Oct-2016

A recommended approach to managing your contaminated sulphur

Contaminated sulphur clean-up can be a daunting task. It is natural to seek the cheapest and quickest methods of removal.

Izza Humayun
Enersul Limited Partnership

Article Summary

Unfortunately, the nature of sulphur can pose a hazard to the environment and requires appropriate and sometimes costly methods for handling, treatment or disposal.

To make a well-informed decision on a method, it is vital to understand the plausibility, process, and potential complications of these options in relation to the specific situation as well as their associated costs.

Due to stricter environmental regulations in recent years, a need exists for the reduction of sulphur pad usage. As a result, many producers are required to remove existing sulphur blocks, including their sulphur base pads.

Contaminated sulphur in the base pad of a storage block usually results from poured liquid sulphur intermingling and solidifying in the soil and bonding with impurities existing there, unless a proper base pad lining was employed. Further contamination can occur during reclamation when the remaining layer of sulphur is being scraped from the ground and the equipment picks up surrounding organic and inorganic materials. Contaminated sulphur may also be gathered during site clean-up and requires similar treatment to base pad sulphur.

Remelting contaminated sulphur poses many difficulties for obtaining consistent results as every case of contamination is different. Results obtained from clean sulphur remelting are incomparable since the success of sulphur remelting depends largely on the level or characteristics of the contaminants. The highest concentration of contaminants in a sulphur block is in the lowest layers.

Every site deals with a unique blend of contaminants. Both organic and inorganic contaminants may be present and though organic contaminants are present in a lesser proportion to inorganic, they can cause greater performance issues. Inorganic contaminants are insoluble materials such as rocks, gravel, shale, clay, silt and sand. Fines such as silt may take an excessive amount of time to settle in the melting tank or may not settle at all; instead staying in the flow of liquid sulphur. Likewise, ash content may remain in the final sulphur product.

Organic contaminants dissolve or react to an extent in liquid sulphur which can generate H2S, cause discolouration of the sulphur and foul the equipment. One example is car-sul, a polymer formed by the reaction between carbon and sulphur, which can foul the process equipment and plug filter surfaces if it is found in excess quantities. Grass, pine needles, cones, leaves and topsoil were found to discolour sulphur after being heated for a few minutes at 250°C or after an extended period at 140°C, which is within the range of normal remelt sulphur temperatures. Upstream plant upsets could also be a cause for high concentrations of alkanolamine or hydrocarbons which would be retained in the sulphur as it is poured into a sulphur block.

Unnecessary mixing of soil and sulphur can occur during reclamation. Organics such as soil and biomass are dug up during base pad reclamation if a concrete or asphalt base was not used. The care which is used in extracting the base sulphur can significantly affect the level of contamination which needs to be dealt with. A poorly prepared sulphur pad and inadequate paving for reclamation equipment will only escalate the contamination. If this occurs, the refuse soil may have high entrained sulphur content when it’s removed from the remelter, thus requiring treatment with an acid neutralising agent such as limestone before disposal.

High moisture content in the raw material greatly affects melting efficiency. Every 1% of moisture adds about 15% of extra useful heat required for remelting. Higher moisture content also leads to more acidic conditions inside the remelter. Likewise, rain water entering a remelter creates higher acidity when mixed with liquid sulphur, which then corrodes the equipment. For this reason, high acidity dramatically affects the equipment life span. Moisture content also creates foam on the surface of the liquid sulphur, which is at risk of overflowing from the remelter if the appropriate head space was not incorporated in the design. In one example of a pit remelter, 4% moisture was found to create as much as 22 inches of foam.

The higher the level of contamination in the remelter feed stock, the more frequently a remelter needs to be cleaned; particularly if it doesn’t have continual refuse removal. As contaminants collect in the bottom of the remelter, the heat transfer from the heating coils is impeded which negatively affects the performance. If the sulphur is extremely contaminated (over 50%), remelting and filtration may still be possible, but may not be practical because the material will not become mobile enough when heated above the melting temperature of sulphur. The size distribution of contamination – coarse versus fine – has a significant impact on the separation efficiency through remelting.

Remelters may be in either a pit or tank form and may use periodic or continual refuse removal. Tank remelters have much more room for variation in design and potential improvements, compared to a pit remelter. However, environmental regulations are the main driving factor for remelting technology selection.

The reality is that disposing of base pad sulphur may be a necessary and unavoidable expense. This is why it is important to have a realistic understanding of what work and resources need to go into a reclamation project, and how to properly plan to avoid escalated costs due to poor handling and improper evaluation of the situation. The first step is to determine if remelting to extract the sulphur is even feasible. By first investing in an analysis of the contaminated sulphur, the best course of action can be determined.

Enersul offers services to perform analyses on sulphur samples. The samples need to be representative of the entire contaminated sulphur source. If significant variation exists in the sulphur source and collecting a representative sample is challenging, multiple samples can be submitted. Tests are conducted on the samples to determine properties such as:
•  H2S content
•  Moisture content
•  Sulphur content
•  Acidity
•  Recoverable sulphur
•  Particle size distribution
•  Fines settling velocity
•  Carbon and ash content
•  Refuse sulphur content
•  Landfill classification

It is possible to then determine whether the contaminated sulphur is capable of being processed and detect potential safety hazards. At this point, client input is required to begin the design basis for information such as:
What is the minimum purity requirement of the output sulphur? What composition standards will the sulphur be marketed or sold to? This is directly linked to the design of the settling tanks, residence times, throughput and filtration.
What is the target throughput? Once the determination of recoverable sulphur is complete, a system throughput requirement (per day) can then be determined. A higher throughput requirement inherently changes the design and cost accordingly.
What standards do we design to? Are there national, regional, or company regulatory requirements, standards and practices to comply with?