mud, (wt%) MgO Na 2 O K 2 O Fe 2 O 3 TiO 2 Al 2 O 3 1.12

6.01

0.8

9.97

2.85

4.78

SiO 2

LOI

14.2

30

CaC 2 and CaF 2 Slags In the steel making process, Arsenic cannot be removed in the usual steps. Arsenic belongs to group V of the periodic table same as phosphorus. They share similar physical properties however As cannot be removed as an arsenic oxide as in the case of dephosphorization of molten iron [12]. This harmful element can be removed by adding calcium or calcium carbide to pig iron or ferronickel [12]. An important step is finding the upper limit amount of the reducing agent to be used during the refining process. When using the lower limit amount, all the reducing agent will be consumed for desulphurization and as such As cannot be removed. Bing et al performed

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dearsenization experiments of low arsenic hot metal with CaC 2 -CaF 2 slag [13]. The experimental results showed a decrease in arsenic content with increasing reaction time using a slag of 60% CaC 2 and 40% CaF 2 [14]. Dearsenization and desulphurization reactions were also studied by Yuanchi et al using CaC 2 -CaF 2 based slags [14]. Dearsenization rate of 80% and desulphurization rate of 98% were achieved with the CaC2-CaF 2 based flux [14]. Experimental The Ni-Co alloy used in this research was obtained as a product from the recycling of spent catalyst from petroleum industry; its chemical composition is listed in Table 2. Table II. Chemical Composition of Alloy used in Present Work (wt%) Ni

Co

50.2 9.6

P

V

Fe

Mo

Si

As

Cu

C

Cr

9.1

9

9

6.4

2.5

0.6

0.4

0.4 0.39 0.15