The High Energy Density Scientific Instrument at the European XFEL.

The European XFEL delivers up to 27000 intense (>1012 photons) pulses per second, of ultrashort (≤50 fs) and transversely coherent X-ray radiation, at a maximum repetition rate of 4.5 MHz. Its unique X-ray beam parameters enable groundbreaking experiments in matter at extreme conditions at the High Energy Density (HED) scientific instrument. The performance of the HED instrument during its first two years of operation, its scientific remit, as well as ongoing installations towards full operation are presented. Scientific goals of HED include the investigation of extreme states of matter created by intense laser pulses, diamond anvil cells, or pulsed magnets, and ultrafast X-ray methods that allow their diagnosis using self-amplified spontaneous emission between 5 and 25 keV, coupled with X-ray monochromators and optional seeded beam operation. The HED instrument provides two target chambers, X-ray spectrometers for emission and scattering, X-ray detectors, and a timing tool to correct for residual timing jitter between laser and X-ray pulses.

Stability of arsenate-bearing Fe(III)/Al(III) co-precipitates in the presence of sulfide as reducing agent under anoxic conditions.

Currently, the co-precipitation of arsenate with ferric iron at molar ratios Fe(III)/As(V) ≥ 3 by lime neutralization produces tailings solids that are stable under oxic conditions. However not much is known about the stability of these hazardous co-precipitates under anoxic conditions. These can develop in tailings storage sites by the action of co-discharged reactive sulfides, organic reagent residuals or bacterial activity. The ferric matrix can then undergo reductive dissolution reactions, which could release arsenic into the pore water. Co-ions like aluminum could provide a redox-immune sink to scavenge any mobilized arsenic as a result of reduction of ferric. As such, in this work Fe(III)/As(V) = 4 and aluminum substituted Fe(III)/Al(III)/As(V) = 2/2/1 co-precipitates were produced in a mini continuous co-precipitation process circuit and subjected to excess sulfide addition under inert gas to evaluate their stability. It was found that the ferric-arsenate co-precipitate could retain up to 99% (30 mg/L in solution) of its arsenic content despite the high pH (10.5) and extremely reducing (Eh < -200 mV) environment. There was no significant reduction of arsenate and only 45% of ferric iron was reduced. Partial aluminum substitution was found to cut the amount of mobilized arsenic by 50% (down to 15 mg/L) hence mixed Fe(III)/Al(III)-arsenate co-precipitates may offer better resistance to reductive destabilization over the long term than all iron co-precipitates.

Stability of continuously produced Fe(II)/Fe(III)/As(V) co-precipitates under periodic exposure to reducing agents.

Arsenic mobilized during ore processing necessitates its effective removal from process effluents and disposal in environmentally stable tailings. The most common method to accomplish this involves co-precipitation with excess ferric iron during lime neutralization. The precipitates produced are stable under oxic conditions. This may not be true, however, under sub-oxic or anoxic conditions. In this context, the potential stabilizing role of ferrous iron on arsenic removal/retention becomes important. As such, this work investigates the removal and redox stability of arsenic with ferrous, ferric and mixtures of both. The stability of produced solids is monitored in terms of arsenic release over time. It was found that ferrous was very effective for arsenic (V) removal with Fe(II)/As(V)=4, reducing its concentration down to <15 ppb via the apparent formation of ferrous arsenate. The presence of Fe(II) seemed to favor an oxidation path toward goethite (and possibly scorodite) formation in the aged bench-scale tailings. When pH and Eh were regularly adjusted with lime and sulfite or sulfide, slightly higher arsenic amounts were released (1-5 mg L(-1)); ferrous again was found to oxidize. Hence, it is concluded that Fe(II)/Fe(III)/As(V) co-precipitates are quite robust against incidental reducing agent exposure.