Mar-2013

Refining refiners’ spent catalysts

Spent catalyst is the raw material for a recycling operation that delivers fresh catalyst and specialist metals

B J STEPHAN
Gulf Chemical and Metallurgical Corporation

Article Summary

Gulf Chemical and Metal-lurgical Corporation (GCMC) provides environmental services to petrochemical refineries through a business designed to treat spent catalysts generated by the refineries in their hydrotreating, hydrodesulphurisation and residual desulphurisation processes. Fully permitted by federal, state and local agencies for receiving, storing and processing both hazardous and non-hazardous spent catalyst at its Freeport, Texas, location, GCMC operates a service for safely and environmentally responsible handling of spent catalysts. The company converts 99% of catalyst and fouling compounds into commodity forms of molybdenum, vanadium, nickel, cobalt and alumina.

Recycling and secondary mining forms the basis of GCMC’s business. Through complete recycling, the company adds value to catalyst consumers by enabling them to use materials with the highest initial activity to increase yields and extend an operating cycle to maximise profitability while minimising the carbon footprint of their production units. This will become more important as the next stage of the shale development relies not only on natural gas, but also on shale oil. These feedstocks will present additional challenges, as the deactivation of catalysts and fouling as a function of time will become non-linear at different points along the operating cycle. Due to these factors, partial treatment or landfilling of refinery wastes is not an environmentally or financially sustainable practice.

Process
There are three distinct plants and several support facilities that operate as one integrated process at GCMC (see Figure 1). They can be operated independently, in sequence, or as batch processes, depending on the types of material being recycled. The logistics, storage, classification and analytical functions tie all of the processes together in order to track, control and optimise the value chain.

Logistics
Materials arrive by truck or rail and are contained in different vessels including bulk railcars and trucks, roll-off boxes, collapsible sacks and a variety of totes. The choice of container depends on the characteristics of the spent catalyst and the method of loading at the customer’s location.

Storage and classification
The entire process begins with spent catalyst being classified and stored according to the characteristics of the material. Table 1 shows a typical composition for hydrotreating catalyst. These characteristics may be inherent to the original catalyst itself, including molybdenum, nickel and cobalt.

Other properties are imparted to the spent material due to additives and materials that are naturally occurring in the crude oil, including volatile organic compounds, phosphorus, nickel, arsenic, vanadium, iron and silicon. GCMC has 
a specialised Resource Conservation and Recovery Act (RCRA) permitted storage management system, including facilities with primary and secondary containment.

Oil recovery
Some spent catalysts, particularly those containing vanadium, also contain residual oil, which is collected and managed using an oil-water separator. Catalyst fines are removed from the oil and recycled in a process described below. The oil is segregated and sent off-site for fuel recovery.

Roasting plant
The roasting plant is used to produce calcine, which provides feedstock for the remaining plants. Spent catalysts are mixed with reagent chemicals and thermally treated using controlled amounts of heat and oxygen to burn residual oil present in the catalysts and to oxidise molybdenum and vanadium to their highest valence states to make them react with the reagents. These reactions produce soluble and insoluble metal compounds, depending on the types of catalyst and their associated contaminants. GCMC uses proprietary multiple hearth furnaces with pyrolysis and oxidation, which provide excellent thermal properties while remaining efficient for the high volumes of material that are processed yearly.

SO2 and particulate removal
The roasting process also liberates particulate matter, hydrocarbons and sulphur from the spent catalyst. The roasters operate in conjunction with a post-combustion chamber (PCC) and two electrostatic precipitators (ESP). During roasting, hydrocarbons are burned off. Part of the sulphur is oxidised to sulphur dioxide and part of it is converted to sodium sulphate. GCMC uses a series of ESPs, high-temperature bag houses and a circulating dry scrubber to capture the particulates and sulphur. In this step, the ESPs are used to collect particulates and recycle them back into the process for valorisation. The off-gases are humidified with water sprays and contacted with hydrated lime in a counter-current entrained bed reactor, where the reaction consumes the water and generates a dry byproduct

Hydrometallurgical plant
The calcine that has been produced during the roasting process is ground and leached to produce a slurry containing all of the remaining metals and contaminants. Grinding is necessary to ensure all of the solids have a uniform particle size distribution to increase the dissolution rate of soluble metals and to improve the efficiencies of mechanical decantation and filtration. Using decantation circuits and filtration, a solid concentrate containing alumina along with oxides of nickel and cobalt is produced. This material provides the base feedstock for the pyrometallurgical plant. The soluble fraction of the slurry is treated to remove phosphorus, aluminium and arsenic waste. The purity of V2O5 and MoO3 is reduced when these contaminants are present, ultimately reducing yield. This comprises the 1% of material that is not currently recycled and is disposed of as hazardous waste (99/100%).

Vanadium and molybdenum are contained in the purified solution and mixed with reagents to precipitate ammonium metavanadate (AMV), which is further calcined at high temperatures to decompose into ammonia and vanadium pentoxide (V2O5). Vanadium is a contaminant found in crude oil and ultimately one of the contributors to fouling of catalysts. It is primarily used for master alloys of steel and chemical production. In higher concentration, it gives steel the toughness necessary to be formed into drill bits and steak knives.